Figure 6. . Relationships between mean and skewness for data on organisms collected, England and Wales, 1991–2011. A) Skewness plotted against log of mean for Cyclospora spp. The full curve is the best fit to the points; the dashed curve corresponds to the negative binomial model; the dotted curve corresponds to the Poisson model. B) Histogram of the parameters corresponding to the best-fit curves for 1,001 organisms; the value −0.5 corresponds to the negative binomial model (equation 2).