measures of population health—a terminological mini-study

prelude

carrying concepts across language barriers requires linguistic equivalents. finding equivalent terms often involves more than consulting a dictionary or artificial intelligence (AI)—particularly in rapidly evolving areas of research. even with terms that have been around for a while, however, one may sometimes have but a vague idea of what they really mean.

terminological equivalence has been defined as a one-to-one mapping of all of the characteristics of a given concept. finding equivalent terms, then, requires comparing the definitions of the source- and target-language terms. in contrast to simple vocabulary lists or AI-generated output, terminological entries generally comprise definitions, synonyms, grammar notes, comments, context examples, or other relevant information.

this terminological mini-study focuses on some of the key terms of population health. it also includes a german term newly coined, apparently in a hurry, when SARS-CoV-2 hit the stage, i.e., Fall-Verstorbenen-Anteil. although in rather frequent use and often accompanied by some purported english-language counterpart, there is a disconnect between the term and what it actually refers to.

in this post, discussed terms are italicized, and the arrow symbol → indicates that the term it precedes has been defined elsewhere in the post.

CASE

is defined by a set of standard criteria for classifying whether a person has a particular disease, syndrome, or other health condition [CDC 2012]

GOOD TO KNOW

A case definition consists of clinical criteria and, sometimes, limitations on time, place, and person. The clinical criteria usually include confirmatory laboratory tests, if available, or combinations of symptoms (subjective complaints), signs (objective physical findings), and other findings. Case definitions used during outbreak investigations are more likely to specify limits on time, place, and/or person than those used for surveillance [CDC 2012].

Example:

Listeriosis—surveillance case definition
Clinical description
Infection caused by Listeria monocytogenes, which may produce any of several clinical syndromes, including stillbirth, listeriosis of the newborn, meningitis, bacteremia, or localized infections
Laboratory criteria for diagnosis
Isolation of L. monocytogenes from a normally sterile site (e.g., blood or cerebrospinal fluid or, less commonly, joint, pleural, or pericardial fluid)
Case classification
A confirmed case is one that is compatible with the clinical description and laboratory-confirmed

Listeriosis—outbreak case definition
Clinically compatible illness with L. monocytogenes isolated (a) from a normally sterile site, (b) in a resident of Winston-Salem, North Carolina, (c) with onset between 24-Oct-2000 and 04-Jan-2001 [MMWR 1997]

CONTEXT

» A death due to COVID-19 is defined for surveillance purposes as a death resulting from a clinically compatible illness, in a probable or confirmed COVID-19 case, unless there is a clear alternative cause of death that cannot be related to COVID disease (e.g. trauma). There should be no period of complete recovery from COVID-19 between illness and death. « [WHO 2020a]

Fall

ist eine Person als Element einer bestimmten Menge von Personen, der bestimmte Merkmale (Fallkriterien) zugeordnet werden können [RKI 2015]

WISSENSWERT

Für Zwecke der epidemiologischen Untersuchung und Dokumentation stehen der Fall einer bestimmten Infektion oder der Fall einer bestimmten Krankheit zur Verfügung. Der Sicherung und Objektivierung dient eine Falldefinition. Die Falldefinition ist ein Instrument zur Einordnung eines Falles in ein spezielles Krankheitsgeschehen. Die Falldefinition kann klinische Symptome, Laborbefunde, personenspezifische Merkmale und epidemiologische Kriterien enthalten. Bei Ausbruchsuntersuchungen kann mit mehreren Falldefinitionen gearbeitet werden (z. B. für Verdachtsfälle und bestätigte Fälle). Eine sensitive Falldefinition erfasst möglichst alle relevanten Fälle, eine spezifische Falldefinition soll nur diagnostisch gesicherte Fälle einer bestimmten Krankheit erfassen [RKI 2015].

INCIDENT CASE

is an individual who changes from a non-disease to a disease state over a specific period of time [Alexander 2015]

Example:

In a study on the incidence of pneumonia within a 1-year time period, persons who develop pneumonia over the course of the 1-year study period are considered incident cases.

Inzidenzfall

ist eine Person, die in einem bestimmten Zeitraum in einer Bevölkerung vom Zustand ‚gesund‘ (bzw. von einer bestimmten Erkrankung oder Störung unbetroffen) in den Zustand ‚krank‘ (bzw. betroffen) wechselt [RKI 2015]

WISSENSWERT

Bei wiederholbaren Krankheiten wird in der Regel auch jeder Wiedererkrankungsfall als Inzidenzfall gezählt.

RATIO

is the relative magnitude of two quantities or a comparison of any two values • the numerator and denominator need not be related, which is why one could compare apples with oranges or apples with number of physician visits [CDC 2012]

EQUATION

Example:

Of the men enrolled in the National Health and Nutrition Examination Survey (NHANES) follow-up study, 3151 were nondiabetic and 189 were diabetic. Therefore, the ratio of non-diabetic to diabetic men was = 3151 / 189 x 1 = 16.7:1 [CDC 2012].

GOOD TO KNOW

In certain ratios, the numerator and denominator are different categories of the same variable, such as males and females. In other ratios, the numerator and denominator are completely different variables, such as the number of hospitals in a city and the size of the population of that city.

In epidemiology, ratios are used as both descriptive measures and as analytic tools:

• As a descriptive measure, ratios can describe the male-to-female ratio of participants in a study, or the ratio of controls to cases (e.g., 2 controls per case)
• As an analytic tool, ratios can be calculated for occurrence of illness, injury, or death between two groups. These ratio measures include risk ratio (relative risk), rate ratio, and odds ratio [CDC 2012]

Quotient

ist das relative Verhältnis zweier Größen zueinander, das Ergebnis einer Division 

PROPORTION

is the comparison of a part to the whole • a type of → ratio in which the numerator is included in the denominator [CDC 2012] •  a specific type of → ratio in which the numerator is a subset of the denominator but time is not a factor [Lang 2006]

EQUATION

GOOD TO KNOW

In epidemiology, proportions are used most often as descriptive measures. For example, one could calculate the proportion of persons enrolled in a study among all those eligible, the proportion of children in a village vaccinated against measles, or the proportion of persons who developed illness among all passengers of a cruise ship.

