Aggregation 3
Aggregation 3 : A distribution of parasites amongst hosts is said to be aggregated, or overdispersed, if parasites are found to co-occur in particular hosts more often than if the parasites were distributed at random amongst all hosts. A more precise test would be if the distribution differed significantly from a Poisson distribution; a good heuristic is whether the variance to mean ratio is bigger than unity (in appropriate units). Macroparasites are typically aggregated in their host populations, so that the majority of hosts harbor few or no parasites while a few hosts harbor large parasite burdens. Aggregated distributions are often well described empirically by the negative binomial distribution
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