Measures of Disease Frequency, Association and Impact: Incidence, Prevalence, RR, OR, Attributable Risk (NEET PG PSM)

Start Here — Why This Chapter Pays

🎯 Exam priority: High-yield (Tier 1). Community Medicine is one of the heaviest subjects in NEET PG, and epidemiology calculations are its most reliable source of questions. Reviews of the 2026 paper reported PSM questions on attributable risk and case-fatality — exactly this chapter. The arithmetic is easy; the marks are lost by putting the wrong thing in the denominator.

CBME mapping: CM 7 (Epidemiology). Every definition here was checked against the CDC's Principles of Epidemiology in Public Health Practice (3rd edition), listed under Sources.

Ratio, Proportion and Rate

• Why the distinction matters: an "attack rate" is not a true rate — it has no time unit. It is the proportion of a population that becomes ill during an outbreak, and its more accurate name is incidence proportion. Case-fatality "rate" and prevalence "rate" are also proportions.

• The maternal mortality "rate" is really a ratio: its denominator is live births, not women at risk.

Measures of Morbidity: Incidence and Prevalence

• Incidence counts only new cases; incidence proportion is a measure of risk, and a risk ratio is simply the ratio of two incidence proportions.

• Prevalence counts all existing cases (new and pre-existing), so the date of onset is not needed.

• Prevalence depends on incidence and duration. High prevalence can mean high incidence, long survival without cure, or both; low prevalence can mean low incidence, a rapidly fatal course, or rapid recovery. A new treatment that keeps patients alive without curing them raises prevalence.

• Prevalence is preferred for chronic diseases with an unclear onset (diabetes, osteoarthritis).

• Secondary attack rate measures spread from primary cases to contacts (households, wards): new cases among contacts ÷ total contacts. Exclude the primary case(s) from both numerator and denominator.

• Person-time rate handles people followed for different lengths of time: 10 people followed 2 years each and 20 followed 1 year each contribute 40 person-years.

Measures of Mortality

• Case-fatality rate = deaths among people with the disease ÷ people with the disease × 100. It reflects severity (and quality of care), not how common the disease is.

• Proportionate mortality tells you what share of all deaths a cause accounts for; it rises if other causes fall, even when the cause's own risk is unchanged — a classic trap.

• Death-to-case ratio uses deaths in a period over new cases in the same period; the deaths need not come from those new cases, so it is a ratio but not a proportion.

• The infant mortality rate = neonatal (under 28 days) + post-neonatal (28–364 days) deaths per 1,000 live births.

Measures of Association: The 2 × 2 Table, RR and OR

Worked example — cohort study

1,000 smokers are followed for 5 years and 60 develop disease X; 2,000 non-smokers are followed and 20 develop it.

• Risk in exposed = 60/1,000 = 0.06; risk in unexposed = 20/2,000 = 0.01.

• Relative risk = 0.06 ÷ 0.01 = 6 — smokers are 6 times as likely to develop X.

• Attributable risk = 0.06 − 0.01 = 0.05 = 50 per 1,000 smokers over 5 years.

• Attributable risk percent = 0.05 ÷ 0.06 × 100 = 83.3% — about five-sixths of the disease among smokers is due to smoking (assuming a causal link and comparable groups).

Worked example — case-control study

Of 80 cases, 50 were exposed; of 120 controls, 30 were exposed. So a = 50, b = 30, c = 30, d = 90.

• Odds ratio = ad ÷ bc = (50 × 90) ÷ (30 × 30) = 4,500 ÷ 900 = 5.

• You cannot calculate incidence or relative risk from a case-control study, because the investigator chose how many cases and controls to study. The odds ratio approximates the relative risk when the disease is rare.

Measures of Public-Health Impact and Vaccine Efficacy

• Attributable proportion (attributable risk percent) estimates how much disease in the exposed group would disappear if the exposure were removed. It assumes the unexposed group shows the baseline risk and works best when one risk factor is responsible.

• Vaccine efficacy is measured under ideal (trial) conditions; vaccine effectiveness under routine field conditions. Both are the proportionate reduction in disease among the vaccinated.

Worked example — vaccine efficacy

Attack rate 30% in unvaccinated children and 6% in vaccinated children.

• VE = (30 − 6) ÷ 30 × 100 = 80%; equivalently RR = 6/30 = 0.2 and VE = 1 − RR = 0.8.

