Why is random assignment important? random assignment ensures that the groups are matched on some variable before the experiment. random assignment helps to make the groups equal on a variety of variables. random assignment ensures that each member of the population has an equal chance of being chosen to be in the experiment. random assignment increases the test-retest reliability of the experiment?

The answer is: Random assignment helps to make the groups equal on a variety of variables. 

This is because there are still many confounding variables present throughout a random sample that you can't control. By using random assignment, the sample is randomly sorted into groups, decreasing error of the results due to a confounding variable.