Efficacy estimates from parasite count data that include zero counts

Because of the positive skewness of parasite distributions and the greater constancy of percentage of response of therapy in animal populations, paras ite count data are conventionally transformed logarithmically before combin ing results from different animals, either all controls or all treated. Obs ervations of zero counts raise difficulties, since the logarithm of zero is not useful. In this study, several types of zero count adjustments are com pared. Two systems for assigning values to zero counts were considered: a f ixed system, which assigns the same value to all zero counts regardless of the proportion of such counts in a treatment group, and a variable system, which replaces zero counts with a value based on the proportion of zero cou nts in the group. The values assigned by either system are then adjusted to reflect aliquot size. An evaluation was performed by using 32 compound Poi sson lognormal distributions, three sample sizes, and three representatives of each zero count adjustment system. The Poisson lognormal distribution p rovides a convenient method with which to provide variability greater than Poisson. Expected values of the sample estimate of the (known) population m ean were calculated for each of the 576 combinations of these factors, and the bias associated with each combination was derived. The bias associated with the three representatives of the variable adjustment system was simila r. The variable adjustment system had a lower overall bias than any represe ntatives of the fixed adjustment system. (C) 2000 Academic Press.