dataset or data set

[stateworker]

We're discussing which way to write this for a report.

As two words "The data set included xx,xxx individuals" or
as one word "The dataset included xx,xxx individuals."

Which way do you write it and why?

[Doc_Duke]

I prefer "data set" if for no other reason than it is the way I learned it.

http://en.wikipedia.org/wiki/Data_set

The two word approach is also used in the Federal Register, so I need to use it that way to quote things.