Question

To study the causes of an outbreak of aflatoxin poisoning in Africa, investigators conducted a case-control study with 40 case-patients and 80 controls. Among the 40 poisoning victims, 32 reported storing their maize inside rather than outside. Among the 80 controls, 20 stored their maize inside.

a. Set up a 2 x 2 table and calculate the odds ratio for the association between inside storage of maize and illness.

b. Interpret the odds ratio.

Answer 1

Case control study definition

A study that compares patients who have a disease or outcome of interest (cases) with patients who do not have the disease or outcome (controls), and looks back retrospectively to compare how frequently the exposure to a risk factor is present in each group to determine the relationship between the risk factor and the disease.

A) Set up a 2 x 2 table and calculate the odds ratio for the association between inside storage of maize and illness.

Sl.no	Cases (ill)	Controls(not ill)	Total
Exposed( Maize stored Inside)	32 (A)	20 (B)	A+B=52
Unexposed( Maize Stored outside)	8 (C)	60 (D)	C+D=68
Total	A+C= 40	B+D=80	A+B+C+D=120

Odd ratio = (A*D) / (B*C)

= (32*60) / ( 20*8)

= 1920 / 160

= 12

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B) Odds ratio

Definition

The Odds Ratio is a measure of association which compares the odds of disease of those exposed to the odds of disease those unexposed.

Interpretation of odds ratio

• OR of 1 would suggests that there is no difference between the groups; i.e. there would be no association between the suggested exposure (fish) and the outcome (being ill)
• OR of > 1 suggests that the odds of exposure are positively associated with the adverse outcome compared to the odds of not being exposed
• OR of < 1 suggests that the odds of exposure are negatively associated with the adverse outcomes compared to the odds of not being exposed. Potentially, there could be a protective effect

In the example above, we can conclude those who stored maize inside( exposed) were 12 times ( OR=12) more likely to aflatoxin poisoning compared to those who stored maize outside ( unexposed ).