Selection Effects
Selection effects refer to when a study uses non-equivalent experimental and control groups, who respond differently to the intervention due to their preexisting characteristics.
Selection effects often cause researchers to believe that the intervention had an effect, whereas the difference is due to differences between the groups themselves.
Assessment
It is important to determine if pt groups have similar demographics or clinical characteristics at baseline. When evaluating selection effects you can usually find them on group differences at baseline on table demographics (usually table 1). If selection effects are present to determine the potential impact of these differences on pt outcomes when interpreting study results.
Prevention / Minimize
Researchers an minimize selectione effects by randomly assigning pts to a treatment or a control group so that any preexisting characteristics that may impact treatment outcomes will be equally distributed between groups. Although this doesnt perfeclty prevent selection effects, it is more likely that groups will be equivalent as sample size increases.