Anteil

ist ein relativer Teil eines Ganzen, dessen Elemente eine bestimmte Merkmalsausprägung oder übereinstimmende Identifikationskriterien (sachlich, räumlich und zeitlich) aufweisen [Minitab 2024]

GLEICHUNG

WISSENSWERT

Mit Anteilen können Gruppen ungleicher Größe verglichen werden. Ein Anteil kann als Prozentsatz (50 %), als Dezimalzahl (0,5) oder als Bruch (1/2) angegeben werden.

RATE

is the number of events that occur in a defined time period, divided by the average number of people at risk for the event during the period under study [Katz 2014] • a measure of the frequency with which an event occurs in a defined population over a specified period of time [CDC 2012] • a specific type of → ratio in which there is a distinct relationship between the numerator and the denominator and time is an intrinsic part of the denominator [Lang 2006]

GOOD TO KNOW

Epidemiologists generally restrict use of the term rate to measures that are expressed per unit of time [CDC 2012]. In this sense, a rate describes how quickly disease occurs in a population, e.g., 70 new → cases of breast cancer per 1000 women per year. This rate is an → incidence rate, and it is a measure that conveys a sense of the speed with which disease occurs in a population.

Others use the term rate more loosely, referring to → proportions as rates.

• For example, if 20 of 80 guests develop diarrhea after attending a reception, some describe this as the attack rate (or case rate) being 25%. However, because no unit of time is given, the accurate term to use here is → incidence proportion
• Sometimes the term prevalence rate is used to refer to the proportion of the population that has a health condition at a given point in time. However, the term to accurately describe this situation would be point → prevalence
• Similarly, → case-fatality rate is often used to describe the → proportion of persons with a disease who die from it [CDC 2012]. The puristically correct term to use is case-fatality proportion, or simply → case fatality

To be valid, a rate must meet certain criteria with respect to the correspondence between numerator and denominator. First, all events counted in the numerator must have happened to persons in the denominator. Second, all the persons counted in the denominator must have been at risk for the event in the numerator. For example, the denominator of a cervical cancer rate should contain no men. Rates are usually miltiplied by a constant multiplier—100, 1000, 10,000, or 100,000—to make the numerator larger than 1 [Katz 2014].

Rate

ist in der Epidemiologie ein Maß für die Häufigkeit des Eintretens eines Ereignisses in einem definierten Zeitraum bezogen auf eine Grundgesamtheit (Population), in der solche Ereignisse möglich sind [RKI 2015]

WISSENSWERT

Eine Wachstumsrate etwa stellt die relative Zunahme einer Größe innerhalb eines bestimmten Zeitraumes dar. So liegt die Wachstumsrate eines Unternehmens, das 2022 einem Umsatz von EUR 400.000 und im Jahr 2023 und einem Umsatz von EUR 500.000 erwirtschaftete, bei 25 %. Die Angabe einer Rate ist nur möglich, wenn auch der Zeitrahmen genannt wird, auf den sie sich bezieht.

Der Begriff Befallsrate oder Erkrankungsrate (engl., attack rate, case rate) hingegen bezeichnet eigentlich den → Anteil der Bevölkerung, der etwa im Rahmen einer Epidemie oder einer Veranstaltung eine Krankheit entwickelt. Wenn beispielsweise 20 von 80 Gästen nach einem Empfang Durchfall bekommen, läge die Befallsrate bei 25 %. Allerdings wird in diesem Fall der Zeitraum durch bestimmte Umstände (hier die Dauer des Empfangs) und nicht durch eine bestimmte Zeiteinheit definiert, weshalb die Befallsrate keine → Rate im eigentlichen Sinn ist, sondern eine → kumulative Inzidenz.

MORBIDITY

is any departure, subjective or objective, from a state of physiological or psychological well-being [Last 2001]

GOOD TO KNOW

In practice, morbidity refers to the presence of disease, injury, or disability. Measures of morbidity frequency characterize the number of persons in a population who become ill (→ incidence) or are ill at a given time (→ prevalence). Although morbidity often means the number of persons who are ill, it can also be used to describe the periods of illness that these persons experience or the duration of these illnesses [CDC 2012].

Morbidität

ist die Häufigkeit des Vorkommens von Erkrankungen in einer bestimmten Population, die sich aus den ständigen Zugängen und Abgängen von Kranken ergibt [RKI 2015]

WISSENSWERT

Im Gegensatz zu den Begriffen → Mortalität und → Letalität ist der Begriff Morbidität nicht allgemein verbindlich definiert [Habermehl 1986]. Zwei häufig für die Beschreibung der Morbidität verwendete Maßzahlen sind → Inzidenz und → Prävalenz.

Die Zugänge (Neuerkrankungen) entsprechen der → Inzidenz. Die Abgänge ergeben sich aus Heilung, Tod, Berichtigung der Diagnose oder Ortsveränderung. Der Bestand als Resultat der Zu- und Abgänge, beeinflusst von der Krankheitsdauer, wird als → Prävalenz angegeben. Damit ist die Morbidität ein zusammenfassender Begriff für das Erkranken (→ Inzidenz) oder das Kranksein (→ Prävalenz) [RKI 2015].

MORBIDITY RATE

a term, preferably avoided, used indiscriminately to refer to the → incidence rate or the → prevalence of a disease [Last 2001]

Morbiditätsrate

unpräziser Begriff, der entweder die → Inzidenzrate oder die → Prävalenz einer Erkrankung meint

INCIDENCE

refers to the occurrence of new → cases of disease or injury in a population over a specified period of time [CDC 2012]

GOOD TO KNOW

Commonly used types of incidence are → incidence proportion, → incidence rate, and → incidence density. They are similar in that the numerator for all three is the number of new cases that developed over a specified period. They are different in how they express the dimension of time [LaMorte 2021].

Incidence, of course, only reflects reported and diagnosed cases of a disorder. The actual size of the problem is likely to be higher. The same is true for → prevalence.

