People will argue about this one.
How Many Data or How Much Data?
Now let's get back to our original question, is data singular or plural? Or, more accurately, is data a mass noun — remember, a mass noun always takes a singular verb — or is data a count noun,‡ the plural of datum.§
As I said, both usages are standard. The count noun datum and its plural data, meaning "a given fact or assumption," were adopted from Latin into English by the seventeenth century (2); however, it wasn't till the late nineteenth century that data took on the modern sense of facts and figures. This shift in meaning also led some to start treating data as a mass noun.**
So if data is correct as both a count noun and as a mass noun, which should you use? It comes down to style and personal preference. Many academic and scientific fields, as well as many publishers and newspapers, still insist on the plural count noun use of data, as in The data are compelling, but it is more commonly used as a singular mass noun, as in The data is compelling.
If you write for an organization or discipline that insists on the plural count noun usage, pay attention to other words in the sentence that are sensitive to number. For example, an author might write the following sentence:
Much of this data is useless because of its lack of specifics.
If the publisher allows for the singular mass noun usage, that is an acceptable sentence. If, however, the publisher insists on the plural count noun usage, an author might change the verb is to are, making the sentence read as follows:
Much of this data are useless because of its lack of specifics.
That change, however, makes the sentence ungrammatical. Note that the author wrote MUCH of THIS data. Count nouns answer how many, not how much. It should be changed to MANY of THESE data. The sentence also reads because of ITS lack of specifics; the author here should use the plural pronoun their, because of THEIR lack of specifics. Thus, the correct sentence should be as follows:
Many of these data are useless because of their lack of specifics.
If that sounds odd to you, as it does to me, then you probably use data as a mass noun and would treat data as singular — and there's nothing wrong with that. Just be aware that if you do write or edit for a publisher or in a discipline that insists on plural data, you should make sure the surrounding words properly reflect the plural treatment of the word data. Even if you don't have a style guide insisting on the plural usage but you decide to use it anyway because you like Latin plurals, be sure to do it consistently throughout the document — in other words, don't mix up your datas, using it as a count noun in one place and as a mass noun in another.