Selection Bias: Cohort Studies/RCTs
{"Image DocumentName": "Selection Bias: Cohort Studies/RCTs", "Image ImageDescription": "Most common form of selection bias does not occur with the process of initial selection of subjects. Instead, selection bias most commonly caused by forces that determine length of participation (who ultimately stays in the analysis; losses) When those lost to follow-up have a different probability of the outcome than those who remain (i.e. informative censoring) in at least one of the exposure groups. AND. Rate of informative censoring differs across exposure groups. Selection bias results. Selection bias among initially selected subjects in longitudinal studies, however, is not the primary source of selection bias in these studies. Instead, the primary source of selection bias comes in the form of how long subjects participate, in other words, how much loss-to-follow-up occurs which is not administrative. Hence, it is not just who you start with; it is also who you end up with. Selection bias occurs when those persons lost to follow up have a different probability of the outcome than those persons who remain in the analysis - remember we called this informative censoring - in at least one of the exposure groups. AND. When the degree or magnitude of informative censoring differs across exposure groups. Note that this all starts with those who are lost having a different incidence of the outcome than those who remain. If losses to follow up are simply by random chance and those persons who are lost have the exact same outcome experience as those who stay, then all you lose is statistical power. This won\u2019t produce any bias in either the incidence of the outcome in any of the exposure groups or in the measure of association between groups. The problem, of course, is that we rarely know the outcome experience of those who leave, other than in studies of all cause mortality where we can use the National Death Index.", "Image XResolution": "37", "Image YResolution": "37", "Image ResolutionUnit": "Pixels/Centimeter", "Image Software": "SlidePlayer", "Image YCbCrPositioning": "Centered"}