Inzidenz

bezeichnet die in einer Population innerhalb einer bestimmten Zeitspanne neu auftretenden Krankheitsfälle oder Ereignisse • misst die innerhalb eines bestimmten Zeitraums neu auftretenden Krankheitsfälle (bzw. gesundheitsbezogenen Ereignisse) in einer definierten Gruppe von Personen, die zu Beginn des Beobachtungszeitraums frei von der zu untersuchenden Krankheit waren [Dreier 2012]

Synonym: Neuerkrankungswahrscheinlichkeit

WISSENSWERT

Genau definierte Maßzahlen der Inzidenz sind → kumulative Inzidenz, → Inzidenzrate und → Inzidenzdichte.

Die Inzidenz von Todesfällen wird als → Mortalität bezeichnet.

INCIDENCE PROPORTION

is the proportion of an initially disease-free population that develops disease, becomes injured, or dies during a specified period of time [CDC 2012] • the denominator is the number of persons in the population at the start of the specified period

Synonym: cumulative incidence [Last 2001, Cummings 2019], incidence risk

EQUATION

GOOD TO KNOW

Incidence proportion is a → proportion because the persons in the numerator (i.e., those who newly develop a condition) are all included in the denominator (i.e., the entire population) [CDC 2012]. It is a measure of the risk of disease or the probability of developing the disease during the specified period. This period is generally the same for all members of the group, but, as in lifetime incidence, it may vary from person to person without reference to age [Last 2001].

Incidence proportions range from 0 to 1. They are meaningless without relating them to a time interval. A woman may have a risk of developing breast cancer during the next 3 years equal to 0.01, but her risk of developing breast cancer in the next day is much lower and her risk of developing breast cancer in the next 20 years is much higher. Incidence proportions are customarily considered to have no units; they are a count of people, with the outcome divided by the count of people under surveillance [Cummings 2019].

Incidence proportion takes no notice of the decline in person-time at risk

The incidence proportion does not account for the decline in person-time at risk that necessarily occurs during the follow-up period as the first outcome occurs (even in the absence of competing risks or dropouts), thereby underestimating the true rate. Once a woman develops her first breast cancer, she is no longer at risk for another first breast cancer. The incidence proportion of 0.01 is not based on the follow-up of 10,000 women free of breast cancer for 3 years; 10,000 is the number free of breast cancer at the start of follow-up, but that number declines as cases of breast cancer occur [Cummings 2019].

Incidence proportions are not → rates

The analytic methods for → proportions (which range from 0 to 1) and → incidence rates (which range from 0 to infinity) are different. Also different is the interpretation of the results. Using the word rate for statistics that are proportions causes unnecessary confusion. For example, if we are following a group of persons over time, e.g., in a clinical study, the term drop-out rate could mean an incidence proportion (count of drop-outs / count of original population) or an → incidence rate (count of drop-out events / person-time at risk for drop-out) [Cummings 2019].

• In an outbreak setting, the term attack rate (→ rate) is sometimes used to describe what is actually an incidence proportion (→ proportion)
• Similarly, many authors refer to a response rate to describe the → proportion of contacted individuals who completed a questionnaire
• Also, → case-fatality rate is often used to describe the → proportion of persons with a disease who die from it. The count of persons who die of a disease divided by the number who acquire the disease is an incidence proportion. Therefore, the correct term to use for the concept behind → case-fatality rate actually is case-fatality proportion, or simply → case fatality [CDC 2012]

The incidence proportion approximates to the → incidence rate multiplied by the time duration when that product is small [Last 2001].

Kumulative Inzidenz

gibt den → Anteil der Personen an, die in einem definierten Zeitraum mindestens einmal an einer Krankheit erkranken [Sachs 2018] • kann auch als personenbezogenes Risikomaß für die Wahrscheinlichkeit definiert werden, dass eine Person in einem Zeitraum erkrankt bzw. von dem betrachteten Problem betroffen wird [RKI 2016]

Synonym: Inzidenzanteil, Inzidenzrisiko, Risiko

FORMEL

WISSENSWERT

Die Bezugsbevölkerung der kumulativen Inzidenz entspricht allen Personen, die zu Beginn des festgelegten Zeitraumes ein Erkrankungsrisiko haben. Zur Berechnung der kumulativen Inzidenz für Ovarialkarzinom etwa werden Männer aus der Bezugsbevölkerung ausgeschlossen.

Die kumulative Inzidenz wird vor allem in Längsschnittstudien verwendet und wird als → Quotient aus der Anzahl der im betrachteten Zeitraum erkrankten Personen und dem Anfangsbestand (gesunder) Personen errechnet [RKI 2016]. In Querschnittstudien könnte die kumulative Inzidenz etwa bestimmt werden, indem man die Studienteilnehmer befragt, ob sie im vorangegangenen Jahr an einer bestimmten Krankheit erkrankt waren. Der Anteil der diese Frage bejahenden Personen wäre hier die kumulative Inzidenz.

Die kumulative Inzidenz ist ein Anteil und nimmt entsprechend einen Wert zwischen 0 und 1 an. Sie trägt keine Einheit, aber die Angabe einer kumulativen Inzidenz ohne Nennung eines Zeitraums ist sinnlos, da die kumulative Inzidenz mit der Zeit zunimmt. Wird kein Zeitraum angegeben, ist meistens ein Zeitraum von einem Jahr gemeint. Diese Zeitspanne hat den Vorteil, dass sich jahreszeitliche Schwankungen weitgehend ausmitteln [wikipedia].

Beispiel:

Im Jahr 2005 mussten sich 16 Personen pro 100.000 Einwohner erstmals einer Dialysebehandlung unterziehen.
Kumulative Inzidenz = 16 : 100.000 = 0,00016 = 0,016 %

Der Begriff Befallsrate (siehe → Rate) bezeichnet den Anteil der Bevölkerung, der etwa im Rahmen einer Epidemie die Krankheit entwickelt. Allerdings wird der Zeitraum durch bestimmte Umstände (etwa die Dauer der Epidemie) und nicht durch eine bestimmte Zeiteinheit definiert, weshalb die Befallsrate keine → Rate im eigentlichen Sinn ist, sondern eine kumulative Inzidenz.

