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