"as ... as" expression

Hi,

I have difficulty figuring out how to write the word that follows something like an  'as much/twice as something' or 'twice as something' expression -- should it be in a 'plain' form or in the possessive? I feel some people are unable to correctly the right situations for either usage.

This year's figure for sales is twice the number as last year (or last year's)

Instinctively, I feel it should be 'last year's' because I think we are talking about numbers -- last year's number and this year's number  that is; but I feel in a lot of cases, a sentence without the apostrophe is accepted. Why? How about this?

The year's membership figure is as many as last year (or last year's), maybe more.    

Your instinct is right--the phrase refers to "last year's figure" but the word "figure" is understood and so dropped.  So grammatically the spelling should be year's. However, it has become common especially in conversationto drop the 's, and that has slipped into common usage in many cases.  It's common for this to happen, as a language is a constantly changing thing.  But careful writers will still, I think, make that a possessive form.  A similar thing has happened with a sentence like, "James is taller than I."  The pronoun "I" is correct because the entire sentence would be "James is taller than I am," but when the understood verb "am" is dropped, many speakers will say, "James is taller than me."  That construction, too, has slipped into common usage in writing for many people.  In fact, those who insist on the grammatically correct form are thought of as old-fashioned, perhaps, by many.