members of the population. In Fig. 161 the line 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.
In Fig. 162 a study has been made to see how the gasoline consumption of motor trucks varies in trucks of different sizes. The horizontal scale shows the rated size in pounds of the trucks under consideration. On the vertical scale, the cost of gasoline is given in cents per car mile. The data of the different motor trucks were indicated by separate dots on the chart. The solid line was then drawn through a point which represents the center of gravity of all the dots on each vertical line. The total number of dots on this chart is rather small. Too much dependence cannot be placed in the resulting curve, as special conditions may have affected some of the records so as to cause the dots to be misleading. Thus, considering the two dots which are given for trucks of 2,000-pounds capacity, it will be noticed that both of these dots are far below the position on the chart which one would expect the average to occupy if one should judge by the general tendency of the curve as a whole. It may have happened that the particular trucks which these two dots represent were run with very light loads, thus making the gasoline consumption lower than would naturally be expected for trucks of that size. In Fig. 162 the object is to determine what relation, if any, exists between the cost for gasoline and the size of the truck.
"Correlation" is a term used to express the relation which exists between two series or groups of data where there is a causal connection. In order to have correlation it is not enough that the two sets of data should both increase or decrease simultaneously. For correlation it is necessary that one set of facts should have some definite causal dependence upon the other set, as seen in Fig. 162.
Correlation studies can frequently be of assistance in business problems. A manufacturer of machinery has recently revised many of his manufacturing and selling policies from the information obtained from a chart showing the relations of cost and selling price of his equipment to the actual size of the equipment. On the horizontal scale of charts used for this study the size of the apparatus was shown according to its actual working capacity. In a vertical direction a scale was selected for the cost of the apparatus and for its selling price. Dots were then placed on the chart in a manner similar to
