Language, above all else, should be used to make one’s thoughts clear to others. The pairing of a plural noun and a singular verb form is distracting, if not confusing. Even though datum is seldom used by Americans, it remains the singular foundation of data, which is the plural form. Data, therefore, never “is”; they always “are.
Latin plurals sometimes become singular English words (e.g., agenda, stamina) and data is often so treated in U.S.; in Britain this is still considered a solecism…. (H.W. Fowler, A Dictionary of Modern English Usage, second edition, p.119)
But Follett is better on the subject:
Those who treat data as a singular doubtless think of it as a generic noun, comparable to knowledge or information…. [A generous interpretation: ED.] The rationale of agenda as a singular is its use to mean a collective program of action, rather than separate items to be acted on. But there is as yet no obligation to change the number of data under the influence of error mixed with innovation. (Wilson Follett, Modern American Usage, pp. 130-1)
To say “data are” is to present oneself as a learned person of high standards. That, unfortunately, is an “old fashioned” attitude in this day of dumbed-down vulgarity.