Usage of an

In a discussion about statistics, my friend said, “I used an SRS of 1,000.”

SRS stands for Simple Random Sample, so wouldn’t one say “a SRS,” or is there a rule that one must use an with acronyms, or letter names, e.g. “He drives an S500.”

The rule is quite simple. If the next word starts with a vowel sound, then “an” is the proper word. Notice that I said “vowel sound”. “S” is pronounced “ess”, so even though it’s not a vowel, it starts with a vowel sound. Similarly, “hour”, while not starting with an actual vowel, still takes the article “an” because the “h” is silent. Words such as “yolk” and “historian”, however, do not take the article “an”: “y” in this case is not acting as a vowel, and the “h” is not silent.

I would think that the usage of “an” is determined by the proceeding sound not the character. If it is a vowel then you use “an”, if not then “a”.

in “SRS” the “S” is pronounced “Es” making it a vowel sound.

Maybe somebody could clarify this for me.