Intervening variable used in the book are classified into independent and dependent variables. Independent variables are those manipulated by the experimenter. For ex- ample, not allowing rats to have any water for several hours would create an independent variable called hours of deprivation. Dependent variables are those observed by the ex- perimenter. For example, one could observe how much water a rat drinks.
Science tries to explain the world by relating independent and dependent vari- ables. Intervening variables are abstract concepts that link independent variables to dependent variables. The principle of parsimony, maintains that researchers should apply the simplest explanation possible to any set of observations. For instance, psychologists try to explain results by using well-accepted theories instead of elaborate new hypotheses.
Parsimony prevents psychologists from inventing and pursuing outlandish theories. In the book on page 15 figure 1.3 it shows a direct and an indirect way to relate an independent variable, hours of deprivation, to a dependent variable, rate of bar pressing. The dependent variable is obtained by placing a rat into a small chamber where it can press a bar to obtain drinking water. The experimenter observes the rate (how many presses per minute) at which the rat presses the bar to get water. The direct relationship uses only one arrow to link hours of deprivation to rate of bar pressing. After doing the experiment, we could build a mathematical formula that directly relates hours of deprivation to rate of bar pressing. The indirect method in figure 1.3 uses two arrows. The first arrow relates hours of deprivation to thirst, an intervening variable. The second arrow relates the intervening variable, thirst, to the rate of bar pressing. Since the indirect method is more complicated, requiring an