Aggregated distribution

These regions may be widely separated, particularly within animal distributions that are influenced by prey or host densities.

This distribution makes sampling difficult and invalidates commonly-used parametric statistics.

The zoologists M. P. Hassell and R. M. May noted that predators and parasites, too, might aggregate themselves where prey was abundant, choosing some response curve: they observed for example that redshanks (predatory birds) adopted a sigmoid (s-shaped) response to the density of Corophium (amphipod) prey per square metre of mudflats.

[2] Aggregation is seen in directly transmitted parasites (those not using a vector) from many groups: ectoparasites like lice and mites, marine parasites like copepods and cyamid amphipods, and many kinds of nematode, fungi, protozoa, bacteria, and viruses.

Log-transformation of data before the application of parametric test, or the use of non-parametric statistics is recommended by several authors, but this can give rise to further problems, so quantitative parasitology is based on more advanced biostatistical methods.

Parasites are distributed very unevenly among their hosts, most hosts having no parasites, and a few hosts having many. This skewed distribution makes sampling difficult and requires careful use of statistics.