Third Variable and Directionality Problem  

A) In non-experimental research we always have to worry about the “third variable” and “directionality” problems ... sometimes we can be solve the directionality problem but we can never be sure that we have solved the third variable problem.

B) Third Variable Problem: When two variables appear to be related to each other but there is another unknown variable (the third variable) that is the real source of the link between the first two variables.

Examples: Hair Length and Weight

              Crime and Number of Churches in a Community

              Class Size and Academic Achievement

                              What about study time and GPA ?

 C) When two variables X and Y are related to each other and you make the assumption that changes in one of the variables, X, is producing (causing) changes in the other variable, Y, when in fact the causal relationship goes in the other direction ... Y is causing changes in X.

 Examples: Use of Pornography causes Sexual Deviance

Violent Behavior in Children is caused by Watching Violent Video
Games (violence on TV)

 D) Third variable problems can be very subtle and hard to catch ...

Example:

            Weight Differences in Children are caused by Amount of TV They Watch

           Differences in Parenting Style cause Differences in Child Temperament