You'll read a lot about guessing strategies in various prep books - some prep companies seem to focus more on guessing strategies than on actual content. I've done a lot of research into the effectiveness of some of the guessing strategies I've seen in print, and I've come to the conclusion that if you don't understand a question, there is no way, based just on the structure of the answer choices, to guess any better than randomly. To list some of the guessing 'strategies' I've seen in print:
* in Quant PS, I've seen many books suggest you try to identify 'twin' answer choices, and guess one of those. For example, in a question like:
At an academic conference, the ratio of the number of men to the number of women was 2 to 3. What percentage of people attending the conference were men?
the reasoning is that the answer choices will include not only 40%, the correct answer, but also the 'opposite' of the correct answer, 60%, which represents the percentage who are women. Thus if there are only two 'twin' answer choices, guessing one of those increases the likelihood of guessing correctly from 1/5 to 1/2, or so these prep books say.
Well I looked through every PS question in both official guides to identify questions with 'twin' answer choices, and worked out how many right answers you'd expect to get if you always guessed a 'twin', and how many right answers you'd expect to get if you simply guessed randomly. It turned out that you'd actually expect to get slightly *more* correct answers by guessing randomly than by guessing a 'twin', though the difference was so small that it was statistically insignificant. In any case, if you're spending time looking for 'twin' answers, you're just wasting your time.
* In Quant PS questions which begin "which of the following..." some books suggest that E is more likely to be correct than any other choice, so you should guess E, the reasoning being that the question designers want you to do the maximum possible amount of work, so they want to force test takers working from A through E to work through as many answer choices as possible. I looked at every official 'which of the following' question I could find (there aren't that many, so I was looking at 31 questions in total), and A or B were correct 10 times, while D or E were correct 12 times - early answer choices were correct essentially just as often as later answer choices. So this kind of 'strategy' is pretty much pointless.
* In Sentence Correction, I've seen some people recommend guessing the shortest answer choice if you need to guess randomly, the reason being that good writing economizes on language, so wordy answer choices are normally wrong. A while ago I looked at every official SC question in the Official Guide to see how often each answer was correct based on the length of the answer choice. I don't have that data in front of me, but if you go only by the lengths of each answer choice, there is only a minuscule advantage to choosing short answers over long ones - if I recall correctly, a shorter answer choice had about a 23% chance of being right, and a longer one about a 17% chance of being right (20% of course being the chance a random answer is right). The result wasn't statistically significant either, so it may just have been coincidence. In any case, if guessing short answers is helpful on GMAT SC, it's only helpful by a negligible amount.
Those are just three examples of guessing 'strategies' I've read about, and which may sound plausible in theory, but which are almost completely useless in practice. The strategies I know of which *are* useful all require some understanding of the content of the question. Estimation, for example, can be helpful in many types of Quant problems. Number Theory can also often be used to eliminate answer choices. Often some answers are completely illogical and can be eliminated immediately (e.g. if a question asks for the remainder when you divide something by 4, the answer can only be 0, 1, 2 or 3, so you can eliminate any other answer choice).
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com
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These are things you should learn using your own experiences. That is the best way to apply an educated guess.
