Is data singular or plural?
The simple answer here is that data is the plural form of datum. Therefore it follows that you would have "a datum" and "some data", just as you would have a stadium and some stadia. The strange thing is that the word datum, meaning a single piece of statistical information, has largely fallen into disuse outside the realm of statistics, and the word data has come to mean facts, figures or information on a subject. In a way, the opposite is true of stadium; stadiums is an acceptable pluralisation and stadia is rarely used.
In scientific writing, the word data is still generally used as a plural: "some data", "the data show" (not shows), "data pertaining to the ant are limited". This does look like changing, however.
So what's the problem?
A few years ago, I noticed an instruction in the style guide to a book I was editing: "Data plural". A truism, I thought – of course it's a plural. So why bother specifying it? It would seem that the plural treatment of data has become contentious, and that writing "this data is interesting" is gaining acceptability.
The point is that data sounds like a singular because it has no "s" on the end, and the part of the brain dealing with syntax automatically treats it as such. In the mind, data is a synonym of information, not statistics.
Sometimes, a living language will through usage add suppleness to the etymological rigidity in a word, and make its morphology fit its own protocols, and it would appear that data is now as acceptable as a singular as it is a plural.
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