INCIDENCE RATE

is a measure of → incidence which, in contrast to the → incidence proportion, incorporates time directly into the denominator [CDC 2012] • the → rate at which new events occur in a population • calculated as the number of → incident cases over a defined study period, divided by the population at risk at the midpoint of the study period [Katz 2014] 

EQUATION

GOOD TO KNOW

As with → incidence proportion, the numerator of the incidence rate is the number of new cases identified during the observation period. However, the denominator differs. Thus, in a dynamic population, the denominator of the incidence rate is the average size of the population, often the estimated population at mid-period (mid-period population, mid-interval population) [Last 2001] or the average of the populations of 2 consecutive years (mean population, average population) [Statistics Finland].

The annual incidence rate of a particular condition is generally expressed as the number of persons affected per 100,000 of the population.

If the incidence rate is low, as with many chronic diseases, it is also a good estimate of the → incidence proportion over a period, e.g., a year [Last 2001].

Inzidenzrate

Maß für die Ausbreitungsgeschwindigkeit einer Krankheit [Sachs 2018] • gibt den Anteil der Menschen an, die in einem definierten Zeitraum neu erkranken und berücksichtigt, im Gegensatz zur → kumulativen Inzidenz, zusätzlich die Dynamik der Populationsgröße im Beobachtungszeitraum. Sie ist der Quotient aus den Inzidenzfällen eines Zeitraumes dividiert durch die (mittlere) Größe der betrachteten Bevölkerung in diesem Zeitraum [RKI 2016]  

Synonyme: Neuerkrankungsrate, Zugangsrate

FORMEL

WISSENSWERT

Außerhalb klinischer Studien können meist nur → Inzidenzfälle erhoben werden, nicht aber die genaue Personenzeit unter Risiko. Auch treten innerhalb des Untersuchungszeitraums in der betrachteten Bevölkerung Ab- und Zuwanderung, Sterbefälle und Geburten auf. Teilt man die Anzahl der → Inzidenzfälle durch die Bevölkerung zur Jahresmitte oder die mittlere Bevölkerung, ergibt sich die Inzidenzrate. Die mittlere Bevölkerung ist die durchschnittliche Einwohnerzahl in einem Zeitraum (Mittel aus dem Anfangs- und dem Endbestand, also dem Mittel aus der Einwohnerzahl zu Beginn und der Einwohnerzahl am Ende der Zeitperiode) [RKI 2015].

Die Inzidenzrate ist ein genaueres Maß als die → kumulative Inzidenz. Sie wird oft pro 1000 oder 100.000 Personen angegeben und ist immer auf einen bestimmten Zeitraum bezogen (häufig 1 Jahr).

INCIDENCE DENSITY

is a measure of → incidence which, in contrast to the → incidence proportion, incorporates time directly into the denominator • is the rate at which new events occur in a population [CDC 2012] • is the number of new events per person-time [Katz 2014].

Synonyms: person-time rate [CDC 2012], person-time incidence rate, force of morbidity [Last 2001]

EQUATION

Example:

Suppose that 3 patients were followed after tonsillectomy and adenoidectomy for recurrent ear infections. If one patient was followed for 13 months, one for 20 months, and one for 17 months, and if 5 ear infections occurred in these 3 patients during this time, the incidence density would be 5 infections per 50 person-months of follow-up, or 10 infections per 100 person-months [Katz 2014].

GOOD TO KNOW

The incidence density is determined by dividing the total number of new cases of an event by the sum of each individual’s time at risk and is expressed as person years at risk. It is often calculated for long-term cohort studies that follow participants over time and document the occurrence of new cases of disease, symptoms, or side effects. Each person in a long-term study is typically observed from a defined starting time until one of the following endpoints is reached, i.e.,

• onset of disease or symptom
• death
• loss to follow-up
• end of study

From then on, participants are no longer at risk and will no longer contribute any person-time at risk.

Similar to the → incidence proportion, the numerator of the incidence density is the number of new cases identified during the observation period. However, the denominator differs—it is the sum of the time each person was at risk and being observed, totaled for all persons [CDC 2012].

Because the incidence density is based on person-time, it has some advantages over an → incidence proportion:

• Person-time is calculated for each person, and it can therefore accommodate persons entering and leaving the study
• The denominator accounts for study participants who are lost to follow-up or who die during the study period
• It allows participants to enter the study at different times

Incidence density is especially useful when the event of interest (e.g., colds, otitis media, myocardial infarction) can occur in a person more than once during the study period [Katz 2014].

A drawback of person-time is that it assumes that the probability of disease during the study period is constant, so that 10 persons followed for 1 year equals 1 person followed for 10 years [CDC 2012]. Similarly, in clinical studies, the risk of adverse reactions may increase (or decrease) with longer treatment durations.

Inzidenzdichte

gibt die Häufigkeit oder Wahrscheinlichkeit an, mit der eine Person unter gegebenen Bedingungen zu einem bestimmten Zeitpunkt oder in einem bestimmten Zeitraum an einer bestimmten Krankheit neu erkrankt und berücksichtigt, im Gegensatz zur → kumulativen Inzidenz, zusätzlich die Dynamik der Populationsgröße während des Beobachtungszeitraumes, also die Tatsache, dass Personen während dieses Zeitraumes die Population verlassen bzw. in sie eintreten [Spektrum 2001, Dreier 2012] • bezieht die Anzahl der → Inzidenzfälle auf die (gesunde) Personenzeit unter Risiko der betrachteten Bevölkerung im betrachteten Zeitraum [RKI 2016]

Synonyme: Personenzeit-Inzidenzrate, Krankheitsstärke [Kreienbrock 2012]

FORMEL

WISSENSWERT

Im Rahmen klinischer Studien wird bei der Erhebung der Inzidenzdichte die Zeit berücksichtigt, in der jede Person tatsächlich unter Risiko stand und beobachtet werden konnte (Risikozeit). Die individuellen Risikozeiten werden zur sogenannten Personenzeit unter Risiko der betrachteten Population im betrachteten Zeitraum aufsummiert. Die Inzidenzdichte ist im Gegensatz zur → kumulativen Inzidenz von der Länge des Beobachtungszeitraums unabhängig, sofern die Dynamik der Erkrankung gleich bleibt. Anders als bei der → kumulativen Inzidenz gehen mehrmalige Erkrankungen derselben Person im Untersuchungszeitraum in der Regel mehrfach in die Berechnung ein. Die Inzidenzdichte ist ein genaueres Maß als die → kumulative Inzidenz und wird meist in Personenjahren angegeben.

