Data x Datum: Origem das palavras
There are several words with Latin and Greek roots whose plural forms ending in A are constantly mistaken for singular ones. See, for instance, criteria and media. "Datum" is so rare now in English that people may assume "data" has no singular form. Fortunately, you seldom need to use the word in the singular. Just remember to say, "the data are printed on page 27 of the report," and you'll be all right. "Data," though it is increasingly treated as singular by scientists and computer scientists, is allowed by the most conservative authorities only when it's a character from "Star Trek: The Next Generation."