Bridge to Screening: Sensitivity, Specificity, Predictive Values

The same 2 × 2 table is used for a test against a reference standard. (The next article covers screening in depth.)

• Sensitivity = a ÷ (a + c): the share of true cases the test detects.

• Specificity = d ÷ (b + d): the share of non-cases the test correctly calls negative.

• Positive predictive value = a ÷ (a + b): the chance that a positive result is a true case.

Worked example

Among 1,000 people, 100 have the disease and 90 of them test positive; of 900 without it, 45 test positive.

• Sensitivity = 90/100 = 90%; specificity = 855/900 = 95%.

• PPV = 90/135 = 67%; NPV = 855/865 = 98.8%. Even a good test gives many false positives when the disease is uncommon in the group tested.

Comparing Populations: Standardization, Population Impact and DALYs

• Why standardize? A crude death rate depends heavily on age structure: an older population has more deaths even if its age-specific risks are lower. An age-standardized (age-adjusted) rate is the rate a population would have if it had a standard age structure, so two populations can be compared fairly.

• Standardization can be direct or indirect. The direct method — applying the study population's age-specific rates to a standard population — is the more frequently used.

• Population attributable risk asks how much disease in the whole population is due to an exposure. It depends on both the relative risk and how common the exposure is. WHO's example: passive smoking raises lung-cancer risk only about 1.5-fold, yet about 20% of cases in a typical high-income population can be attributed to it, because up to half the population is exposed.

• DALY (disability-adjusted life year) = years of life lost to premature death (YLL) + years lived with disability (YLD). YLD multiplies incident cases by average duration and a severity weight from 0 (perfect health) to 1 (dead). One DALY is one lost year of healthy life.

Traps the Examiner Sets

• "Attack rate" and "case-fatality rate" are proportions, not true rates.

• Relative risk needs incidence — so it comes from a cohort study or trial, never a case-control study.

• Attributable risk is an absolute difference; relative risk is a ratio. The first tells a health planner how many cases an intervention could prevent; the second measures strength of association.

• A drug that prolongs life without curing increases prevalence; a disease that kills quickly has low prevalence despite high incidence.

• IMR, NMR and maternal mortality all use live births as the denominator.

• Secondary attack rate leaves out the primary case.

Rapid-Revision Cards

• Attack rate is more accurately called…

  Incidence proportion — a proportion, not a true rate.

• Secondary attack rate formula

  New cases among contacts ÷ total contacts (exclude the primary case).

• Prevalence depends on…

  Incidence and duration of disease.

• Effect of a drug that prolongs life without cure on prevalence

  Prevalence rises.

• Relative risk formula

  Risk in exposed ÷ risk in unexposed (cohort study or trial).

• Odds ratio formula

  ad ÷ bc — the measure of association in a case-control study.

• When does OR approximate RR?

  When the disease is rare.

• Attributable risk

  Risk in exposed − risk in unexposed (absolute excess risk).

• Attributable risk percent

  (Re − Ru) ÷ Re × 100 — also called the attributable proportion.

• Vaccine efficacy formula

  (ARU − ARV) ÷ ARU × 100, or 1 − RR.

• Efficacy vs effectiveness

  Efficacy = ideal (trial) conditions; effectiveness = routine field conditions.

• Denominator of IMR, NMR and MMR

  Live births.

• Neonatal period

  Under 28 days; post-neonatal = 28–364 days.

• Case-fatality rate measures…

  Severity of a disease (deaths among cases ÷ cases × 100).

• Sensitivity / specificity / PPV

  a/(a+c) · d/(b+d) · a/(a+b).

• Age-standardized rate

  The rate a population would have with a standard age structure; direct method is more common.

• Population attributable risk depends on…

  Strength of association AND how common the exposure is.

• DALY

  YLL + YLD; one DALY = one lost year of healthy life.

Sources

Sources

Facts in this article were checked against these documents.

• Basic Epidemiology, 2nd ed. — Chapter 2 (standardization, population attributable risk, DALYs) — World Health Organization, 2006 (link)

• Principles of Epidemiology in Public Health Practice, 3rd ed. (Self-Study Course SS1978), Lesson 3: Measures of Risk — U.S. Centers for Disease Control and Prevention (public domain), 2006, updated 2012 (link)

• Competency Based Medical Education Curriculum 2024, Vol. II — Community Medicine (CM 7) — National Medical Commission, 2024

Test Yourself: Practice Questions

Fifteen questions in NEET PG style. Work each calculation on paper first.

