Ophthalmic statistics note 10: Data transformations

Many statistical analyses in ophthalmic and other clinical fields are concerned with describing relationships between one or more ‘predictors’ (explanatory or independent variables) and usually one outcome measure (response or dependent variable). Our earlier statistical notes make reference to the fact that statistical techniques often make assumptions about data.1 ,2 Assumptions may relate to the outcome variable, to the predictor variable or indeed both; common assumptions are that data follow normal (Gaussian) distributions and that observations are independent. It is, of course, entirely possible to ignore such assumptions, but doing so is not good statistical practice and in medicine; poor statistical practice can impact negatively upon patients and the public.