Most people who shop for groceries no more than three times a month

Question

Most people who shop for groceries no more than three times a month buy prepared frozen dinners regularly. In Hallstown most people shop for groceries no more than three times a month. Therefore, in Hallstown most people buy prepared frozen dinners regularly.

Which one of the following arguments has a flawed pattern of reasoning most like the flawed reasoning in the argument above?

(A) It is clear that most drivers in West Ansland are safe drivers since there are very few driving accidents in West Ansland and most accidents there are not serious.

(B) It is clear that John cannot drive, since he does not own a car and no one in his family who does not own a car can drive.

(C) It is clear that Fernando's friends usually drive to school, since all of his friends can drive and all of his friends go to school.

(D) It is clear that most people in Highland County drive sedans, since most people who commute to work drive sedans and most people in Highland County commute to work.

(E) It is clear that most of Janine's friends are good drivers, since she accepts rides only from good drivers and she accepts rides from most of her friends

GMATWhizTeam · Answer

ANALYSIS OF STIMULUS

•  General Principle : Majority of people shopping for groceries ≤3 times a month- --> buy prepared frozen dinners regularly
• In Hallstown, the first part of the principle is observed (Majority of people shop for groceries ≤3 times a month)

•  Conclusion:  In Hallstown majority of the people buy prepared frozen dinners regularly.

PRE-THINKING

1. The author’s conclusion is based only on the observation that in Hallstown the first half of the general principle is being observed- majority of people shop for groceries ≤3 times a month)

2. The author clearly fills in the rest of the gap.

3. The author assumes two things here which contribute to the flawed reasoning:
  Assumption#1 : The general principle is applicable in Hallstown as is.
  Assumption#2 : The first half of the principle being observed in Hallstown is  sufficient evidence to conclude that the second half of the principle is also taking place

4. So, in the correct answer choice we should look for the following  framework :
• A general principle
• Evidence that first half of the general principle is observed
• Conclusion: the second half of the principle is also observed.

CORRECT ANSWER CHOICE D

•  A general principle : most people who commute to work- -->  drive sedans
•  Evidence that first half of the general principle is observed : most people in Highland County commute to work
•  Conclusion- the second half of the principle is also observed : most people in Highland County drive sedans

None of the other choices fit in the framework for the parallel reasoning

Fdambro294 · Answer

The key to spotting the flawed reasoning is spotting the ever so subtle shift in words the author makes in the conclusion.

The argument appears to follow what is termed the “Repeat Form” in inductive reasoning.

If X ——-> then Y

So and so are X ——-> then so and so must be Y.

But this is not exactly what is said in the passage.

If you are part of “most people who shop for groceries no more than three times a month” ———> then you and the “most people who shop…” buy frozen dinners.

In Hallston, it is true that most people shop for groceries no more than three times for month. Then based on the premise, it could be inferred that THESE people who shop for groceries no more than three times per month will buy prepared frozen dinners regularly.

However, the conclusion made is that out of ALL the people in Hallston (including those who shop for groceries no more than 3 times per month AND those who shop MORE than 3 times per month), most will buy prepared frozen dinners regularly.

The flaw made is that the conclusion should only about the subset of people: those people in Hallston who shop for groceries no more than 3 times per month.

The conclusion should not be about ALL the people in Hallston.

D makes this same error.

We have a subset of people (people who commute to work). This subset of people drives sedans.

Then the inference is applied to ALL the people in Highland County, when it should be applied to only those people who commute to work in Highland County.

I guess it could be raised to see with numbers.

Say most people who shop 3 or fewer times per month (51% of 100 people) ——-> all 51 by prepared frozen dinners.

It’s then not necessarily true that because most people in Hallston shop 3 or fewer times per month, that most of the population in Hallston buy prepared frozen dinners

Out of the 1,000 people in Hallston, most (51%) shop 3 or fewer times per month.

510 people.

So, out of these 510 people who shop 3 or fewer times per month ——-> most buy prepared frozen dinners (51% of 510 = around 260)

However, we can then not say that MOST of the entire population of Hallston buys prepared frozen dinners (we have only 260 out of 1,000)

Posted from my mobile device

hrushij00 · Answer

Can I get a detailed breakdown on why D and E are different? I understand why D is the right answer but E seems to follow the same logic

guddo · Answer

Most people who do A also do B.
 In Hallstown, most people do A.
 Therefore, in Hallstown, most people do B.

This is flawed because “most” plus “most” does not guarantee “most.” Even if most Hallstown residents are in the A group, and most of the A group also does B, the overlap can still be less than half of Hallstown.

Example idea in one line: if 60% of Hallstown shops no more than three times a month, and 60% of those people buy frozen dinners, then only 36% of Hallstown buys frozen dinners, not “most.”

Choice (D) matches this exact pattern:
Most people who commute to work drive sedans.
 Most people in Highland County commute to work.
 Therefore, most people in Highland County drive sedans.
Same flawed “most of a subgroup” to “most of the whole group” jump.

Why the others are not the same pattern:
 
(A) Uses accident frequency to infer driver safety. Different kind of reasoning.
 (B) Uses a universal rule: if you do not own a car, you cannot drive. That is not a “most” overlap flaw.
 (C) “Can drive” plus “go to school” does not imply “drive to school.” Different flaw.
 (E) If she accepts rides only from good drivers, then anyone she accepts a ride from is a good driver. Since she accepts rides from most of her friends, most of her friends must be good drivers. Not the same flaw.

Answer: D

guddo · Answer

D is flawed because it does this:
Most commuters drive sedans.
 Most people are commuters.
 So most people drive sedans.

That does not follow. “Most of a subgroup” does not guarantee “most of the whole group.”

E ( most of Janine's friends are good drivers ) is different because it has a rule:

She accepts rides only from good drivers.
 So everyone she rides with is automatically a good driver.

And she rides with most of her friends.
 So most of her friends are in that “she rides with them” group, and that whole group is good drivers.

So E is valid subset logic.
D is an invalid “most plus most” jump.

Most people who shop for groceries no more than three times a month

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Most people who shop for groceries no more than three times a month

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