Pseudoreplication: a widespread problem in primate communication research

Pseudoreplication (the pooling fallacy) is a widely acknowledged statistical error in the behavioural sciences. Taking a large number of data points from a small number of animals creates a false impression of a better representation of the population. Studies of communication may be particularly prone to artificially inflating the data set in this way, as the unit of interest (the facial expression, the call or the gesture) is a tempting unit of analysis. Primate communication studies (551) published in scientific journals from 1960 to 2008 were examined for the simplest form of pseudoreplication (taking more than one data point from each individual). Of the studies that used inferential statistics, 38% presented at least one case of pseudoreplicated data. An additional 16% did not provide enough information to rule out pseudoreplication. Generalized linear mixed models determined that one variable significantly increased the likelihood of pseudoreplication: using observational methods. Actual sample size (number of animals) and year of publication were not associated with pseudoreplication. The high prevalence of pseudoreplication in the primate communication research articles, and the fact that there has been no decline since key papers warned against pseudoreplication, demonstrates that the problem needs to be more actively addressed.