Example of case-control studies
Let's look at an example of a case-control study. This example is about an outbreak of cyclosporiasis among residents of New Jersey. It was found that 21 of 30 cases and 4 of the controls had eaten raspberries.
We can put these figures into a table. See table a below.
| Eat raspberries | Case (got ill) | Control | Total |
|---|---|---|---|
| yes | 21 | 4 | 25 |
| no | 9 | 56 | 65 |
| Odds | 2.3 | 0.071 |
First of all you need to understand how to calculate odds.
The odds is calculated by dividing the number of times the event happened by the number of times the event does not happen.
Odds of exposure (cases) = No of cases with the exposure/ No of cases without the exposure
= 21/9
= 2.3
Odds of exposure (controls) = No of controls with the exposure/No of controls without exposure
= 4/56
= 0.071
Odds ratio = odds of exposure (cases) /odds of exposure (control)
= 2.3/0.071
= 32.39
The odds ratio for this study concluded that exposure to raspberries was over 30 times higher among cases than controls.