Proper reorganization of your data set will allow any other day to be used as the standard, should you so desire. The grand mean can also be used as the standard. The regression based ratio adjustment described above creates a set of dietary intake data with equal means across survey days. The data set is now ready for the estimation of the usual intake distribution. For nutrients such as vitamins and some micro nutrients, skewness is quite extreme.
Most statistical procedures rely on the assumption of normality.
For example, the method that was proposed by the National Academy of Sciences in 1986 for estimating usual intake distributions, assumes that intake data are normal, and proposes that dietary intake data be transformed prior to analysis so that the normality assumption is satisfied. The C-SIDE procedure transforms adjusted dietary intake data into normality as a part of obtaining estimates of usual intake distributions. The transformation into normality in C-SIDE is done in two steps. In the first step, data are transformed so that their distribution is as close to normal as possible, by using a power transformation similar to the one described earlier. However, power transformed data are not necessarily normal.