Because the meaning data is much like that of the singular noun information, and because its Latin -a plural announces the word’s plural status less plainly than a final s would, it is often treated as if it were singular. This use is extremely common, and few perceive it as wrong these days, especially given the word’s connotation of a collection or single unit made up of many informational subunits.
All the same, in formal English, Our data have been assembled over a number of years would be regarded as correct, and commonly used constructions such as very little data, the data shows…, and a great deal of data would be regarded as incorrect.
<taken from Microsoft Encarta Dictionary>
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