Logical reasoning PrepTest 142 · Section 1 · Question 18

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Errors in Reasoning

Stimulus Summary:
Sample Ð 1 million adults
Results Ð Positive correlation between sleeping more than 8 hours/night and illness
Conclusion Ð Sleep deprivation strengthens the immune system

Answer Anticipation:
This argument starts by establishing a survey was done, and so the two most likely flaws are sampling and correlation/causation.

Here, the sample is 1 million adults. That should be a large/representative sample—at least, there's no reason to think that it isn't. While it does seem that this survey is relying on self-reported data and thus could be flawed, there are no specific red flags raised with the sample, so we shouldn't expect an answer relating to that.

Next, we need to consider a correlation/causation flaw. In concluding that sleep deprivation "probably bolsters" the immune system, the argument is reaching a causal conclusion. And this is based on a correlation—"greater frequency" establishes that. So the argument is jumping from correlation to causation, and the correct answer will almost certainly reflect that.

Answer Explanation:
This answer brings up one of the possible reasons that a correlation/causation jump is flawed—it's possible that there's another factor causing both elements of the correlation. Something could be causing both the bolstered immune system and trouble sleeping.

Key Takeaway:
Language about frequency ("(more) likely", "(greater) frequency") establish correlations. While noting causality in the conclusion is a great way to kick off searching for a correlation/causation flaw, when you start to notice correlation keywords earlier in the argument, you'll know you're on your way to LSAT mastery.

Incorrect. First, the argument doesn't establish that a given factor is causal—it concludes it. Second, the conclusion isn't that sleep deprivation is the only determinant of immune response—it just says that it "bolsters" it.

Incorrect. While the argument does establish some phenomena, it doesn't establish a sufficient/necessary condition relationship, so this answer is out of scope.

Incorrect. First, there should be an observable correlation between two phenomena if there's a causal relationship between them—if two things aren't correlated, it's hard to see how they'd be causally related. Second, the way this answer is phrased is the reverse of the jump—"If there's causality, then there's a correlation" vs. this argument's "if there's correlation, then there's causality."

Incorrect. This is a very tempting answer, as it could be rephrased as, "fails to consider that even if sleep deprivation doesn't cause illness, it may still have other negative consequences" and so saying that it's not unhealthy is wrong. However, those two terms are directly related—if you're unhealthy, then you must have an illness, and if you're ill, then you're unhealthy. Sleep deprivation may have other negative consequences that aren't related to health, but those are out of scope of the conclusion. So either way, this answer choice is incorrect.