Epidemiological Study Designs · 50 MCQs
NEET PG-style questions. Tap an option: you are marked instantly and every option is explained, including why the wrong ones are wrong.
An investigator observes a population without intervening and sets out to test, rather than generate, a specific aetiological hypothesis.
Which category of epidemiological study is this?
Why D is correct: Observational studies are divided into descriptive and analytical. Descriptive studies describe distribution and generate hypotheses; analytical studies (ecological, cross-sectional, case-control, cohort) test them — still without any intervention by the investigator.
Ruling out the others:
- A. Descriptive studies formulate hypotheses; they do not test them.
- B. Experimental studies require the investigator to intervene; here he only observes.
- C. A field trial is an experimental study on healthy people — it involves an intervention.
Experimental studies are described as similar in approach to cohort studies.
What is the essential feature that distinguishes an experimental study from a cohort study?
Why C is correct: In an experimental study the conditions are under the direct control of the investigator: he deliberately applies or withdraws the suspected factor (manipulation). In a cohort study the exposure has occurred naturally and the investigator only observes.
Ruling out the others:
- A. Cohort studies also have a comparison (control) cohort.
- B. Cohort studies also proceed forward, from cause to effect.
- D. Incidence is measured directly in both designs.
A health department chooses one town as the unit of intervention and fluoridates its entire piped water supply. Dental caries in its schoolchildren over the next five years is compared with that in a similar town left unfluoridated.
What type of study is this?
Why D is correct: The intervention is applied to a whole community rather than to individuals, so the community is the unit of study — a community trial (community intervention study). Water fluoridation is the standard example of a measure that can only be applied community-wide.
Ruling out the others:
- A. A clinical trial has patients as the unit and evaluates treatment.
- B. A field trial has healthy individuals as the unit; each person receives or does not receive the intervention.
- C. A cross-sectional study is a one-time observational survey; here the investigator introduced an intervention and followed its effect.
Deaths from a disease fall steadily over five decades, with small year-to-year ups and downs superimposed on the fall.
Which type of time trend does this represent?
Why B is correct: A secular (long-term) trend is a progressive increase or decrease in occurrence over a long period — years or decades. Short-term fluctuations may be superimposed on it.
Ruling out the others:
- A. The best-known short-term fluctuation is an epidemic, over days or weeks.
- C. A seasonal trend recurs within each year.
- D. A cyclic trend shows recurring peaks at intervals (e.g., measles every 2–3 years before vaccination), not a steady one-way change.
A cancer common in its home country becomes less frequent among emigrants settled abroad, and in their children born abroad its rate approaches that of the host population.
What does this pattern chiefly suggest about the cause?
Why A is correct: The migrants' genes do not change in one or two generations, yet their rate moves towards that of the new home. The cause must lie in environment and lifestyle (diet, infections). Migrant studies are one way of distinguishing genetic from environmental factors — stomach cancer in Japanese migrants is the classic case.
Ruling out the others:
- B. A genetic cause would keep the rate high in migrants and their offspring.
- C. Selection could explain a difference in first-generation migrants, not a progressive shift in children born abroad.
- D. Better diagnosis would raise the recorded rate, not lower it.
After a wedding feast, 80 guests develop vomiting and diarrhoea. The epidemic curve rises and falls sharply with a single peak, no secondary wave, and all cases occur within one incubation period.
What type of epidemic is this?
Why C is correct: A common-source, single-exposure (point-source) epidemic follows a brief, essentially simultaneous exposure: the curve rises and falls rapidly with one peak and no secondary waves, cases cluster explosively, and all occur within one incubation period.
Ruling out the others:
- A. Continuous or multiple exposure produces cases beyond one incubation period.
- B. A propagated epidemic spreads person to person, rises gradually and spans several incubation periods.
- D. Slow (modern) epidemics are non-communicable diseases rising over years.
Carcinoma of the cervix was observed to be very rare among nuns.
Which aetiological hypothesis did this descriptive observation eventually lead to?
Why D is correct: This observation about marital status led to the hypothesis that cervical cancer is linked to sexual contact and multiple partners, and then to the suspicion of a venereally transmitted infectious agent (now known to be HPV). It shows how a descriptive 'person' clue starts an epidemiological enquiry.
Ruling out the others:
- A. Nuns are not a genetically distinct group of women.
- B. Nothing in the observation points to diet; the distinctive feature of nuns is celibacy.
- C. The relevant difference is the absence of sexual activity, not the nature of their work.
A single survey examines a sample of 1,000 adults once, at one point in time, for diabetes.
Which measure of disease frequency does this design provide?
Why A is correct: A cross-sectional study is a single examination of a cross-section of the population at one point in time. It is also called a prevalence study.
Ruling out the others:
- B. Incidence needs disease-free people followed over time (a longitudinal design).
- C. Relative risk needs incidence among exposed and unexposed, which a cohort study provides.
- D. Attributable risk also needs incidence rates.
In a one-time survey of 800 adults, both daily salt intake and blood pressure are measured at the same visit. Hypertension is more common among those reporting high salt intake.
What is the main limitation in concluding that high salt intake causes hypertension from this study?
Why A is correct: Exposure and outcome are measured at the same moment, so the survey cannot show which came first. The time sequence essential to causation cannot be deduced from cross-sectional data; the association only suggests a hypothesis for analytical testing.
Ruling out the others:
- B. Differential recall between cases and controls is a case-control problem; no groups were selected by disease here.
- C. A single survey has no follow-up, so there is nothing to lose.
- D. Berkesonian bias comes from hospital-based selection with different admission rates; this is a community survey.
The procedures of a descriptive study are listed in sequence: defining the population, defining the disease, describing by time–place–person, measuring the disease, and comparing with known indices.
What is the sixth and final step?
Why D is correct: The sixth and final procedure of a descriptive study is the formulation of an aetiological hypothesis. It is the end product of descriptive work and the starting point for analytical studies.
Ruling out the others:
- A. Testing a hypothesis is the task of analytical studies, which follow the descriptive phase.
- B. Identifying high-risk groups is part of step 5, comparing with known indices.
- C. Estimating the disease load is step 4, measurement of disease.
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