As you probably know by now, the GMAT is a Computer Adaptive Test ("CAT"). This means that the questions that you see on the exam are selected by the computer based on your performance on earlier questions. For example, if you answer a question correctly, your next question will be harder. If you answer a question incorrectly, your next question will be easier. The exam is trying to gauge your ability level by seeing how well you do with questions (known as "items" in testing parlance) of varying degrees of difficulty. Generally speaking, the harder the questions you answer correctly, the better your score will be.
There are other factors besides difficulty level that influence the selection of items on a particular exam (e.g., question type (data sufficiency vs. problem solving, for example), content (e.g., algebra, ratios, assumptions, etc.), and exposure (i.e., how many times has the question been seen by other test takers already that month?)). But difficulty level is arguably the most important.
The CAT does not "bucket" items into "easy", "medium", and "hard" categories. Instead, each item can be considered easy, medium, or hard depending on the person to whom it is given. Each item is tested out for a period as an unscored "experimental" during the actual exams of people taking the GMAT. After a sufficient sampling of test-takers has answered the items, ETS compares the overall scores of the test-takers with their performance on the experimental items.
If, say, fifty percent of all test-takers scoring in the 600-620 range got a particular experimental item right, that item would be considered of medium difficulty for that ability level. If ninety percent of those scoring in the 700-720 range got the item right, it would be considered easy for that ability level. When the item is then presented as a real scored question on subsequent exams, the computer uses the experimental data to determine whether the item is appropriately difficult for someone performing at a given level thus far in the exam. The computer tries to give you questions that you have a 50/50 shot at, based on your performance up to that point. The better you do, the harder your 50/50 items will be.
Each item has an "item characteristic curve" that graphs the likelihood of answering that item correctly, based on the experimental data. The curve looks like this:
Over the next few weeks, we will discuss in detail the various aspects of the curve, to give you a better sense of how your GMAT exam adapts to your performance level.
