# Measures of Disease Frequency, Association and Impact: Incidence, Prevalence, RR, OR, Attributable Risk (NEET PG PSM) Source: https://tracknation.in/story/measures-of-disease-frequency-association-and-impact-incidence-prevalence-rr-or-attributable-risk-neet-pg-psm (TrackNation โ€” free UPSC notes with PYQs) > NEET PG Community Medicine: ratio vs proportion vs rate, incidence proportion (attack rate), secondary attack rate, person-time incidence rate, point and period prevalence, mortality measures (crude, cause-specific, proportionate, case-fatality, IMR, NMR, MMR), relative risk, odds ratio, attributable risk percent and vaccine efficacy โ€” with worked calculations, the traps examiners use, 15 verified MCQs and rapid-revision cards. ## Start Here โ€” Why This Chapter Pays ๐ŸŽฏ **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 โ€” 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 | **Kind of measure** | **What it is** | **Examples** | | Ratio | One quantity divided by another; numerator need not be part of the denominator | Risk ratio (relative risk), rate ratio, odds ratio, death-to-case ratio | | Proportion | A ratio in which the numerator is included in the denominator; no time unit | Attack rate (incidence proportion), secondary attack rate, point and period prevalence, proportionate mortality, attributable proportion | | Rate | Speed of occurrence: events per unit of population per unit of time | Person-time incidence rate; crude, cause-specific, age-specific mortality; IMR, NMR, maternal mortality 'rate' | - **Why the distinction matters:** an "attack rate" is not a true rate โ€” 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 | **Measure** | **Numerator** | **Denominator** | | Incidence proportion (attack rate, risk) | New cases during a specified interval | Population at the start of the interval | | Secondary attack rate | New cases among contacts | Total number of contacts | | Incidence rate (person-time rate) | New cases during the interval | Summed person-years of observation (or average population) | | Point prevalence | All current cases (new + old) at one point in time | Population at that point | | Period prevalence | All current cases during a period | Average or mid-interval population | - **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 รท 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 | **Measure** | **Numerator** | **Denominator** | **Per** | | Crude death rate | All deaths in the interval | Mid-interval population | 1,000 | | Cause-specific death rate | Deaths from one cause | Mid-interval population | 1,00,000 | | Proportionate mortality | Deaths from one cause | All deaths in the same interval | 100 | | Case-fatality rate | Deaths among cases of a disease | Cases of that disease | 100 (a proportion) | | Neonatal mortality rate | Deaths < 28 days of age | Live births | 1,000 | | Infant mortality rate | Deaths < 1 year of age | Live births | 1,000 | | Maternal mortality 'rate' | Pregnancy-related deaths | Live births (so it is really a ratio) | 1,00,000 | - **Case-fatality rate** = deaths among people with the disease รท people with the disease ร— 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 โ€” 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 (28โ€“364 days) deaths per 1,000 live births. ## Measures of Association: The 2 ร— 2 Table, RR and OR | **** | **Disease present** | **Disease absent** | **Total** | | Exposed / Test positive | a (true positive) | b (false positive) | a + b | | Not exposed / Test negative | c (false negative) | d (true negative) | c + d | | Total | a + c | b + d | a + b + c + d | | **Measure** | **Formula** | **Study design** | **Interpretation** | | Relative risk (risk ratio) | Risk in exposed รท risk in unexposed | Cohort, trial | 1 = no association; > 1 = harmful exposure; < 1 = protective | | Odds ratio | ad รท bc (cross-product) | Case-control (also cohort) | Approximates RR when the disease is rare | | Attributable risk (risk difference) | Risk in exposed โˆ’ risk in unexposed | Cohort | Excess risk due to exposure, in absolute terms | | Attributable risk percent (attributable proportion) | (Re โˆ’ Ru) รท Re ร— 100 | Cohort | Share of disease in the exposed that is due to exposure | | Vaccine efficacy / effectiveness | (ARU โˆ’ ARV) รท ARU ร— 100 = 1 โˆ’ RR | Trial (efficacy) / field (effectiveness) | Proportionate reduction in disease among the vaccinated | ### Worked example โ€” 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 รท 0.01 = **6** โ€” smokers are 6 times as likely to develop X. - **Attributable risk** = 0.06 โˆ’ 0.01 = 0.05 = **50 per 1,000** smokers over 5 years. - **Attributable risk percent** = 0.05 รท 0.06 ร— 100 = **83.3%** โ€” about five-sixths of the disease among smokers is due to smoking (assuming a causal link and comparable groups). ### Worked example โ€” 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 รท bc = (50 ร— 90) รท (30 ร— 30) = 4,500 รท 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 โ€” vaccine efficacy Attack rate 30% in unvaccinated children and 6% in vaccinated children. - VE = (30 โˆ’ 6) รท 30 ร— 100 = **80%**; equivalently RR = 6/30 = 0.2 and VE = 1 โˆ’ RR = 0.8. ## Bridge to Screening: Sensitivity, Specificity, Predictive Values The same 2 ร— 2 table is used for a test against a reference standard. (The next article covers screening in depth.) - **Sensitivity** = a รท (a + c): the