Complete Question Explanation
Parallel Flaw, SN. The correct answer choice is (C)
The stimulus features conditional reasoning, which is identifiable by the use of the sufficient condition indicator “if” in the first sentence. The conditional relationship can be diagrammed as follows:
- FBP = First child born prematurely
SBP = Second child born prematurely
The first clause of the second sentence indicates that the necessary condition has not occurred, as Jackie’s second child was not born before its due date:
On the basis of this information, the author concludes that it is likely that the sufficient condition did not occur, i.e. that Jackie’s first child was probably not born before its due date:
While the conclusion appears to be based on the contrapositive of the conditional relationship, there is a peculiarity in the language that reveals the flaw in the argument, and that you can use to your advantage when attacking the answer choices: the phrase “likely” appears just before the necessary condition in the first sentence, and when this event has been found not to occur, the author uses the same phrase (“likely”) to qualify the conclusion.
The entire form of the argument, then, appears as follows:
- Premise: FBP SBP (likely)
Premise/Conclusion: SBPJackie FBPJackie (likely)
Many students, when examining this argument, do not realize at first that there is a flaw present. The structure appears to be that of a contrapositive, and they simply accept that the argument is sound and move on. But, the question stem clearly notes that the argument is “questionable,” so let us take a moment and see where the error occurs. As mentioned before, the use of “likely” is at the heart of the problem, and in the argument the author does not make a perfect contrapositive but rather shifts the word “likely” from the from one necessary condition to the other necessary condition (as opposed to the sufficient condition, where the “likely” should have moved during the contrapositive). Here’s how the argument would have appeared if the argument was sound
- Premise: FBP SBP (likely)
Premise/Conclusion: SBPJackie (likely) FBPJackie
Note how the “likely” should have moved from the necessary to the sufficient, but in the actual argument the author moved it from the necessary condition to the necessary condition (which was the conclusion).
Because your job is to parallel the argument, you must parallel the conclusion as well as the premises, making sure to “match” the certainty level of all subcomponents in the argument. The correct answer choice should contain a seeming-contrapositive argument form, and also have the same probabilistic language (“likely” or a similar term) modifying both the necessary condition and the conclusion. This knowledge allows you to quickly eliminate answer choices (D) and (E) from consideration, and focus your analysis of the conditional relationships in answer choices (A), (B), and (C).
Answer choice (A): Although this answer choice uses the same probabilistic language as the stimulus (“probably” is synonymous with “likely”), it is incorrect because the argument is in the form of Mistaken Negation:
- Premise: Finish Project Go to Fair (probably)
Premise/Conclusion: Finish ProjectBen Go to FairBen(probably)
Note that this answer choice uses the sufficient condition indicator (“people who”) instead of “if.” This difference is inconsequential and should not be used as grounds for eliminating the answer.
Answer choice (B): Although this answer choice also uses the same probabilistic language as the stimulus, it is incorrect because the argument is in the form of Mistaken Reversal:
- Premise: Resp. Owner Good with Kids (probably)
Premise/Conclusion: Good with Kids Resp. Owner (probably)
As with answer choice (A), the difference between the sufficient condition indicator used in the stimulus (“if”) and the answer choice (“all”) is irrelevant, since they both modify the same type of condition.
Answer choice (C): This is the correct answer choice
. The argument is in a contrapositive form and therefore matches the logical validity of the stimulus. It also uses the identical term “likely” in the necessary condition and a synonymous term “probably” in the conclusion:
- Premise: Movie a Hit Sequel a Hit (likely)
Premise/Conclusion: Sequel a HitHawkII Movie a HitHawkI (probably)
Also, note that this reasoning matches precisely the abstract nature of the argument in the stimulus: if the first occurrence in a series of events has a certain characteristic, the second occurrence in that series is likely to have the same characteristic. Therefore, if the second occurrence did not have that characteristic, it is likely that the first one did not either. You should not be concerned with the fact that the characteristic in the stimulus was a negative one (premature birth), whereas the characteristic in answer choice (C) is positive (box-office hit). Remember—the topic in Parallel Reasoning questions does not matter, as your only job is to identify an argument that has a similar pattern of reasoning.
Answer choice (D): This argument is valid, but it is not in a quasi-contrapositive form. In fact, the conclusion is a Repeat of the premise:
- Premise: Fail (likely) Invest
Premise/Conclusion: FailPS (likely) InvestPS
Also note that the word “likely” here qualifies the sufficient condition of relationship, not the necessary condition as in the stimulus. Furthermore, the conclusion does not feature the same probabilistic language. A savvy test taker who notices this difference may choose not to even diagram this answer choice, and automatically eliminate it from consideration.
Answer choice (E): This answer choice is incorrect because the argumentation is in the form of Mistaken Reversal:
- Premise: SailingTai Nice Weather
Premise/Conclusion: Nice Weather SailingTai (probably)
As with answer choices (A) and (B), the difference between the indicators used in the stimulus and the answer choice is irrelevant. In this case, the phrase “only if” indicates that having a nice weather is a necessary condition for sailing; the fact that the same indicator was not used in the stimulus does not matter.
What matters, is that the necessary condition in the premise does not feature the same probabilistic language we have come to expect. This difference, although minor, is sufficient to eliminate this answer from contention, the Mistaken Reversal error notwithstanding. Remember, all relevant elements of the argument must be paralleled, not just some of the elements. This can save you time, as you need not diagram every single answer choice in order to arrive at the correct answer.