In making the tests which are represented in Fig. 81, different depths below the surfaces were selected and the bacteria determined from the water samples taken at these depths. The depth is here the independent variable, and bacteria per cubic centimeter the dependent variable. The decision as to which is the independent variable and which is the dependent variable rests entirely on how the problem is approached. Numerous samples could have been taken at different depths, and then a curve plotted to determine the depth at which certain numbers of bacteria per cubic centimeter were found. In

80 GRAPHIC METHODS

such a case, bacteria per cubic centimeter would be the independent variable and depth would be the dependent variable. This sort of problem may be attacked from either one standpoint or the other, and it is just a question of convenience as to which method is used and which variable is made the independent variable. Though the problem can be stated in such manner that either one variable or the other can be made the independent variable, after the statement has been made the chart should be consistently drawn so that the independent variable will be used as the horizontal scale and the dependent variable as the vertical scale.

As Fig. 81 is shown it is necessary for the person interpreting the chart to select from the vertical scale some number of feet below the surface and then read the number of bacteria per cubic centimeter by the horizontal distance to the right. It is only after some little puzzling that the reader will notice that the scales for the variables have been reversed and that the chart has been practically turned on its side. How this chart would appear if the horizontal scale were used for the independent variable may be judged by turning the book and looking at Fig. 81 from the left-hand side of the page.