Effects of vitamin A/E supplementation on ERG cone flicker progression rates.
REGRESSION
Modeling of the effects of vitamin A and vitamin E on the rate of ERG progression using the participants from the original vitamin A/E study initially produced different results than the original publication; vitamin A had a beneficial significant effect (Next, the baseline implicit time was added to the regression model as a predictor of rate of progression (Having noted that the original conclusions were not reproduced with these new analyses, we expanded the sample size by adding additional participants taking vitamin A only from the 2 control arms of the subsequent DHA and lutein trials (To assess whether the choice of statistical packages and models contributed to these findings, the analyses were implemented in the R Lmer package (see Methods). Nearly identical results were obtained between SAS and R (In summary, adding additional year 5 and year 6 data, in combination with adding the predictive power of baseline implicit time to adjust for an imbalanced randomization at baseline, removed the evidence for the broad, beneficial effect of vitamin A that was seen in the original study. This lack of a significant vitamin A treatment effect persisted after adding additional participants from a later trial. The negative effect of vitamin E became smaller but remained statistically significant.