One of the toughest grammar rules for many GMAT students to spot is the comparison error. This can be especially difficult for students because many of the errors that pop up are related to context rather than grammatical structure.
If you come across a comparison in a sentence correction problem, you should look for two possible problems.
Look for parallel form
First, you should make sure the items being compared are in parallel form. A comparison is treated grammatically as a list of two items. Therefore, just as you would in a list, you must make sure that the items in the comparison are formatted in the exact same way.
Look for logical similarity
Second, and this is where it can get tricky, items being compared must be logically similar. Consider the sentence, “unlike the hearing of a dog, humans cannot hear very high pitched tones.” In order to determine what the sentence is comparing, you must identify the comparison word. In this case it is “unlike.” You must then ask yourself, “what items are unlike?” Here, it is “the hearing of a dog” and “humans.” Next, ask, “ is hearing similar to humans?” The answer is no. This sentence intends to compare the hearing of dogs to the hearing of humans. To be correct it must compare hearing to hearing or dogs to humans. Thus, either “unlike the hearing of dogs, the hearing of humans cannot detect very high pitched tones” or “unlike dogs, humans cannot hear high pitched tones” are correct on the GMAT.
By watching out for both of these comparison errors, you should be able to spot any comparison problems on test day.