Page:Graphic methods for presenting facts (1914).djvu/169



CUMULATIVE OR MASS CURVES

The curves thus far shown have practically all been of a type in which the thing plotted was a value or a rate per week, per month, or per year. The tendency of such curves is to follow a horizontal direction unless affected by conditions which cause seasonal fluctuations or gradual increases or decreases. In this chapter we shall consider curves in which the data plotted concern total output, rather than the rate of output. With cumulative or mass curves, such as are considered here, each point on the curve represents a total output up to the time for which the last point is plotted. The figure for each successive period of time is added to the total already recorded, and the new total point is plotted. Because the figures relating to the last unit of time are always added to the total figures already recorded, curves of this type are called cumulative curves or mass curves. The tendency of a cumulative curve is to start at the lower left-hand corner of the co-ordinate ruling and move toward the upper right-hand corner of the sheet, according to the scale which may have been selected.

Fig. 134 is a half-tone illustration of a cumulative-curve chart representing the output of an automobile factory for one fiscal year of the business. In a conference between the sales, engineering, and manufacturing heads of this business, it was decided that the quantity of automobiles desired was fifty per week until the first of April, then sixty per week until the first of June, and seventy per week thereafter, giving a total production of three thousand two hundred automobiles for the whole season. It was thought that the rate of production on the new model automobile could be increased after the factory had been put into thoroughly good running shape, and the schedule rate of production was accordingly increased gradually in the manner shown. On account of the delay in getting drawings