Test of treatment effect in pre-drug and post-drug count data with zero-inflation.

A procedure for testing for treatment effect in data similar to the data on premature ventricular contractions (PVC) is presented. We consider a zero-inflated beta-binomial model. Based on this model, we develop score tests to test for treatment effect in the data in which observations in the form of counts are recorded before and after applying a therapy. Results of a small simulation experiment, to study small sample behaviour of a score test and a likelihood ratio test, are reported and the PVC data are analysed. Both the score and the likelihood ratio tests show good level properties. Either the score tests or the likelihood ratio tests can be used for testing the presence of treatment effect. The score tests, however, may be preferable because they use estimates of the parameters only under the null hypothesis and in the important range pi<0.5 power of the score test statistic S1 is slightly better than the likelihood ratio statistic LR1.