There are several words with Latin or 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. Many American usage communities, however, use “data” as a singular and some have even gone so far as to invent “datums” as a new plural. This is a case where you need to know the patterns of your context. An engineer or scientist used to writing “the data is” may well find that the editors of a journal or publishing house insist on changing this phrase to “the data are.” Usage is so evenly split in this case that there is no automatic way of determining which is right; but writers addressing an international audience of nonspecialists would probably be safer treating “data” as plural.
List of errors