Editing Effect size (section)

==== Odds ratio ====

The [[odds ratio]] (OR) is another useful effect size.  It is appropriate when the research question focuses on the degree of association between two [[binary data|binary variables]].  For example, consider a study of spelling ability.  In a control group, two students pass the class for every one who fails, so the odds of passing are two to one (or 2/1 = 2).  In the treatment group, six students pass for every one who fails, so the odds of passing are six to one (or 6/1 = 6).  The effect size can be computed by noting that the odds of passing in the treatment group are three times higher than in the control group (because 6 divided by 2 is 3).  Therefore, the odds ratio is 3. Odds ratio statistics are on a different scale than Cohen's ''d'', so this '3' is not comparable to a Cohen's ''d'' of&nbsp;3.