Berksonian bias, Pygmalion effect and Hawthorne effect
As these are difficult to understand and find examples of, i have compiled some info in easiest way i could. I hope it helps you all :) Hawthorne effect: the alteration of behaviour by the subjects of a study due to their awareness of being observed. Pygmalion effect: Investigator inadvertently conveys his high expectations to subjects, who then produce the expected result. A " self-fulfilling prophecy ". Berkson bias: usually occurs when cases and controls are selected from hospital inpatients.More specifically, when both the exposure and outcome affect the selection and leads to a false negative association.It looks confusing but just look at this example: Consider an investigator studying a relation between diabetes and CHD.He goes to a hospital and gets a list of people admitted with CHD and he selects equal number of controls(inpatients not having CHD). Let us create a 2x2 table here CHD + CHD - Exposure (DM) + a b