Statistical analysis was performed on the data obtained from the virus growth curve studies and the evaluation of cpe in PS cells. For growth curves, the data were plotted to include the standard error of the mean (SEM) for each data set. At any given time point divergence by at least 2 SD from the mean, between wild-type and mutant viruses, was taken as significant. Measurement of cpe was done visually by three independent evaluators in a 'blind' manner. The cpe of viruses were estimated on a scale of 1-4 corresponding to 20-40, 40-60, 60-80 and 80-100% of monolayer destruction following microscopic examination. The interevaluator consistency error was verified using F-test which revealed no one evaluation was significantly different from that of the others.