A Cautionary Note on Linear Measurements and Their Ratios in Taxonomy

Statisticians work with figures, whereas scientists work with estimated quantities. Every direct (physical) measurement has some degree of uncertainty. Single numbers pose no problems, and an implied range can always be specified. Difficulties arise when those numbers or sets of numbers are used to calculate derived figures. Statistical measures such as ratios can be skewed if uncertainty about the actual measurements used to derive those quantities is not taken into account. This lack of consideration may lead to incorrect figures being used and calls into question the criteria used to diagnose, identify or delimit new species. In this case study, I use data gathered from the literature on different species of the clade Hydrachnidia (Acari, Parasitengona) to show how range ratios of important characters differ when uncertainty is considered. I outline the successive steps taken during the measuring process—from microscope calibration to the calculation of several statistical values from the direct measurements—and suggest some corrections. I anticipate that the results and recommendations presented here will be applicable to other taxonomic groups for which linear measurements play a central role in the description and identification of species.