PrepTest 158

“Presupos[ing] the truth” of a conclusion refers to circular reasoning, a flaw in which the conclusion merely restates a premise. That is not the case here, as the premise is about the last two years, and the conclusion is about this year.

Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Errors in Reasoning

Stimulus Summary:
After two years of Browning residents experiencing more rain in September than July, this amateur meteorologist is predicting Browning residents will probably once again experience more rain in September than July.

Strategy Overview:

1. Argument or facts? Always argument, so identify premises and conclusions of argument
2. Anticipate why the premises are not enough to prove that the conclusion is true 
   1. Checking to see if a common logical fallacy is committed is very helpful
3. Use anticipations to select the answer choice that:
   1. Accurately describes the reason why the premises are not enough to prove that the conclusion is true

Answer Anticipation:
On an Errors in Reasoning question, we should take the time to articulate why the premise or premises aren’t enough to prove that the conclusion is true. To help organize our thought process, we can think about the common fallacies that arguments commit over and over again.

When premises bring up past patterns, an experienced LSAT-taker will immediately ask whether the argument commits a temporal fallacy. Specifically, the shift-in-time period version of a temporal fallacy, in which the author uses premises about the past to support a conclusion about the present or future.

And that certainly is the case in this argument. The author takes what happened in the last two years as enough evidence to conclude that things will probably happen the same way this year. But, of course, things can change. Maybe Browning will experience some irregular weather patterns this year (or perhaps the last two years were actually the irregular patterns, and this year will return to the norm). The correct answer may describe this flaw (something like, “uses premises about the past few years to make conclusions about the future”). Or it may describe the assumption the author is making (something like, “takes for granted that the next year will follow a similar pattern of rainfall as the prior two years”). Or it could describe the problem with that assumption (something like, “overlooks that rainfall patterns can change dramatically from year to year”). We should look for one of those.

Note: As is the case for many Errors in Reasoning questions, we could classify this flaw in this argument in multiple ways. If you noticed that this author was only looking back at the two prior years and thought this was a sampling flaw, you would be on point.

Answer Choice Explanation:
This describes the flaw perfectly. The author is drawing an inference about a future event (the amount of rainfall over Browning this year) from premises about a “very limited number of instances of related past events” (two past events: the amount of rainfall over Browning in the last two years). This answer choice accurately describes the temporal flaw discussed above by describing the shift from one time period to another. And, if you thought of this as a Sampling flaw, the characterization of the premises as “very limited” also references that.

Key Takeaway:
Experienced test takers look for clues in the arguments of Errors in Reasoning questions to help them find the flaw committed. Premises about past patterns are generally a clue that the argument is committing a temporal flaw.

Like many incorrect answer choices to Errors in Reasoning questions, this answer choice misconstrues the conclusion drawn in this argument. The author's conclusion was not about “average rainfall statistics” in Browning or anywhere else. That was not a point the author was trying to make, so there’s no way the author could have “overemphasize[d]” how much abnormal rainfall could skew the average.

When an answer choice uses abstract, nonspecific language like this, try to fill in the vague terms with concrete details from the passage. If you’re unable, or find yourself straining, to find something from the passage that aligns with a vague term, then there’s a good chance that the answer choice misdescribes the argument and is wrong.

In this case, what are the “two phenomena” that conclusion claims are “associated"? The conclusion arguably describes Browning rainfall in June and Browning rainfall in September. Still, the author doesn’t claim these phenomena are “associated” — just that there's probably going to be more rainfall in June than September. Moreover, the premises never assert that there are many instances when both these phenomena are present (we’d probably have to bend the space-time continuum to get rain in June and September at the same time). Since we can’t fill in the vague terms from this answer choice with concrete details from the passage, this answer choice misdescribes the argument.

There’s no reason to believe that we can’t verify the evidence used in this argument. As long as someone or something is keeping track of the rainfall in Browning, we can double-check these figures. Moreover, this answer choice gets perilously close to undermining a premise, which is not something a correct answer on an Errors in Reasoning question will do. (Remember, we’re trying to identify why the conclusion may not be true, even if the premises are true; we’re not trying to show why the premises may be false.)