Plural form of abbreviations

[nyggus]

Consider some abbreviation, say, ANOVA (analysis of variance). If I am talking about several analyses of variance, should I use "several ANOVA's" or just "several ANOVAs"? The same with mathematical symbols. Consider several variables denoted Xi. Should I use "X's", or just "Xs"? I have met both these forms in the literature and I am wondering whether they both are correct.

Thanks in advance!
Nyggus

[Tdol]

I wouldn't use an apostrophe in either case because the lower case makes it clear. I'd only use an apostrophe with a lower case- x's. However, many use an apostrophe with acronyms, numbers and letters.