Figure 2. Intercalibration between qualitative (perceived abundance) and quantitative (observed relative composition) data in the period 1901–1925.

(a) Cumulative frequency distribution of the number of species against qualitative classes of perceived abundance; (b) cumulative frequency distribution of the number of species against quantitative observed relative composition. Red dots represent the class limits that subdivide the species into four groups whose cumulative frequency distribution is the same of the frequency distribution of naturalists' classes of perceived abundance. Black dots represent the class weights associated to each class of perceived abundance.