Practice Questions (NEET PG pattern)

Original questions written in the style of the exam: +4 for a correct answer, −1 for a wrong one. Every answer was checked against the sources listed above. NEET PG papers are not published by NBEMS, so these are not past-paper questions.

Q1. In an outbreak of food poisoning after a wedding lunch, 40 of 200 guests fell ill. The figure 20% is best described as the

• (a) person-time incidence rate

• (b) attack rate (incidence proportion)

• (c) secondary attack rate

• (d) point prevalence

Q2. Two days after the index case of measles in a hostel, 6 of the 30 susceptible room-mates develop measles. The measure that describes this spread is the

• (a) secondary attack rate

• (b) proportionate mortality

• (c) case-fatality rate

• (d) period prevalence

Q3. A new drug prolongs survival in a chronic disease but does not cure it. If incidence stays the same, the prevalence of the disease will

• (a) stay the same

• (b) fall to zero

• (c) decrease

• (d) increase

Q4. Which measure is most appropriate for a chronic disease like type 2 diabetes in a community survey?

• (a) Prevalence

• (b) Secondary attack rate

• (c) Case-fatality rate

• (d) Attack rate

Q5. In a cohort study the incidence of lung cancer is 0.06 among smokers and 0.01 among non-smokers. The relative risk is

• (a) 6

• (b) 0.05

• (c) 5

• (d) 83%

Q6. Using the same data (0.06 in smokers, 0.01 in non-smokers), the attributable risk percent among smokers is about

• (a) 17%

• (b) 83%

• (c) 6%

• (d) 50%

Q7. Attributable risk percent is also known as the

• (a) relative risk

• (b) proportionate mortality

• (c) odds ratio

• (d) attributable proportion

Q8. In a case-control study: exposed cases 50, unexposed cases 30, exposed controls 30, unexposed controls 90. The odds ratio is

• (a) 5

• (b) 0.2

• (c) 3

• (d) 1.7

Q9. Which measure of association can be calculated directly from a case-control study?

• (a) Relative risk

• (b) Incidence rate

• (c) Attributable risk

• (d) Odds ratio

Q10. During an epidemic the attack rate was 40% in unvaccinated and 8% in vaccinated children. Vaccine effectiveness is

• (a) 80%

• (b) 5%

• (c) 32%

• (d) 20%

Q11. Vaccine efficacy, as opposed to vaccine effectiveness, refers to protection measured

• (a) only in the elderly

• (b) only after a booster dose

• (c) under ideal conditions, as in a clinical trial

• (d) under routine field conditions

Q12. The denominator of the infant mortality rate is

• (a) live births in the same period

• (b) children under 1 year at mid-year

• (c) mid-year population

• (d) all births including stillbirths

Q13. The maternal mortality 'rate' is more correctly a ratio because

• (a) it is expressed per 100

• (b) it has no numerator

• (c) its denominator is live births, not women at risk

• (d) it excludes deaths after delivery

Q14. Of 50 patients admitted with acute snakebite envenomation, 5 died. The figure 10% is the

• (a) crude death rate

• (b) attack rate

• (c) proportionate mortality

• (d) case-fatality rate

Q15. Two districts have very different age structures. To compare their death rates fairly you should use

• (a) proportionate mortality

• (b) age-standardized rates

• (c) crude death rates

• (d) case-fatality rates

Q16. Passive smoking carries a relative risk of only about 1.5 for lung cancer, yet accounts for a large share of cases in many populations. The best explanation is that

• (a) passive smoking is a confounder

• (b) the exposure is very common, so population attributable risk is high

• (c) relative risk is not a valid measure

• (d) lung cancer is rare

Q17. Disability-adjusted life years (DALYs) are the sum of

• (a) incidence and prevalence

• (b) life expectancy and infant mortality

• (c) years of life lost and years lived with disability

• (d) quality-adjusted life years and YLL

Q18. A screening test detects 90 of 100 people with a disease and correctly calls 855 of 900 healthy people negative. Its sensitivity is

• (a) 90%

• (b) 67%

• (c) 99%

• (d) 95%

Active Recall

Without looking, define incidence proportion, secondary attack rate, prevalence and case-fatality rate; write the formulas for RR, OR, attributable risk percent and vaccine efficacy; and say which study design each needs.