The slope of a curve plotted on paper with ordinary co-ordinate ruling depends largely on whether the data of the curve are in large figures, so as to bring the curve near the top of the chart, or in small figures, bringing the curve near the bottom of the chart. In Fig. 98 the fact that curve A slopes more than curve C is due to the fact that curve A is placed higher on the vertical scale of the chart than curve C. A little study will show that the reduction in mortality portrayed by curve A is much less on a percentage basis than that depicted by curve C, yet curve A has the steeper slope. The slope of these two

COMPARISON OF CURVES

curves can best be compared if a pencil line is drawn in such manner that the peaks above the pencil line are approximately equal to the valleys below the pencil line for each curve.

Fig. 99 contains some interesting information. Though the chart proves fairly well the close dependence of the price of cast-iron pipe upon the price of the pig iron from which it is made, the chart is nevertheless misleading in that the first glance would indicate a much greater fluctuation in the price of pig iron than actually occurred.

nJ^-^C -- .:S

~° ^^

-c:/^.i i 1

H"t Y rSWAtf MsU [4=\=H

M T T\4vf rM-UJ in

- T TT TN-l 1 l\ Lo

mUAJWn

[yjj 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 FTYiy 1 VI