If dropping out of college was a watershed moment for the likes of Bill Gates, Michael Dell and Mark Zuckerberg, then staying in was equally transformative for Houston, particularly his fraternity experience.
This could in principle introduce bias if dropping out of the survey was to any extent contingent on drinking levels.
If drop-out were to be higher during the apprenticeship then the firm would incur greater net costs and presumably it would be encountering a higher risk on the investment.
Inadequate retention can reduce the power of a trial and introduce bias, particularly if drop-out differs across trial arms.
These assessments were given at baseline and at 6m, 12m and 24m (or earlier and at intervening periods if drop-out or loss to follow-up was likely).
If drop-out was associated with both exposure and outcome, there would be a risk of differential non-response, which could bias the estimates.
If drop-out is dependent on measured parameters (a "missing at random" mechanism"), several analytical approaches can provide unbiased effect estimates; however, if drop-out is dependent on unknown or unmeasured parameters, there is no easy solution for bias correction (Table 1).
Finally, we conducted simulations to explain the impact of selective drop-out on the strength of prediction if drop-out, predictor and criterion variables are correlated to varying degrees.
If drop-out rates were far higher in placebo than hypnotic groups, a greater incidence of depression in hypnotic groups might be attributable to longer exposure, but the study of Krystal et al. did not report an important difference in overall drop-out rates [ 9].
If drop-outs are excluded this could unfairly bias the trial, however, if drop-outs continue in the study after having treatment with antibiotics and systemic corticosteroids, this could also bias the results.
