Selection bais is due to -

Ans is 'a' i.e., Procedure used to select subjects & 'b' i.e., Factors that influence study participation Bias* Selection biases are distortion that result from procedure used to select subjects and from factors that influence study participation. Groups to be compared are differentially susceptible to the outcome even before the experimental maneuver is performed.* Selection bias may be of following typesi) Surveillance!detection bias# It results from the use of a particular diagnostic technique or type of equipment.ii) Neyman survival bias (incidence-prevalence bias)# This type of bias is due to missing of fatal cases, mild cases or cases of shorter duration from the study.iii) Referrel bias (volunteer bias)# Volunteer or referral bias occurs because people who volunteer to participate in a study (or who are referred to it) are often different than non-volunteers/non referrel.# This bias usually favors the treatment group, as volunteers tend to be more motivated and concerned about there health.iv) Response bias# This type of bias occurs when those who respond to a survey differ in important ways from who do not respond.v) Berkesonian bias# Berksonian bias results from the greater probability of hospital admission for people with two or more disease than for people with one disease. So, it is also known as admission rate bias.