In Fig. 161 we have the curves for two widely separated years plotted side by side, so that we may tell from the general shape of the curves whether the distribution of wealth is approaching uniformity of tending in the direction of great concentration in the hands of a few people. The more nearly the curve approaches a straight line the more nearly wealth is distributed uniformly among all the

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Rating of Truck in Pounds 9 Report by Operator a Estimcrte by Manufacturer

* Average reported Figure

H. F. Thomson. Massachusetts Institvte of Technology, Vehicle Research

Fig. 162. Cost of Gasoline in Cents per Truck-Mile for Different Sizes of Motor

Trucks

Since motor trucks are rated by the manufacturers in multiples of 1,000 pounds, the dots representing the records of different trucks naturally fall on the lines spaced by 1,000-pound intervals on the chart. The solid line is drawn through points at the "center of gravity" of all dots for any vertical line on the chart and represents the average condition as reported by truck operators. The dotted line is drawn through points on the chart for data furnished by the manufacturer as his estimate of good practice

A curve of this general type, proving a close relationship between two variables, may be called a correlation curve

CORRELATION 199

members of the population. In Fig. 161 the hne was more bowed in the later year than in the earlier year, and the conclusion may accordingly be drawn that wealth in Prussia tended toward further concentration in those years intervening between 1892 and 1901.