The case-control study is the observational study that recruits persons with the disease of the interest (known as cases)and a suitable control group of persons without the disease(known as controls).The relation of a putative risk factor to the disease is examined by comparing the frequency of the putative risk factor between cases and controls.The case-control study is the observational study that recruits persons with the disease of the interest (known as cases) and a suitable control group of persons without the disease (known as controls) . The relation of a putative risk factor to the disease is examined by comparing the frequency of the putative risk factor between cases and controls