The difference in average annual income in (OG18) help

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The difference in average annual income in favor of employees who have college degrees, compared with those who do not have such degrees, doubled between 1980 and 1990. Some analysts have hypothesized that increased competition between employers for employees with college degrees drove up income for such employees.

Which of the following, if true, most seriously undermines the explanation described above?


A) During the 1980s a growing percentage of college graduates, unable to find jobs requiring a college degree, took unskilled jobs.
B) The average age of all employees increased slightly during the 1980s.
C) The unemployment rate changed very little throughout the 1980s.
D) From 1980 to 1990 the difference in average income between employees with advanced degrees and those with bachelor's degrees also increased.
E) During the 1980s there were some employees with no college degree who earned incomes comparable to the top incomes earned by employees with a college degree.

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by ceilidh.erickson » Fri Nov 02, 2018 1:08 pm
If we want to WEAKEN an argument, we must first find the logical flaws - the missing information - between the premises and the conclusion.

Premises:
The difference in average annual income in favor of employees who have college degrees, compared with those who do not have such degrees, doubled between 1980 and 1990.

Conclusion:
Increased competition between employers for employees with college degrees drove up income for such employees.

Logical gap:
1. There is a mathematical disconnect here! Just because the DIFFERENCE between two income groups increased, that does not mean that the ACTUAL amounts both increased. What if in 1980, non-college-graduates made $50k on average and college graduates made $60k. Then in 1990, non-college-graduate incomes dropped to $40k. The DIFFERENCE would have increased, but one amount dropped and the other remained unchanged. In order to conclude that "increased difference between non-college-graduates & college graduates = higher income for college graduates," we'd need to know that non-college-graduate incomes did not go down.
2. We are also concluding (without justification) that the reason these incomes went up is competition between employers. The logical gap here is: we're ignoring any other possible explanation. Even if college graduate incomes went up, do we know WHY?

In order to WEAKEN this hypothesis, we need to attack one of these logical gaps.

A) During the 1980s a growing percentage of college graduates, unable to find jobs requiring a college degree, took unskilled jobs.
This undermines the notion that employers are generally competing for college graduates by paying them more and more. We can't necessarily assume that "unskilled" means "lower paying," so we can't draw conclusions about changed in actual income amounts. We know that the difference between income averages changed, but this gives us reason to think that the explanation is NOT that employers in general are wanting to pay college graduates more and more.

B) The average age of all employees increased slightly during the 1980s.
We have no reason to believe that age has any bearing on income. We're only interested in comparing college graduates to non college graduates.

C) The unemployment rate changed very little throughout the 1980s.
This gives us no additional information to disrupt the connect between "income gap widened = employers competing for college grads."

D) From 1980 to 1990 the difference in average income between employees with advanced degrees and those with bachelor's degrees also increased.
Irrelevant. We only care about those who had college degrees (a group that includes advances degrees) and those who do not.

E) During the 1980s there were some employees with no college degree who earned incomes comparable to the top incomes earned by employees with a college degree.
A few outliers don't reverse a trend. Any time we're talking about AVERAGES within a population, there may well be outliers who behave differently. This wouldn't alter our conclusion about general trends for most people. (If our conclusion had said "all college grads are better off than all non-college-grads," that would be different).

The answer is A.
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education

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by ceilidh.erickson » Fri Nov 02, 2018 1:30 pm
This type of logical flaw is very common on GMAT CR. I like to call it "Mixed Metrics" - conflating one mathematical concept (difference in incomes increased) with another (actual incomes increased). Any time you see one statistic / metric / mathematical concept swapped in for another, ask yourself if there's a discrepancy between the two.

Here are several other examples along the same lines:

https://www.beatthegmat.com/the-proport ... tml#820184
https://www.beatthegmat.com/critical-re ... tml#725229
https://www.beatthegmat.com/lyme-diseas ... tml#714758
https://www.beatthegmat.com/passenger-j ... tml#743749
https://www.beatthegmat.com/a-cr-less-a ... tml#776214
https://www.beatthegmat.com/statistics- ... tml#564609
https://www.beatthegmat.com/total-books ... tml#680834
https://www.beatthegmat.com/cr-evaluate ... tml#558393
https://www.beatthegmat.com/i-m-doubtin ... tml#551227
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education