Bias in Which variation occur due to different disease treated in different hospitals:-

Berkesonian bias is a special type of bias, named after Dr. Joseph Berkeson, who recognized this problem due to different rates of admission to hospitals for people with different diseases Neyman bias: due to missing of fatal cases, mild/silent cases and cases of sho duration from the study. Attention bias or Hawthorne effect: study subjects may systematically alter their behavior when they know they are being observed. Recall bias: cases are more likely to remember exposure more correctly than controls.