Beispiel:

In Stadt X ereigneten sich 1973 unter 40–44-jährigen Männern (41.532 Personenjahre) 29 Herzinfarkte. Damit betrug die Inzidenzdichte = 29 / 41.532 = 0,00071 pro Jahr [Sachs 2018].

PREVALENCE

is the proportion of persons in a population who have a particular disease or attribute over a specified period or at a specified point in time [CDC 2012]

Sometimes (and wrongly) used synonymously: prevalence rate [Katz 2014]

EQUATION

GOOD TO KNOW

Prevalence differs from → incidence in that prevalence includes all new and preexisting cases in the population, whereas → incidence is limited to new cases.

• Point prevalence refers to the prevalence measured at a particular point in time. It is the proportion of persons with a particular disease or attribute on a particular date
• Period prevalence refers to prevalence measured over an interval of time. It is the proportion of persons with a particular disease or attribute at any time during the interval

Prävalenz

Angabe der Bestandshäufigkeit, des Maßes einer Verbreitung, des Bestands an Erkrankten, Infizierten oder Merkmalsträgern in einer bestimmten Bevölkerung zu einem bestimmten Zeitpunkt oder innerhalb einer bestimmten Zeitperiode [RKI 2015]

Synonym: Krankenstand

FORMEL

Beispiel:

2015 erhielten 80 Personen pro 100.000 Einwohner eine Dialysebehandlung.
Prävalenz = 80 : 100.000 = 0,0008 oder 0,08 Prozent

WISSENSWERT

• Punktprävalenz: Prävalenz zu einem bestimmten Zeitpunkt
• Periodenprävalenz: Prävalenz innerhalb eines Zeitabschnitts (zum Beispiel Jahres- oder Lebenszeitprävalenz)

PREVALENCE RATE

is actually a → proportion and not a → rate [Katz 2014] • term sometimes used to refer to the proportion of the population that has a health condition at a given point in time, although the accurate term to describe this is point → prevalence

Prävalenzrate

ist eigentlich keine → Rate, sondern ein → Anteil • unpräziser Begriff, der in der Regel die Punktprävalenz meint (→ Prävalenz)

MORTALITY

is the frequency of death in a population [Britannica 2019] • retrospective information on the number of deaths caused by the health event under investigation that allows for continuous evaluation of the efficacy of either a specific health care system or an implemented intervention in place [Hernandez 2022]

Mortalität

ist die bevölkerungsbezogene Betrachtung der Sterblichkeit [RKI 2016]

Synonym: Sterblichkeit

WISSENSWERT

Zwischen Mortalität und → Letalität besteht bei gleichbleibender Erkrankungshäufigkeit folgender Zusammenhang [Habermehl 1986]:

Mortalität = → Inzidenz x → Letalität

MORTALITY RATE

is a measure of the frequency of occurrence of death in a defined population during a specified interval [CDC 2012] • an estimate of the portion of a population that dies during a specified period [Last 2001]

Synonym: death rate [Last 2001]

EQUATION

GOOD TO KNOW

The denominator of the mortality rate most commonly used is the the mid-interval population (i.e., the size of the population at the middle of the time period) or the mean population, i.e., the average of the populations of 2 consecutive periods.

The crude mortality rate is the mortality rate from all causes of death for a population.

Example:

In the United States in 2003, a total of 2,419,921 deaths occurred. The estimated population was 290,809,777.
The crude mortality rate in 2003 was, therefore, (2,419,921 / 290,809,777) x 100,000, or 832.1 deaths per 100,000 population.

The crude mortality rate does not reflect the burden of death in particular groups of populations, such as in different age groups. Thus, a higher mortality rate among one population than among another may simply reflect the fact that the first population is older than the second. The age-adjusted, or age-standardized, mortality rate is a mortality rate statistically modified to eliminate the effect of different age distributions in the different populations.

Example:

The mortality rates in 2002 for the states of Alaska and Florida were 472.2 and 1,005.7 per 100,000, respectively. The reason that Alaska’s mortality rate is so much lower than Florida’s is that Alaska’s population is considerably younger. Indeed, for 7 age groups, the age-specific mortality rates in Alaska are actually higher than Florida’s. To eliminate the distortion caused by different underlying age distributions in different populations, statistical techniques are used to adjust, or standardize, the rates among the populations to be compared. These techniques compare the population in question with a standard population in which the age and sex composition is known precisely as a result of a census or by an arbitrary means, e.g., an imaginary population, the ‘standard million,’ in which the age and sex composition is arbitrary [Last 2001]. Mortality rates computed with these techniques are age-adjusted or age-standardized mortality rates. Alaska’s 2002 age-adjusted mortality rate (794.1 per 100,000) was higher than Florida’s (787.8 per 100,000), which is not surprising given that 7 of 13 age-specific mortality rates were higher in Alaska than Florida [CDC 2012].

The mortality rate and the → case fatality provide different information.

Example:
Picture 2 populations.

Population A consists of 100 people, 30 of whom are diagnosed with disease A, and 15 die from it.
Mortality rate = 15 / 100 = 0.15, or 15%.
Case fatality = 15 / 30 = 0.5, or 50%.

Population B consists of 100 people, 6 of whom are diagnosed with disease B, and 5 die from it.
Mortality rate = 5 / 100 = 0.05, or 5%.
Case fatality = 5 / 6 = 0.83, or 83%.

Thus, the incidence of death from disease A is higher than that from disease B.
However, the severity of disease B is greater than that of disease A.

bezeichnet die Sterblichkeit in einer Population in einem bestimmten Zeitraum [RKI 2015] • ist die auf die Gesamtbevölkerung bezogene Anzahl der im Bezugszeitraum Verstorbenen [Habermehl 1986]

CASE FATALITY

is the proportion of individuals with a particular condition who die from that condition [CDC 2012] • the count of deaths divided by the original number of cases of new illness over a given follow-up time interval [Cummings 2019]

Synonyms: case-fatality proportion [Cummings 2019, Kelly 2013], fatality proportion [Cummings 2019, Kelly 2013], case-fatality risk [Cummings 2019, Kelly 2013], lethality [Souris 2020]

Wrongly used synonymously: case-fatality rate (CFR), case-fatality ratio

EQUATION

Example:

Around 1850, the case fatality of cholera was up to 40%. This means that out of each 100 cases of newly diagnosed cholera, 40 would eventually die due to the disease, usually within 2 weeks after onset. In comparison, the case fatality of tuberculosis in those times was almost 100% within the first 2 years after diagnosis [Bosman 2014].

