You know...in terms of pure probability it's probably not going to matter either way, whether you go completely random or you just pick B every time you have to guess. The reason you've probably heard that is that, if you see the test as random, then the answer should be B 1/5 of the time. People who then decide that they'll pick the same letter every time they guess are doing so because they're worried about "missing" their guesses each time. But in terms of pure probability it won't make a difference.
What we'd advise:
1) Work on pacing so that you minimize the amount you have to guess. Hopefully you're only *random* guessing 2 times or less per section. And then...you won't ever have to guess that many times in a row, so the above "strategy" is moot.
2) When you've at least had a second to read the question, make an educated guess. You can get some odds back by doing things like:
-If the question asks for a smaller/larger number (for example "a rope is cut in half...what is the length of the shorter piece?") try picking an answer toward that end. The "other" answer is likely to be a trap, so if the question asks for the shorter/younger variable, it's really unlikely that the biggest number will be correct.
-If it's Data Sufficiency, you can often eliminate two answers by checking the "easier" statement within a few seconds. Even if the full question is difficult, often one of the two statements isn't all that bad.
-With Sentence Correction, you can often find one glaring mistake in 1-2 answer choices and cut those down quickly before the true investment of time would have to come in. Or you can play the odds on some of the GMAT "tricks" (they're not always true, but the word "being" is usually wrong, and if any answer choices have semicolons in them, the correct answer probably requires a semicolon).
-With Critical Reasoning "Inference" and "Assumption" questions, words like "always", "never", "all", etc. are usually wrong.
Now...the above list isn't by any means an actual strategy list. The GMAT breaks those tendencies frequently so if you have time to "do" the question, please, please do it. But if you only have 10-15 seconds to register a guess, you can play the odds a little on "this is usually true", and that will give you a better probability of correct answers than you'd get by picking a random strategy.
One other thought on this - it's far better to guess on and miss a question that you would have gotten wrong anyway than one that you should have gotten right. We all miss questions on the GMAT - the fact that it's an adaptive test means that we'll all come face to face with our maximum threshold of question difficulty. So another good guessing strategy is to guess mid-test when you see a question that might as well be written in a foreign language. Even if you spent 5 minutes on that one, you'd probably get it wrong anyway, so while a random guess in 20 seconds will probably result in a wrong answer, at least you're getting that wrong answer with time to spend on future questions that you should get right. So if you know that your pacing is likely to mean that you'll need to guess 3-4 times, try to use them when you see near-impossible questions. The way that the GMAT scoring algorithm works, you get punished a good amount for missing questions below your ability level, so if you know you'll have to guess at some point, guess when it's a question that you probably won't get right anyway.
Brian Galvin
GMAT Instructor
Chief Academic Officer
Veritas Prep
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