Controlling extraneous variables by holding them leads to more sensitive experiments with less external validity

............... is an important technique for randomly assigning subjects to groups because it produces groups that are of equal size while controlling for time related variables.

.............. occurs when the effect of one independent variable differs depending on the level of a second independent variable.

................ psychology was advanced in the late 1990 by Seligman during his term as president of American Psychological Association.

These tests are based on the assumption of normality 

In .................... subjects are first divided into groups, such that within each group the subjects are relatively homogeneous in respect to some selected variable.