http://grammar.quickanddirtytips.com/is-data-singular-or-plural.aspx wrote:
Is "Data" Singular or Plural?
Episode 139: October 3, 2008
by [guest-writer] Charles Carson, managing editor of the journal American Speech
We've received several requests to address Latin plurals, so today we're going to tackle a tricky one. A listener called with this request:
Hi, Grammar Girl. This is Adam from Peoria. I was wondering if you could go over the usage of the word data, as in The data are correct or The data is correct. Thanks.
The question seems easy enough: is data singular or plural? Unfortunately, the answer is that both usages are standard (1). However, before we can address this fully, we need to review a couple of linguistic concepts.
Mass Nouns Versus Count Nouns
Count nouns are used for objects that can be counted; that is, they're distinct objects that can be numbered. For example, in my refrigerator there are eggs, apples, and lemons. These are all count nouns. Count nouns can be singular or plural, and when you use them as the subject of a sentence, the verb must correctly reflect that number, as in The last apple IS on the bottom shelf or The eggs ARE fresh.
Mass nouns, on the other hand, are used for things that don't have a natural boundary and can't be counted. Also in my fridge are butter, iced tea, and bacon. These are all mass nouns. Mass nouns always take a singular verb, as in The iced tea IS already sweetened and They say bacon IS bad for you, but I love it.*
How Many or How Much?
An easy way to tell these two types of nouns apart is to ask yourself how many or how much. If it makes sense to ask how many there are of a noun, as in how many cars or how many people, then it's a count noun. If, however, it makes more sense to ask how much there is of a noun, as in how much butter or how much rain, then it's a mass noun.†
The use of many and much parallels the use of fewer and less: many and fewer are used with count nouns (like items in a grocery cart) and much and less are used with mass nouns, like tea or bacon.
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 17th century (2); however, it wasn't till the late 19th 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.
A Quick and Dirty Way to Check Your Writing
Here's a quick and dirty tip to check your own use of data. If you wish to use data as a singular mass noun, you should be able to replace it in the sentence with the word information, which is also a mass noun. For example,
Much of this information is useless because of its lack of specifics.
If, however, you want to or need to use data as a plural count noun, you should be able to replace it with the word facts, which is also a plural count noun. For example,
Many of these facts are useless because of their lack of specifics.>>