The fairly uniform slope of the curve for the first six years after graduation indicates that the men were receiving almost uniform raises in pay each year. It must be remembered that a straight-line curve simply indicates that the amounts of the increases year by year are uniform in numerical value. If a curve were started at the lower left-hand corner of the chart and drawn diagonally across each of the rectangles of the chart, it would be seen at once that there would be a straight line indicating an increase in salary of $500 per year. With such a straight line across the chart, the increase in salary for the first year would be $500. As compared with a zero beginningwage there would be an increase of an infinite percentage at the end of the first year. The next year the increase would again be $500. Compared to the $500 salary, the increase would be 100 per cent. The third year the increase would be $500, and compared to a $1,000 salary the increase would be only 50 per cent. A curve of uniform slope on any chart of rectangular co-ordinate lines indicates only that there has been a uniform increase or decrease in actual numbers, not

Fig. 69. Average Income of 155 Princeton Graduates of the Class of 1901 for Ten Years After Graduation

Note the effect of the 1907 panic on incomes in 1908

GRAPHIC METHODS

values given in Millions of UollaJ^

•PRODUCTION

a uniform rate of change on a percentage basis. A plotted line representing a uniform rate of increase from year to year on a percentage basis may be seen in the curve given in Fig. 121.