The size of mammals is often given as the head and body length (HBL). The condylobasal length of the skull (CBL) is also used as a measure of the size of mammals. The HBL in small mammals is mostly measured from the tip of the nose to the root of the tail. In species like whales, human beings and elephants, the measurements are not comparable with those from small mammals which in fact does not matter. On the contrary, it is of prime importance for the measurements taken within the same species to be comparable. If we deal with incomparable data from different authors or museums, it may result in false conclusions. In the present paper this problem is illustrated by the red fox Vulpes vulpes. The HBL in Scandinavian red fox is 4.43 to 4.54 times the CBL. Data in the literature indicates that European foxes outside Sweden have a HBL 4.54 to 4.96 times their CBL. The difference is probably an artifact of different measuring techniques. Therefore we believe that CBL gives better information about the size of the foxes. However, the length of the skull is far from ideal here, since the proportion HBL/CBL seems to vary geographically. We suggest that what has been measured and how the measurements have been taken must be carefully reported by the authors. The scientists would know then which data can certainly be used for an extended meta-analysis. How to cite this article: Englund J.K.Å., Elmeros M., Österdahl L.E.W. 2020. The need for consistent data collection for large-scale comparative studies illustrated by the study of morphology of the red fox Vulpes vulpes // Russian J. Theriol. Vol.19. No.1. P.99–103. doi: 10.15298/rusjtheriol.19.1.11.