Page:Graphic methods for presenting facts (1914).djvu/104



CURVE PLOTTING CONTINUED

There are so many different applications of curves and such varied yet convenient methods of plotting curves, that it seems worth while to take up some of these in detail, and point out certain advantages and disadvantages of different curve-plotting schemes.

Practically all curves display relations existing between different sets of data which we may call "variables". One of the variables is used as a standard or measure by which to interpret the facts under consideration, and it may be called the "independent variable". The other variable, which is interpreted from the independent variable, is called the "dependent variable". For example, in a bacteriological examination of a pond at varying depths, distance below the surface would be the independent, and number of bacteria per cubic centimeter the dependent variable. In a seasonal gauging of a stream the dates of observation would be the independent and cubic feet per second of flow the dependent variable. Sometimes we consider more than two variables simultaneously, and we then have two or more independent variables from which to consider a dependent variable.

It is difficult to make a general rule for determining in any case which is the independent variable and which is the dependent variable. The decision depends entirely on how any set of data is approached and on the habits of mind of the investigator. When time is one of the variables it is usually, but not always, the independent variable. If we consider values or quantities at different dates, as in Fig. 80, time is very obviously the independent variable. If, however, we are interested in the length of time required to do different operations, as in Fig. 85, Fig. 86, and Fig. 87, our data are expressed in length of time and time is the dependent variable. This example is an exceptional case and it is named here only to show that, although time is