In spite of the increasing focus on missing data theory and methodology, it is still not often implemented in research beyond reporting missing percentages and correlations between missingness and variables in the study (McKnight et al., 2007; Schafer and Graham, 2002).
The principal limitation of this work is the exclusion of some cases for missing data, but the methodology allows for assessment of the influence of BPT upon outcome independently of other predictors in a relatively large cohort comparable in size to studies in recent literature.
Finally, during 10-year follow-up there were up to 20% missing data and an imputation methodology was used.
Where it was possible, without risking bias, missing data was imputed using standard methodologies [25].
Approximately 60% of patients who entered ULTRA 3 and had achieved remission and mucosal healing at year 1 maintained these end points at year 4, when assessed using a conservative NRI methodology for missing data.
However, with the method as presently implemented in PsN, it is only possible to impute missing values for one covariate at a time while other software implementations of the MI methodology impute missing data for all partly missing variables at the same time (10– 10).
Where no access to the missing data was provided, the following methodology was applied: in cases where the mean was not reported, the median was used; in the absence of SDs, interquartile ranges (IQRs) were used to estimate SDs using a normal approximation, and, in the remaining cases, the maximum of clinical trial SDs was used.
This was a RCT with a good sample size; a similar compliance in both arms; the use of a robust methodology under missing data of the MAR type; and no indication of a source of bias in the investigation of the missingness mechanism.
There are interesting similarities between the missing data and the bootstrap regression modeling methodologies.
Several methodological issues (missing data and model stability) which are not accounted for by the standard statistical methodology are expected to complicate this matter.
