6.2 The explanatory variable and variation in the response

The explanatory variable may be associated with changes in the values of the response variable. However, it may not; after all, determining this is the purpose of the study.

If nothing else influenced the values of the response variable, life would be easy: Any change of a given size in the value of the explanatory variable would always results in change of the same size in the value of the response variable.

Example 4.1 (Explanatory variable) In the typing-speed study (Example 5.3), the explanatory variable is the sex of the person. If nothing else influenced typing speed, all females would record the same typing speed every time, and all males would record the same typing speed every time. This is clearly unreasonable.