Q: Residents of 3 villages with 3 different types of water supply were asked to participate in a study to identify cholera carriers. Because several cholera deaths had occurred in the recent past, virtually everyone present at the time submitted to examination. The proportion of residents in each village who were carriers was computed and compared. This study is a:

a. Cross-Sectional Study                b. Case-control stuy    

c. Con-current Cohort Study          d. Non-concurrent cohort study

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Ans: d

If all people in a village are asked for disease present / absent only, it is cross-sectional study.

If diseased people in one village are compared with neighbouring village, it has to be an analytic design – Cohort or Case Control.

The Cause and effect diagram here will be :

Residence in a particular village ---> Cholera carrier state

If those with carrier state are compared with those without carrier state for village of residence, then it will be case-control study.

If those residing in one village are compared with another village for number of people with carrier state, then it will be cohort study.

Now cohort study can be of 2 types:

1. Concurrent (Prospective): Study starts when cause variable has occurred but effect variable is yet to occur

2. Non-concurrent (retrospective): Study starts after both cause and effect have occurred.

Thus, I will mark option d – Non concurrent Cohort study as the best answer.