The GMAC is constantly mixing new questions into the test pool and eliminating old ones. I don't know what the lifespan of a typical test item is, but I'd be surprised if it's more than a year or so.
That doesn't mean that the test changes very much. The specific problems may differ, but the format, the content, and the underlying patterns all stay the same.
Hints From the GMAC
As one piece of evidence, consider the Official Guide. The current, 11th, edition was published in September of 2005. A new edition is due soon, but the GMAC doesn't find it necessary to alter their study materials more than once every few years.
As a bit of further, more powerful, evidence, perhaps you've seen the 10th edition of the Official Guide. A large number of those questions reappear in the supplemental Official Guide books, and they still accurately represent what's on the test. (I don't recommend using the 10th edition, but it doesn't make a huge difference.)
From my own perspective, I could've written most of what is in The GMAT Math Bible five or ten years ago. There are some things I wouldn't have focused on as much, or others I might have highlighted more, but the content I covered would've been nearly identical.
In other words, if you prepared for the GMAT using only those materials that were available in 2000, you wouldn't sacrifice very much test-readiness in September 2008. You might not even notice the difference.
The GMAT Has Changed...A Little
That isn't to say the test is identical. It isn't. One noticeable difference is the difficulty level. The test is scored on a curve, and admissions keep getting more competitive, so every year, it's a little harder to get your target score.
That doesn't mean the content has changed much, though. I started working with GMAT students in 2000, and the topics that people worried about back then (combinations, probability, standard deviation, number properties) are the same one that cause concerns today.
The main difference is the increased pool of difficult questions. If the average test taker is getting stronger, the GMAT must have more items at the high end to distinguish between 730, 740, and 750 scorers. That, I think they've done.
What the Changing GMAT Means To You
It may come as no suprise that the GMAT is getting harder. There's not much you can do about that except to study harder. But apart from a few question types I highlighted in a previous article, you shouldn't alter your preparation strategy as if the GMAT is a different test every month.
If anything, you can use the predictability of the exam to your advantage. When you see a question that looks completely unfamiliar, you can rest assured (at least if you are well prepared) that it relates to something you already know.
The GMAT is a standardized test, and that means you don't have to worry about sudden shifts in content or question types. The stakes may be high, but so is the exam's consistency.
