"Data is", or "Data are"?

I guess I'm a pedant traditionalist when it comes to the word "data": one "datum", several pieces of "data", etc.

As with many matters relating to the use of language, though, this one isn't open and shut, by any means.

And so a few days ago we saw The Wall Street Journal, The Economist, and The Guardian grappling with this issue once again.

However, I'm going to stick with my guns, dust off my slide-rule, and also continue to use the "Oxford comma"!

© 2012, David E. Giles

Local vs. Global Approximations

Approximating unknown (continuously differentiable) functions by using a Taylor (MacLaurin) series expansion is common-place in econometrics. However, do you ever pause to recall that such approximations are only locally valid - that is, valid only in a neighbourhood of the (possibly vector) point about which the approximation is made?

Unlike some other types of approximations - such as Fourier approximations - they are not globally valid.

Does this matter? Is it something we should be concerning ourselves with?