GOOD TO KNOW

Being a proportion, the case fatality is usually expressed as per 100 or per 1000. The case fatality can be seen as a → cumulative incidence [Bosman 2014]. People enter the population to be followed when they are newly diagnosed with a potentially lethal illness, and they are followed until they die or recover. A true case fatality can only be calculated once all → incident cases have been resolved through either death or recovery.

Also, being a → proportion, the numerator of the case fatality is restricted to deaths among individuals included in the denominator. However, the time periods for the numerator and the denominator do not need to be the same: the denominator could be cases of HIV/AIDS diagnosed during the calendar year 1990, and the numerator, i.e., deaths among those diagnosed with HIV in 1990, could be from 1990 to the present [CDC 2012].

Just like the → case-fatality ratio, the case fatality is often referred to as the → case-fatality rate. This is incorrect, however, because the case fatality is not expressed per unit of time, i.e., time is not part of the denominator.

The case fatality provides different information than the → case-fatality ratio

• The → case-fatality ratio is the number of cause-specific deaths that occurred during a specified time period, divided by the number of new cases of that disease that occurred during that time period. The deaths included in the numerator are not restricted to the new cases included in the denominator; in fact, for many diseases, the deaths are among persons whose onset of disease was years earlier. This is consistent with the general definition of → ratio, whereby the numerator and denominator need not be related
• In contrast, in case fatality, the deaths included in the numerator are restricted to the cases in the denominator [CDC 2012]

CONTEXT

» Sometimes the length of follow-up is so short and the brevity of the illness so well known, that the case fatality is presented without stating the actual follow-up time. However, some time interval is always used in practice. Even for a disease with a short duration, the incidence of death may vary considerably with time. For example, after a car crash, about 50% of the deaths occur at the scene, 90% within the first 24 hours, 99% within the first 10 days, and nearly all the rest prior to 30 days. This variation might be important for some studies. Imagine that we wished to compare hospitals regarding their case-fatality proportions for victims of motor-vehicle crashes. If some hospitals receive most patients soon after a crash, while other hospitals receive most crash victims much later after a crash, the case-fatality proportions will tend to be lower in the latter group of hospitals, because many with the worst injuries died before they reached the hospital. For chronic conditions that are invariably fatal, the case-fatality proportion until all deaths have occurred may not be of much interest as it will equal 1 if follow-up is sufficiently long. « [Cummings 2019]

Letalität

Anteil der an einer Krankheit Verstorbenen an den in einem definierten Zeitraum (durch Genesung oder Tod) abgeschlossenen Krankheitsfällen [Habermehl 1986]

Synonym: Fallsterblichkeit, Sterberisiko, Tödlichkeit

FORMEL

WISSENSWERT

Letalität ist das krankheitsbezogene Sterberisiko und ist ein Maß zur Charakterisierung der Schwere einer Krankheit [RKI 2015]. Alle diagnostizierten Fälle müssen bis zum Tod oder zur definitiven Genesung des einzelnen Patienten verfolgt werden.

CASE-FATALITY RATE

is a → case fatality expressed over time [Bosman 2014]

Abbreviation: CFR

Example:

To stay with the same example as under → case fatality, around 1850, most cases of cholera had either recovered after 2 weeks or had died. Once recovered, a person is no longer a case and the person time of that person no longer contributes to the denominator.

Assuming that of the 100 cases of newly diagnosed cholera, 40 patients die due to the disease after 2 weeks and the rest, i.e., 60 patients, recover from the disease after the same amount of time, then the case-fatality rate for cholera is 40 per 200 person-weeks (=1 per 5 person-weeks = 4 per 5 person-months = 10 per 1 person-year).

Of the 100 cases of newly diagnosed tuberculosis, 50 die in the first year and 50 die in the second year. This results in a case-fatality rate of 1 per 2 person-years for the first year after diagnosis.

This example illustrates the fundamental difference between → case fatality and case-fatality rate: tuberculosis is deadlier than to cholera (because → case fatality is 100% for tuberculosis and 40% for cholera), but Mycobacterium tuberculosis kills much slower than Vibrio cholerae does [Bosman 2014].

GOOD TO KNOW

For case-fatality rate to be a true rate, time has to be included in the denominator. It can be expressed as the number of deaths among cases per 100 or 1000 person-years. Depending on the disease, it may also be expressed per person-weeks or person-months. As a rate, it reflects the dynamic of the fatality over time among cases.

The term case-fatality rate is often wrongly used to refer to either → case fatality or → case-fatality ratio.

LETALITÄTSRATE

Synonym: Fallsterblichkeitsrate

CASE-FATALITY RATIO

is the number of deaths attributed to a particular disease during a specified time period, divided by the number of new cases of that disease identified during the same time period [CDC 2012] 

Synonyms: death-to-case ratio,

Wrongly used synonymously: → case fatality, case-fatality propotion, or → case-fatality rate (CFR)

EQUATION

GOOD TO KNOW

Although the case-fatality ratio is often referred to as the → case-fatality rate (CFR), the terms are not interchangeable, because → rate denotes a time component, which is absent from the concept behind case-fatality ratio. The case-fatality ratio is a → ratio, but not necessarily a → proportion, because some of the deaths that are included in the numerator may have occurred among persons who developed disease in an earlier period and are therefore not included in the denominator. The case-fatality ratio, therefore, is not bound by the numerator being a subject of the denominator (i.e., the definition of a proportion) [WHO 2020b].

