Logistic regression calculates the probability ofsuccess over the probability of
failure;the results of the analysis are in the form of an odds ratio. The odds ratio is a
measure of effect size (Westergren A. 2001), describing the strength of association or
non-independence between two binary data values. It is used as a descriptive statistics,
and plays an important role in ‘logistic regression’. Unlike other measures of
association for paired binary data such as the relative scores, the odds ratio treats the
two variable being compared symmetrically, and can be estimated using some type of
non-random samples.