Abstract
Crossover can bias clinical outcomes of randomized clinical trials by increasing the risk of both type I (false positive) and type II (false negative) errors. To show how crossover can increase type I error, we provide computer simulation and review herein illustrative examples (iniparib, olaratumab) of recently reported RCTs that demonstrated false-positive treatment efficacy signals due to crossover. The ethical issues associated with crossover are also discussed.