BOGUS CONTROL BIAS:

In an experimental study, when patients who are allocated to the experimental treatment group, die or experience an adverse event during treatment, which results in them being removed from the treatment group or re-assigned to the control/comparator group, the experimental treatment will appear spuriously superior¹. Also see: Withdrawal Bias, Carryover Bias, and Selection Bias.

Reference:

1. Sackett DL. Bias in analytic research. J Chronic Dis. 1979;32 (1-2):51-63. (Link to Reference)