The term ellipsis, or elliptical construction, describes the omission of words in order to make a sentence more concise. Ellipses are used frequently with comparisons. An ellipsis may eliminate a subsequent occurrence of a word or words stated previously in a sentence only when the word or words are exactly the same each time they appear.
No ellipsis: During the 17th century, Britain experienced some types of political turmoil, and France, Spain, and Germany experienced others.
Correct ellipsis: During the 17th century, Britain experienced some types of political turmoil, and France, Spain, and Germany others.
The ellipsis omits experienced, which appears in this exact form earlier in the sentence. This ellipsis is correct.
Incorrect ellipsis: I walk as fast now as [I walked] when I was young.
This is not correct because “I walk” and “I walked” are not identical.
Correct ellipsis: I walk as fast now as I did [walk] when I was young.
In this case, “did” indicates the tense shift and stands in for “did walk.”
Incorrect ellipsis: I walk faster than Brian [walks].
Although we say this all the time in everyday English, this sentence is technically not correct. “Walk” and “walks” are not identical, so “walks” should not be omitted. Remember that a word or phrase can be omitted only if it exactly repeats a previous word or phrase.
Correct ellipsis: I walk faster than Brian does [walk].
In GMAT-land, the switch in verb tenses is not OK when you’re omitting verbs, but the switch in a comparison (I walk faster than Brian [walks] ) is sometimes acceptable if it is in the best option and it’s clear what the two things compared are.
On the GMAT, your job is to recognize when an ellipsis is not used correctly. An ellipsis can never be used when parallel construction is an issue, as it is in correlative conjunctions. For example, “I like to swim neither in pools nor lakes” is not correct. The item after “neither” starts with “in,” so the item after “nor” must also start with “in.” This is not a correct elliptical construction. If an ellipsis makes the meaning of the sentence unclear, then it is not correct. Take a look this sample question.
To develop more accurate population forecasts, demographers have to know a great deal more than now about the social and economic determinants of fertility.
(A) have to know a great deal more than now about the social and economic
(B) have to know a great deal more than they do now about the social and economical
(C) would have to know a great deal more than they do now about the social and economical
(D) would have to know a great deal more than they do now about the social and economic
(E) would have to know a great deal more than now about the social and economical
First, it’s important to recognize that “have to know” and “would have to know” would both be correct here. They have different meanings of course, but each meaning could work. The “have to know” and “would have to know” split is pretty much just a diversion from the real errors in the sentence. We can immediately eliminate choices B and C; the word “economical” is not correct because its meaning is not logical in this context. We need the word “economic” to describe the “determinants of fertility.”
Next, we can eliminate choices A and E because they both contain ambiguous comparisons. Omitting “they do” is an incorrect ellipsis here because it changes the meaning of the comparison. The comparisons in A and E seem to compare how much demographers (would) have to know, an amount of knowledge, with the present time period (“now”).
D is the correct answer because it repeats “they do” to make the comparison clear.