A(n) N Problem

In writing the previous post, I initially wrote (emphasis added):

It's a great example of an MCP aspect of English.

But I paused for a second when using the word "an" instead of "a." It seems that it is still entrenched in my mind that you put "a" in front of words that start with consonants, and "an" in front of words that start with vowels.
This doesn't word for all sorts of words that I've already automated because I know the sound is a vowel sound or a consonant sound, such as: a union, a eulogy, but an honor, etc.

The natural unfamiliarity that arises in acronyms and initialisms might have something to do with my hesitation. To investigate, I googled some various word combinations:

	W/ "a"	W/ "an"	f¹
MBA	688,000	1,310,000	1:3
NBA	271,000	1,090,000	1:5
Honor	309,000	1,470,000	1:6
USA	1,600,000	144,000	11:12
Union	1,250,000	201,000	6:7
Eulogy	420,000	39,200	11:12

(Note¹: f is the likelihood of "a" occuring)
It's clear that some of the results, particularly those with 'a,' are not exact examples of what I was searching for, but, this should still give enough of an indication as to what is happening with people's fingers on the internet. There are some discrepencies; For example: X-Ray is much more common with an, but F-Stop was very close (about 4:9), slight preference for an.

Even more interestingly, I get comparable results for LSAT, but there are about three times as many instances of a SAT over an SAT. This can be explained by some instances of homophony, such as SAT meaning "saturday" or "satisfiability" and quite possibly pronounced [sæt]. To combat this, I did searches for "a SAT score" and "an SAT score" with about five times as many results for "a SAT score." Almost ironic, no?