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Disease frequency measures and standardization
Incidence counts new cases in the population at risk over time, prevalence counts all existing cases at a point in time, and age standardization lets us compare populations whose age structures differ.
Why it matters in the exam
This is the basis of almost every quantitative public-health question: you compute an incidence or prevalence rate from a table, identify the numerator and denominator, and explain why prevalence rises when a new treatment prolongs life without curing. Other questions show two populations whose crude death rates point the opposite way to their standardized rates and ask you to explain the gap or to choose between direct and indirect standardization. Attack rate in an outbreak, secondary attack rate among contacts and case fatality are also tested. Knowing the numerator, the denominator and the time unit of each measure solves most items without complex formulas, and shows when a comparison between countries or periods misleads because the age structure changed. When answering standardization items, remember that a standardized rate is an artificial number meant only for comparison and does not describe the real situation in the population, so service planning uses crude and age-specific rates. When two populations differ in age structure, an older population looks "sicker" in the crude rate even if its rates are lower in every age group. This is the paradox exams like to show in a table of two cities or two countries.
Key points
- Incidence measures new cases, prevalence measures all existing cases.
- In a steady state prevalence roughly equals incidence times mean disease duration.
- Incidence density uses person-years when follow-up is uneven.
- Direct standardization needs age-specific rates; indirect standardization produces an SMR.
- Case fatality measures short-term severity, and its denominator is patients only.
Summary table
| Measure | Numerator | Denominator | When used |
|---|---|---|---|
| Cumulative incidence (risk) | New cases during the period | Population at risk at the start of the period | Closed group with short follow-up, such as an outbreak |
| Incidence density | New cases | Total person-years of follow-up | Open cohort with people entering and leaving |
| Point prevalence | All existing cases at a given moment | Whole population at that moment | Service planning and chronic diseases |
| Attack rate and secondary attack rate | Ill people in an exposed group, or contacts who fell ill | The exposed, or the susceptible contacts | Outbreaks and how infectious an agent is |
| Case fatality rate (CFR) | Deaths from the disease | All people with the disease | Severity of an acute illness in the short term |
| Standardized mortality ratio (SMR) | Observed deaths | Expected deaths from reference-population rates | Indirect standardization, small groups |
Free exam questions
מה מבין הבאים נחשב כמדד הטוב ביותר לצורך השוואת הסיכון לחלות בסרטן הריאה בשתי מדינות? (the question as examined, in Hebrew)
- א. שיעורי הימצאות, מתוקננים לגיל, ל- 100,000
- ב. שיעורי תמותה מתוקננים לגיל, ל - 100,000
- ג. שיעורי הישרדות יחסית תוך 5 שנים
- ד. שיעורי היארעות מתוקננים לגיל, ל- 100,000
בין 1 בינואר 2003 ועד 31 בדצמבר 2017 ,טופלו בבוסטון, מסצ'וסטס 60,000 חסרי - ב ית, ו- 7,130 ,מהם נפטרו בתום התקופה. מאלה 1,727 נפטרו בגלל מנת- יתר של סמים. מספר הנפטרים ממנת- יתר של סמים היה גבוה פי 12 ממספר הנפטרים ממנת- יתר של סמים שהיה צפוי באוכלוסייה זו על פי השיעור הידוע לכלל מדינת מסצ'וסטס. מה משמעותו של היחס האמור (פי 12 ?) (the question as examined, in Hebrew)
- א. שיעור תמותה מתוקנן לגיל(age-standardized mortality rate)
- ב. יחס תמותה מתוקנן(standardized mortality ratio, SMR)
- ג. יחס תמותה פרופורציוני(proportional mortality ratio)
- ד. שיעור קטלניות(case-fatality rate)
( "מה משמעות המושג של "סיכון מיוחס attributable risk ?) (the question as examined, in Hebrew)
- א. הפרופורציה או הכמות מתוך היארעות המחלה (או סיכון) אשר מיוחס לחשיפה ספציפית
- ב. החלק היחסי של גורם סיכון ספציפי מכלל גורמי המוות בקרב אוכלוסייה
- ג. היחס בין ה היארעות.בחשופים לגורם הסיכון ובין ההיארעות בלא חשופים לאותו גורם סיכון
- ד. הסיכון המצטבר של גורם סיכון לכל המחלות להן הוא תורם במצטבר
In the full version for subscribers
- A simple algorithm
- Memory aid
- Exam traps
- 5 review cards
- Enough for IMLE
The full version is open to subscribers of: Public health, Medical licensing exam (IMLE), Family medicine.
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Sources
- Gordis, Epidemiology, 6th ed., 2019 — Measuring the occurrence of disease: morbidity and mortality
- Rothman, Epidemiology: An Introduction, 2nd ed., 2012 — Measuring disease occurrence and causal effects
- CDC, Principles of Epidemiology in Public Health Practice, 3rd ed., 2012 — Summarizing data: measures of risk
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.