A LIKELIHOOD RATIO TEST FOR CANCER SCREENING TRIALS

In randomized cancer screening trials, mortality rates for the screene d group relative to those of the control group are not likely to be co nstant as a function of years from randomization due to the inherent l ag between initiation of screening and any putative effects of screeni ng on mortality. In this situation, a log rank test for differences in mortality between the randomization groups will not be optimal. Altho ugh optimality could potentially be recovered by use of a weighted log rank statistic, the optimal weights are difficult to specify a priori and the potential loss of power by use of poorly specified weights is great. We describe a likelihood ratio test with two degrees of freedo m for use in this situation which is based on a fit of a weakly struct ured full model. Computation of an approximate significance level for this test is described and a large sample justification for this appro ximation is given. Size and power properties of the proposed statistic are compared to that of several other statistics in a small simulatio n study and the statistic is applied to data from the HIP Breast Cance r Screening Trial.