Question

1. Public health officials (PHO) noted an increased incidence of acute lymphocytic leukemia (ALL) among children aged 3-15, many whom seek services in a public health clinic in a neighborhood exposed to chemical waste from a nearby factory. The PHO believe the chemical waste is causing the leukemia and plan a study to determine if this is true. Which of the following subjects are the PHO most likely to use as a control group?

a. Children exposed to the chemical waste who do not suffer from ALL      

b. Children from the public health clinic who do not suffer from ALL

c. Children not exposed to the chemical waste who suffer from ALL

d. Children who suffered from ALL but were cured

Answer 1

Answer is option (b) Children from the public health clinic who do not suffer from ALL

Case Control study is a retrospective study which is used to determine whether the exposure is associated with the outcome.

There are 2 study groups in a case control study

1) Cases: Group known to have the outcome.
Here, children who have ALL are the cases.

2) Controls: Group known to free of outcome.
Free of outcome means children who do not have ALL.

Controls are selected from the children who are coming to public health clinic who have ALL.

After selecting cases and controls, we study backwards about the exposure of members of both groups.

**We are taking control group irrespective of their exposure. So option (a) is wrong.

**All Children who suffer from ALL are taken as cases. So option (c) cannot be true.

**This is a backwards study to know the association between exposure and outcome. So already cured patients cannot be controls. Option (d) is also incorrect.