Analysis of ordinal dental date: Evaluation of conflicting recommendations

Although recommendations for the appropriate analysis of non-normal and ord inal-scaled data have appeared in the dental research literature for many y ears, there is no consensus. When one is conducting statistical tests for d ifferences between groups, the central concern is whether it is safe to use parametric tests (e.g., analysis of variance), or if only non-parametric r anking tests should be considered. Relevant statistical and scientific issu es associated with non-normality and measurement scale are reviewed, and th ree conclusions are reached regarding the analysis of dental data: (1) Para metric tests are sufficiently robust relative to typical violations of norm ality; (2) presumed statistical prohibitions against the application of par ametric methods to ordinal data do not actually exist; and (3) 'ordinal' de ntal indices have sufficient quantitative meaning to be considered quasi-in terval. For these reasons, parametric tests should not be avoided; they wil l be valid and usually more powerful and more easily applied to complex des igns than non-parametric alternatives.