ABSTRACT
Count data include number of fronds, offspring, juveniles, leaves, or roots, i.e., non-negative integers. In contrast to binomial data, the experimental design imposes no a priori upper limit on the counts. For instance, reproduction data, which are commonly obtained from chronic toxicity tests in ecotoxicology, are often counts of the number of offspring present at the end of the test period. Dose-response analysis of count data is based on statistical models that describes the mean trend on the scale of the counts, not on the scale of the logarithm-transformed mean as would typically be the case for a generalized linear model for count data. There may be several ways to analyze such count data, depending on the distributional and modeling assumptions one is willing to make. Counts are often assumed to follow a Poisson distribution, although this distribution is not always sufficiently flexible to describe the variation in the count dose-response data.
