- It is cheapest, easy to conduct and quick.
- It is always done for rare diseases.
- It is the study of cases and controls. It is the comparison of cases and controls with reference to exposure status. It is also called retrospective study which means “looking backward”. It means that you tell about the agent by taking history.
- Multiple risk factors can be studied by taking history for all.
Individual having disease is called case.
Any healthy individual is called control. Or any person having disease other than case.
How to Conduct the Study?
There are three steps in conducting case and control study;
Selection of Cases… Cases are selected and taken from different hospitals.
Selection of Controls... Ideal study is conducted by selecting healthy individuals from the population. Controls from hospital do not represent the whole population.
History and Data… In this step, history and data from the diseased people and the healthy individuals is taken.
Odd’s Ratio… Now odd’s ratio is calculated by the following formula;
OR = odd’s of exposure in cases/odd’s of exposure in controls
Odd’s of exposure = exposed/ non-exposed. So, formula for OR will be;
OR = cases exposed/cases non-exposed/controls exposed/controls non-exposed.
If OR is more than 1, your hypothesis is correct.
If OR is less than 1, your hypothesis is wrong.
If OR is 1, there is no relationship b/w components of your hypothesis.
Example… if answer comes 16, it will be written as;
There are 16 times more chances of lung cancer in smokers as compared to non-smokers.
- Incidence can not be calculated b/c it is the study of looking backward.
- Selection of cases and controls is difficult.
- Chances of recall bias (error, mistake) are present.