Odds ratio: an ecologically sound tool to compare proportions

The correct use and interpretation of statistical measures is often challenging for field-oriented ecologists. One such basic measure is the odds ratio (OR), which enables the comparison of two proportions. Odds ratio is the pivotal concept in the simple analyses of proportions in two-way contingency tables, as well as in complex logit model approaches. Here, we clarify the use and interpretation of the odds ratio in ecological research. We show that the odds ratio is both a statistical and an ecological solution to quantifying the direction and magnitude of discrepancy between proportions. To enhance comparison of suppressing (with OR below one) and promoting (with OR above one) factors, we propose that the odds ratio should always be reported as a value above one, together with an exponent (1 or -1) to denote the direction of the effect. The odds ratio supports powerful ecological interpretations in the comparison of proportions and thus should become a standard concept in ecological papers.