The case-fatality ratio is generally used for studying infectious diseases, poisonings, chemical exposures, or other short-term deaths. With chronic diseases, the case-fatality ratio has limited usefulness because

• the time of onset of a chronic disease may be difficult to determine and
• the time from diagnosis to death is much longer

Therefore, deaths that occur in the current observation period may have little relation to the number of new cases that occur in the same observation period, e.g., because different prevention and treatment strategies may meanwhile have been implemented that may have an impact on prognosis. When reporting the case-fatality ratio, therefore, the time element should always be clearly specified.

Example:

In the United States in 2002, a total of 15,075 new cases of tuberculosis were reported. During the same year, 802 deaths were attributed to tuberculosis. The tuberculosis death-to-case ratio for 2002 can be calculated as 802 / 15,075.

Dividing both numerator and denominator by the numerator yields 1 death per 18.8 new cases. Dividing both numerator and denominator by the denominator (and multiplying by 100) yields 5.3 deaths per 100 new cases. Both expressions are correct. Of note, many of those who died had presumably actually contracted tuberculosis years earlier. Thus, many of the 802 in the numerator are not among the 15,075 in the denominator [CDC 2012].

In an ongoing epidemic, reliable estimates of the case-fatality ratio are difficult to obtain. This is because, early in an outbreak, surveillance tends to focus on symptomatic patients who require medical care (e.g., hospitalized cases), whereas milder cases and asymptomatic infections are less likely to be detected—a situation that results in overestimation of the case-fatality ratio. Reliable case-fatality ratios that can be used to assess the deadliness of an outbreak and the effectiveness of public-health measures can only be obtained after the end of an outbreak, i.e., after all cases have either died or recovered. In this later phase, the case-fatality ratio will approximate the → case-fatality proportion. The simplest estimate of case-fatality ratio during an ongoing epidemic is to divide the cumulative number of deaths by the cumulative number of cases at a given time. This measure is referred to as the → crude case-fatality ratio [Thomas 2021].

• Alternatively, the case-fatality ratio could be the comparison of 2 → case fatalities, expressed as a → ratio. For example, the cholera:tuberculosis case-fatality ratio would be 40:100 (or 4:10). The greater killer is usually mentioned first, so the tuberculosis:cholera case-fatality ratio is 2.5:1. In this sense, it is a comparison between 2 populations, similar as we do with odds ratio, risk ratio, or sex ratio. The case-fatality ratio can also be used to assess the impact of an intervention [Bosman 2014]
• A term from the field of oncology that actually refers to the same concept as case-fatality ratio but is used mainly as a measure of data quality is mortality-incidence ratio (also mortality-to-incidence ratio, M/I ratio, M:I ratio, MIR) [Ellis 2019, Porta 2014]

CONTEXT

» Consistent with a living language, there is an evolution in usage. On 22 February 2013, searches of text strings in titles/abstracts in PubMed resulted in the following numbers of hits: “case fatality rate” = 3030; “case fatality ratio” = 332; and “case fatality risk” = 20. Although case fatality rate has been used traditionally, case fatality ratio started to appear only in the 1970s and the articles reporting case fatality risk were all published since 1990. « [Kelly 2013]

COMMENT: On 30 May 2022, a search similar to the one reported by Kelly (and again without regard for how these terms were actually defined) yielded the following numbers:

—case-fatality rate, n=6533 (first occurrence: 1937)
—case-fatality ratio, n=770 (first occurrence: 1972)
—case-fatality risk, n=120 (first occurrence: 1987)
—case-fatality proportion, n=55 (first occurrence: 1994)

» The case-fatality rate is the ratio between the number of deaths due to the disease and the number of closed cases (i.e. recovered or dead). It is estimated by the healthcare system based on the reporting of these two values. The case-fatality rate should not be confused with the mortality rate, which is the ratio of the number of deaths to the total population, or also with the morbidity rate, which is the ratio of the number of cases to the total population. Mortality and morbidity rates depend on the extent of disease in a population, unlike case-fatality rates. « [Souris 2020]

COMMENT: The → case-fatality rate is not a ratio. Believe it or not, it’s a → rate.

Fall-Verstorbenen-Anteil

→ Quotient aus gemeldeten Todesfällen und gemeldeten Infektionen [Masser 2022] • Zahl der gemeldeten verstorbenen Fälle dividiert durch die Zahl der gemeldeten Fälle in einer Population [RKI 2020]

CAVEAT

Die Benennung Fall-Verstorbenen-Anteil ist eine Wortneuschöpfung, die im Zusammenhang mit SARS-CoV-2 entstand. Bis inklusive 2019 liefert eine Google- bzw. Bing-Suche keine Fundstelle zu Fall-Verstorbenen-Anteil – die ersten Funde datieren aus dem Jahr 2020. Häufig stellen deutschsprachige Autoren dem Begriff Fall-Verstorbenen-Anteil den englischen Begriff → case-fatality rate (CFR) als vermeintliches Äquivalent bei. Der Begriffsinhalt von → Anteil ist mit dem Begriffsinhalt von → rate allerdings nicht deckungsgleich.

Das deutsche Robert-Koch-Institut (RKI) selbst definiert den Fall-Verstorbenen-Anteil etwas vage als “Anteil der Verstorbenen an den gemeldeten Fällen” [RKI 2024a], “Anteil der COVID-19-Todesfälle an allen COVID-19-Fällen” [RKI 2024b] bzw. “kumulativen Anteil der gemeldeten Fälle, der verstorben ist” [RKI 2021].

Der Begriff → Anteil aber setzt voraus, dass die Anzahl der Personen im Zähler eine Teilmenge der Anzahl der Personen im Nenner ist. Dazu müssten sämtliche im Nenner abgebildeten diagnostizierten Fälle bis zum Tod oder zur definitiven Genesung jedes einzelnen Patienten verfolgt werden. De facto ist in der Praxis mit dem Begriff Fall-Verstorbenen-Anteil der → Quotient aus zu einem bestimmten Zeitpunkt gemeldeten Todes- und Erkrankungsfällen gemeint. Eine präzise, wenn auch in der Praxis nicht gängige, alternative Benennung für Fall-Verstorbenen-Anteil wäre daher etwa Fall-Verstorbenen-Quotient oder Fall-Todesfall-Verhältnis, was dem englischen case-fatality ratio entspräche.

