CR# 81

A recent report determined that although only 3 percent of drivers on Maryland highways equipped their vehicles with radar detectors, 33 percent of all vehicles ticketed for exceeding the speed limit were equipped with them. Clearly, drivers who equip their vehicles with radar detectors are more likely to exceed the speed limit regularly than are driver who do not.

The conclusion drawn above depends on which of the following assumptions:

(A) Drivers who equip their vehicles with radar detectors are less likely to be ticketed for exceeding the speed limit than are drivers who do not.
(B) Drivers who are ticketed for exceeding the speed limit are more likely to exceed the speed limit regularly than are drivers who are not ticketed.
(C) The number of vehicles that were ticketed for exceeding the speed limit was greater than number of vehicles that were equipped with radar detectors.
(D) Many of the vehicles that were ticketed for exceeding the speed limit were ticketed more than once in the time period covered by the report.
(E) Drivers on Maryland highways exceeded the speed limit more often than did drivers on other state highways not covered in the report.

I was split between choice A and B. I diagrammed right in less than 45 sec but ended by choosing the incorrect answer B. Can you focus on the these two answer choices and explain how we could have eliminated the incorrect answer using “boundary words” or “out of scope” techniques.

Source: The official Guide for GMAT Review 11th Edition

Thank you,

An assumption that supports a conclusion, though unstated, is NECESSARILY TRUE. In other words, the conclusion must directly depend on the information from the assumption. In this argument, the conclusion states that drivers who equip their vehicles with radar detectors are more likely to exceed the speed limit regularly than are driver who do not. The only factual basis (i.e. premise) for this conclusion is that 33% of vehicles ticketed for exceeding the speed limit were equipped with radar detectors, while only 3% of drivers on Maryland highways equip their vehicles with radar detectors.

The only information provided is about the percentage of vehicles equipped with radar and the percentage of ticketed vehicles equipped with radar; the conclusion, however, makes a sweeping generalization about the regular driving behavior of individuals with radar-equipped vehicles. When the GMAT creates such a "jump" or "gap" in the logic of an argument, the assumption will often "fill in" the missing information.

In this case, it seems that you did choose the correct answer (B). In order for the conclusion to be valid, it must be true that these ticketed drivers exceed the speed limit on a regular basis.

Dan,

Absolutely love the class recording..

I have spent a lot of time trying to understand this logic, but I am simply unable to grasp this. I think my struggle is that the answer choice (B) has no information about radar detector. How are we connecting the dots? Can you give a analogous example? Is there one in OG problem set?

A missing assumption fills in a 'hole', anywhere in an argument. In this case, the 'hole' was in the portion of the argument that doesn't deal with radar detectors at all.

The passage equates being ticketed for speeding with speeding regularly. Read through the text again and see how the passage makes this jump - which isn't justified without the assumption stated in (b).

An as analogy: 'The number of reported laptop thefts on campus increased by 10% from last year to this year. Therefore, more laptops have been stolen from campus this year than last year.'
The assumption here is that REPORTED laptop thefts are meaningfully correlated with ACTUAL laptop thefts. Without this association, the logic falls apart. The same is true for the tickets issued vs. regular speeding in this problem.