share of true cases the test detects. - **Specificity** = d รท (b + d): the share of non-cases the test correctly calls negative. - **Positive predictive value** = a รท (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 โ€” applying the study population's age-specific rates to a standard population โ€” 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** โ€” 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 - Incidence proportion โ€” a proportion, not a true rate. - New cases among contacts รท total contacts (exclude the primary case). - Incidence and duration of disease. - Prevalence rises. - Risk in exposed รท risk in unexposed (cohort study or trial). - ad รท bc โ€” the measure of association in a case-control study. - When the disease is rare. - Risk in exposed โˆ’ risk in unexposed (absolute excess risk). - (Re โˆ’ Ru) รท Re ร— 100 โ€” also called the attributable proportion. - (ARU โˆ’ ARV) รท ARU ร— 100, or 1 โˆ’ RR. - Efficacy = ideal (trial) conditions; effectiveness = routine field conditions. - Live births. - Under 28 days; post-neonatal = 28โ€“364 days. - Severity of a disease (deaths among cases รท cases ร— 100). - a/(a+c) ยท d/(b+d) ยท a/(a+b). - The rate a population would have with a standard age structure; direct method is more common. - Strength of association AND how common the exposure is. - 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. โ€” Chapter 2 (standardization, population attributable risk, DALYs) โ€” World Health Organization, 2006 (link) - Principles of Epidemiology in Public Health Practice, 3rd ed. (Self-Study Course SS1978), Lesson 3: Measures of Risk โ€” U.S. Centers for Disease Control and Prevention (public domain), 2006, updated 2012 (link) - Competency Based Medical Education Curriculum 2024, Vol. II โ€” Community Medicine (CM 7) โ€” 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, โˆ’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 **Answer: (b)** โ€” An attack rate is the proportion of a population that develops illness during an outbreak โ€” its more accurate name is incidence proportion. It has no time unit. **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 **Answer: (a)** โ€” The secondary attack rate = new cases among contacts รท total contacts (6/30 = 20%), excluding the index case. **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 **Answer: (d)** โ€” Prevalence depends on both incidence and duration. Longer survival without cure lengthens duration, so more people have the disease at any time. **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 **Answer: (a)** โ€” Prevalence is often measured for chronic diseases with a long duration and an unclear date of onset. **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% **Answer: (a)** โ€” Relative risk = risk in exposed รท risk in unexposed = 0.06 / 0.01 = 6. (0.05 is the attributable risk.) **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% **Answer: (b)** โ€” Attributable proportion = (0.06 โˆ’ 0.01) / 0.06 ร— 100 โ‰ˆ 83%: about five-sixths of the disease in smokers is due to smoking. **Q7.** Attributable risk percent is also known as the - (a) relative risk - (b) proportionate mortality - (c) odds ratio - (d) attributable proportion **Answer: (d)** โ€” The attributable proportion is also called the attributable risk percent โ€” a measure of the public-health impact of a cause. **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 **Answer: (a)** โ€” OR = ad / bc = (50 ร— 90) / (30 ร— 30) = 4,500 / 900 = 5. **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 **Answer: (d)** โ€” Case-control studies start from disease status, so incidence (and therefore RR and attributable risk) cannot be computed; the odds ratio can. **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% **Answer: (a)** โ€” VE = (40 โˆ’ 8) / 40 ร— 100 = 80%; equivalently 1 โˆ’ RR = 1 โˆ’ 0.2 = 0.8. **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 **Answer: (c)** โ€” Efficacy is used for ideal (trial) conditions and effectiveness for typical 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 **Answer: (a)** โ€” IMR = deaths under 1 year of age รท live births in the same period ร— 1,000. **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 **Answer: (c)** โ€” Pregnancy-related deaths are divided by live births, which is not the population at risk, so it is really a ratio. **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 **Answer: (d)** โ€” Case-fatality rate = deaths among people with the condition รท people with the condition ร— 100 โ€” a measure of severity. **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 **Answer: (b)** โ€” An age-standardized rate is the rate a population would have with a standard age structure, removing the effect of different age distributions. **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 **Answer: (b)** โ€” Population attributable risk depends on both the strength of association and how common the exposure is; WHO notes up to half the population is exposed. **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 **Answer: (c)** โ€” DALY = YLL (premature death) + YLD (disability, weighted by severity). One DALY is one lost year of healthy life. **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% **Answer: (a)** โ€” Sensitivity = a / (a + c) = 90 / 100 = 90%. (95% is specificity; 67% is PPV.) ### 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.