At the end of an earlier post, my friend and former boss Rick Lempert touched on a pet peeve of mine: treating "data" as a singular noun (as in, "Our data is...," or "The data suggests..."). "Data," I learned early on in my graduate training, is the plural of "datum," and so is akin to "facts"; the only correct form is therefore the plural ("Our data are...," "The data suggest..."). Any other usage -- including that in a number of posts on this very blog -- is simply wrong.
Sadly, my smug self-satisfaction at relentlessly educating all those singular-data cretins has come to an end. As it happens, the cretins were right; or, at least, they weren't as wrong as I'd thought. Language, as we all know, evolves; the growing consensus is that data is a mass noun, and so admits to a singular usage. In this respect, data is undergoing the same transition that agenda -- formally, the plural of agendum -- underwent a long time ago: Few people I know would say (e.g.) "The meeting's agenda allocate ten minutes for opening statements." My reading of a number of discussions of this issue around the blogosphere also suggests that there is a generational component to the change, with younger folks being more willing to accept -- and in some cases, even argue strenuously for -- data as singular.
So, with a heavy heart, I'll be scratching data-as-singular-noun from my pet-peeve list (which includes, among other things, reporting "p = 0.000" and stating that one's parameter estimates are "significant in the wrong direction"). But with that, a plea: keep treating data as plural anyway. It's still correct, and (in this blogger's opinion) it just sounds better.