Comparing parasite numbers between samples of hosts

The comparison of parasite numbers or intensities between different samples of hosts is a common and important question in most parasitological studies. The main question is whether the values in one sample tend to be higher (or lower) than the values of the other sample. We argue that it is more appropriate to test a null hypothesis about the probability that an individual host from one sample has a higher value than individual hosts from a second sample rather than testing hypotheses about means or medians. We present a recently proposed statistical test especially designed to test hypotheses about that probability. This novel test is more appropriate than other statistical tests, such as Student's t-test, the Mann-Whitney U-test, or a bootstrap test based on Welch's t-statistic, regularly used by parasitologists.