Saylor URL: http://www.saylor.org/books Saylor.org
Figure 2.2.2
Still another possible explanation for the observed correlation is that it has been produced by the
presence of a common-causal variable (also known as a third variable). A common-
causal variable is a variable that is not part of the research hypothesis but that causes both the
predictor and the outcome variable and thus produces the observed correlation between them. In
our example a potential common-causal variable is the discipline style of the children’s parents.
Parents who use a harsh and punitive discipline style may produce children who both like to
watch violent television and who behave aggressively in comparison to children whose parents
use less harsh discipline:
Figure 2.2.2