Though engineers have used curves for many years to represent data relating to the laws of physics, the engineer has made practically no use of frequency curves such as are used by the biologist. This is probably due to the fact that the engineer can determine the laws of physics from mathematical computations based on a relatively small number of observations, while the biologist must deal with statistical averages based upon observations and measurements in thousands of different cases. The laws of biology are not so definitely mathematical as those of engineering and physics. The biologist must have more observations than the engineer if he is to draw accurate conclusions.

A frequency chart is based on the number of times a certain characteristic is found repeated in a large number of observations. The number of repetitions is referred to as the "frequency". A comparison

C. B. Davenport, in Popular Science Monthly

Fig. 140. Frequency Chart Based on the Number of Ribs in Scallop Shells. Shells Are Sorted into Different Piles According to the Number of Ribs, the Piles (from Left to Right) Having Respectively 15, 16, 17, 18, 19 and 20 Ribs

The heights of the different piles show the relative frequency of shells having the different numbers of ribs. Seventeen-rib shells were found much more commonly than shells with any other number of ribs. K a line were drawn through the tops of the different shell piles, we should have a frequency curve

FREQUENCY CURVES

relating to the frequency with which different characteristics or items are found repeated is commonly referred to by the biologist as "frequency distribution", and the charts showing frequency are quite often called "distribution charts" or "distribution curves".