#975 Operational Bias in assessing time to progression (TTP) G. Williams, K. He, G. Chen, G. Chi, R. Pazdur Food and Drug Administration, Rockville,MD Purpose: To assess the operational bias associated with visit schedules and censoring patterns for TTP. Method: 5000 simulations of TTP were performed assuming 1:1 randomization, a Poisson accrual process, exponential TTP, non-informative censoring, and 24 mos. Study duration. The true hazard ratio (THR) for treatment (T) versus placebo (P) was specified as 0.80 with median TTP of 7.5 and 6 mos. respectively. . Asymmetry of visit intervals or censoring patterns between study arms caused significant bias: prolonging visit intervals from 1 month to 3 months caused moderate bias; however, increasing the degree of symmetric censoring from 5% to 20% caused minimal bias. Conclusions: Asymmetric visit schedules may introduce significant bias into the evaluation of TTP. When studies compare TTP of treatment regimens given by different schedules, tumor progression should be evaluated on symmetric schedules. TTP results associated with asymmetric censoring may also yield biased results. Remember we are NOT Doctors and have NO medical training. This site is like an Encylopedia - there are many pages, many links on many topics. Support our work with any size DONATION - see left side of any page - for how to donate. You can help raise awareness of CAM.