Power and sample size requirements for two-part models

Two-part models assume the data has a probability mass at zero and a continuous response for values greater than 0. We construct tests based on the proportion of zeros and the difference among the positive values of a response giving rise to a two degree of freedom chi (2) test. This note gives the non-centrality parameter and finds the power for the test. The power is compared to the simulation results in a recent paper. We derive sample size calculations. We note that some modifications of this procedure may provide better tests if the non-zero responses are discrete. Published in 2001 by John Wiley & Sons, Ltd.