The short answer: VERY. Admittedly, idioms alone are not the single biggest tested topic on the GMAT Sentence Correction. Of course, I would recommend studying all grammatical and syntactical rules, not merely idioms in isolation. Nevertheless, there are rumors swirling around on the web to the effect that the GMAT "no longer emphasizes idioms" or even "no longer tests idioms." Buffalo feathers!! That's pure nonsense!!
Idioms in GMAT Sentence Correction
Here is the breakdown by topics in the Sentence Correction section in the OG 13. For each concept, I list the number of Sentence Correction questions in the Official Guide which test that concept, and then express this number as a percentage of the 140 SC questions in the Official Guide.
Agreement = 28 questions (20%)
Diction = 16 questions (11.4%)
Grammatical Construction = 34 questions (24.3%)
Idiom = 36 questions (25.7%)
Logical Predication = 60 questions (42.9%)
Parallelism = 39 questions (27.9%)
Rhetorical Construction = 46 questions (32.9%)
Verb Form = 27 questions (19.3%)
This is the list of topics given in the "Basic English Grammar Rules", at the beginning of the Sentence Correction section in the OG13: all of these terms are explained there. Notice, also, the percentages do not add up to 100% because multiple concepts are tested on each question.
Idioms are tested on one quarter of all the Sentence Correction questions in the Official Guide. If the OG is any guide, you would expect that, of the 13-14 Sentence Correction questions you see on the GMAT Verbal Section, about 3-4 of them should test idioms. If we interpret 25.7% as a probability, then the chance that, of 14 Sentence Correction questions on the real GMAT, you would not see an idiom question is about 1 in a 100. The odds are overwhelming that you will have to deal with idioms at least once on the GMAT!
Idioms are tested on 36 questions, and Parallelism is tests on 39 ---- only three more. If anyone said that the GMAT doesn't test parallelism on the Sentence Correction, we would be ready to lock that person up on a padded express to the funny farm. Clearly, parallelism is one of the GMAT's very favorite topics! Well, again, if numbers in the Official Guide are any indication, idioms are almost as important as parallelism!
Should I study idioms?
Yes, yes, and yes. Idioms are alive and well on the GMAT Sentence Correction. Here's a SC question on which to practice your knowledge of idioms.
What are some idioms you find tough? Let us know!
This post was written by Mike McGarry, GMAT expert at Magoosh, and originally posted here.