Sophisticated methods of assay, such as immunosorbent assay, have now
been developed for some vitamins. However, it is probable that values quoted
in most tables have been obtained using more traditional methods such as
microbiological assay. Although these methods are accurate for some vitamins,
for others (e.g. folic acid) this may not be the case. Tables usually quote
only single values for each vitamin, whereas analytical surveys can show wide
variations between samples of the same ingredient. Although some of this
variation will be attributable to analytical error, it is undoubtedly true that
individual vitamin contents of ingredients do vary. In setting diet standards,
therefore, some account must be taken of the likelihood and extent of individual
batches of a feedstuff having a vitamin content lower than the table average
value.