For case control studies the rate of disease in the population is unknown so the relative risk cannot be calculated. Results for men. The example showed that cigarette smoking is associated with the development of lung cancer. This did not necessarily prove that smoking caused lung cancer. BIAS A bias is a factor which leads to results which show a systematic departure from the truth.
Random error affects all observations equally and can be overcome by increasing the sample size.
Bias results from factors which have different effects on diseased and undiseased or on exposed and unexposed.