KONTEXT

» Fall-Verstorbenen-Anteil (engl. case fatality rate, CFR): Für den Fall-Verstorbenen-Anteil teilt man die Zahl der gemeldeten verstorbenen Fälle durch die Zahl der gemeldeten Fälle in einer Population, z. B. in China. Alternativ wird durch die Zahl der Fälle mit bekanntem Endpunkt (genesene und verstorbene Fälle) geteilt. Ersterer Quotient würde den endgültigen Anteil unterschätzen (da noch nicht von allen Patienten der Endpunkt bekannt ist und Patienten mit längerem Krankheitsverlauf häufiger tödlich verlaufen), bei letzterem Quotient würde der endgültige Anteil überschätzt werden. Am 27.02.2020 z. B. war der erste Anteil für die von China gemeldeten Fälle 3,5 % (2.747 / 78.514), und der zweite 7,7 % (2.747 / (32.926 + 2.747)). Beide Anteile nähern sich einander an, wenn von immer mehr gemeldeten Fällen der Endpunkt bekannt ist. In den anderen chinesischen Provinzen lag der Fall-Verstorbenen-Anteil deutlich niedriger (am 26.02.2020 0,8 % (103 Todesfälle / 13.004 Fälle), außerhalb Chinas (1,5 % (44 / 2.918)). Der Fall-Verstorbenen-Anteil bei den Passagieren des Kreuzfahrtschiffs „Princess Diamond“ wurde auf 2,3 %, der Infizierten-Verstorbenen-Anteil auf 1,2 % geschätzt. « [RKI 2020]

ANMERKUNG: Nur im Falle des Kreuzfahrtschiffs „Princess Diamond“ geht es tatsächlich um einen → Anteil, da die Gruppe der Passagiere und die Subgruppen der Erkrankten und Verstorbenen hier individuell bekannt und die Personen im Zähler tatsächlich in der im Nenner abgebildeten Personengruppe enthalten sind.

» Die infection fatality ratio (IFR; Infizierten-Verstorbenen-Anteil) gibt an, wie wahrscheinlich es ist, an einer Infektionskrankheit nach erfolgter Ansteckung zu sterben. Daneben gibt die case fatality ratio (CFR; Fall-Verstorbenen-Anteil) Auskunft darüber, wie viele Personen nach Ausbruch einer Krankheit versterben. Die CFR rechnet also nur mit Patienten, die einen Krankheitsverlauf zeigen, während die IFR asymptomatische sowie symptomatische Krankheitsverläufe mit einbezieht. Die IFR ist somit schwieriger zu ermitteln, gibt jedoch eine realere Angabe über die Gefahr einer Infektionskrankheit an und ist daher eine der wichtigsten epidemiologischen Kennzahlen. Eine kürzlich veröffentlichte Metastudie, nennt einen Wert der IFR von 0,68 Prozent über verschiedene Populationen hinweg. Die IFR wird im Folgetext als Mortalitätsrate oder Sterblichkeit angegeben. Eine Mortalitätsrate von 0,68 Prozent bedeutet, dass von 10 000 infizierten Personen im Durchschnitt 68 versterben. Das erscheint erst mal ziemlich ungefährlich – aber Achtung: Die Sterblichkeitsrate stellt keine intrinsische Eigenschaft des Virus dar, sondern hängt stark von unserer Fähigkeit ab, mit Covid-19 umzugehen. So würde die Mortalitätsrate beispielsweise drastisch abnehmen, sobald ein wirksames Medikament vorhanden wäre, obwohl die Eigenschaften des Virus unverändert bleiben. Das bedeutet aber leider auch, dass die Mortalitätsrate wieder steigen kann. « [Füglistaler 2021]

ANMERKUNG: Ein Vorzug dieses Kontextbeispiels ist, dass darin der treffende englische Begriff case-fatality ratio anstelle des in diskutiertem Zusammenhang oft fälschlicherweise genutzten Begriffs → case-fatality rate verwendet wird. Schade allerdings ist, dass nicht auch gleich die mißverständliche Benennung Fall-Verstorbenen-Anteil etwa in Fall-Verstorbenen-Quotient korrigiert wird. Schließlich wird Fall-Verstorbenen-Anteil gar mit → Mortalitätsrate gleichgesetzt, obgleich beiden Benennungen unterschiedliche Begriffsinhalte zugrunde liegen: erstere Benennung setzt die Zahl der Todesfälle in Bezug zu den Inzidenzfällen, letztere zur Gesamtbevölkerung unter Risiko (siehe → Mortalitätsrate).

CRUDE CASE-FATALITY RATIO

cumulative number of deaths divided by the cumulative number of cases at a given time [Thomas 2021]

Synonym: naive case-fatality ratio [Thomas 2021]

GOOD TO KNOW

The simplest estimate of the → case-fatality ratio during an ongoing epidemic is to divide the cumulative number of deaths by the cumulative number of cases at a given time, a measure referred to as the crude case-fatality ratio [Thomas 2021].

Example:
The figure below [Our World in Data 2024] depicts the → ratio between confirmed deaths and confirmed cases and refers to this ratio as the → case-fatality rate. If what is shown is a ratio, why not refer to it as such?
What the graph actually shows is the crude case-fatality ratio.

The crude case-fatality ratio is biased by time lags in report dates for cases and deaths.

• One way of correcting crude case fatality ratios is by performing time-shifted distribution analyses [Thomas 2021]
• Another way of mitigating the bias due to delays to case resolution during an ongoing outbreak is to restrict the analysis to resolved cases only, once patients have either died or recovered, which would essentially be equivalent to the → case fatality. A drawback of this method is that it requires individual-level data, which are less accessible in real time than aggregate case and death counts [WHO 2020b]

Roher Fall-Verstorbenen-Anteil

ist die kumulierte Anzahl der Verstorbenen dividiert durch die kumulierte Anzahl labordiagnostisch bestätigter Krankheitsfälle zu einem bestimmten Zeitpunkt [Brockow 2021] 

CAVEAT

• Der → Fall-Verstorbenen-Anteil ist kein → Anteil, sondern ein → Quotient. Eine präzisere, wenn auch in der Praxis nicht gängige, Benennung wäre daher etwa Fall-Verstorbenen-Quotient, was dem englischen case-fatality ratio entspräche

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