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Difficulty: MediumPercentages vs. Numbers Flaws

Energy Analyst: In the year 2010, coal power plants generated 40 percent of the electricity consumed in our country. Last year, coal power plants generated only 25 percent of the electricity consumed. It is clear, then, that the country's coal power plants generated a smaller absolute amount of electricity last year than they did in 2010.

Which one of the following most accurately describes a flaw in the energy analyst's reasoning?

  1. A
    fails to distinguish between the final conclusion regarding absolute electricity production and an intermediate conclusion used to support it.
  2. B
    draws a sweeping conclusion about all power plants based on a trend that applies to only some power plants.
  3. C
    depends on the assumption that no new alternative energy sources were introduced between 2010 and last year.
  4. treats a decrease in a specific energy source's percentage of total electricity as sufficient to conclude that the absolute amount of electricity generated by that source has decreased.Answer
  5. E
    presumes that a reduction in the proportional share of coal power caused a decrease in the country's total electricity consumption.

Answer

The correct answer is the option stating that the argument treats a decrease in a specific energy source's percentage of total electricity as sufficient to conclude that the absolute amount of electricity generated by that source has decreased.
The argument concludes that the absolute amount of electricity generated by coal decreased, basing this claim entirely on the fact that coal's percentage of the total electricity consumption decreased. This is a classic 'Percentages vs. Numbers' flaw. To determine an absolute number from a percentage, one must know the total size of the whole. If the country's total electricity consumption increased substantially between 2010 and last year, the absolute amount generated by coal could have actually increased, even though its proportional share of the total decreased.

Here is a hypothetical scenario demonstrating the flaw:

YearTotal Electricity ConsumedCoal's PercentageAbsolute Coal Electricity
2010100 units40%40 units
Last Year200 units25%50 units (An increase!)
The author completely overlooks this possibility, erroneously assuming the total remained static.

Step-by-Step Solution

1
Identify the argument's premises.
Premise 1: In 2010, coal generated 40 percent of the total electricity. Premise 2: Last year, coal generated only 25 percent of the total electricity.
Deconstructing the argument is the critical first step in identifying structural flaws.
2
Identify the argument's conclusion.
The absolute amount of electricity generated by coal was smaller last year than it was in 2010.
Understanding exactly what the author is trying to prove is necessary to evaluate the logic.
3
Compare the nature of the evidence to the nature of the conclusion.
The evidence consists purely of percentages (proportions of a whole), while the conclusion is a definitive claim about an absolute numerical value (amount of electricity).
A mismatch in the type of information presented in the premises versus the conclusion usually points directly to the logical flaw.
4
Test the validity of the inference using hypothetical numbers.
If the total electricity consumed in 2010 was 100 units, coal generated 40 units (40%). If the total electricity consumed last year was 200 units, coal generated 50 units (25%).
Using concrete numbers proves that a percentage can decrease while the absolute number simultaneously increases, provided the total size of the group grows.
5
Select the option that matches this flaw.
The correct option correctly identifies that the author improperly infers a decrease in absolute amount based solely on a decrease in percentage.
This is a textbook 'Percentages vs. Numbers' flaw, a common pattern on the LSAT.

Key Concept

Percentages vs. Numbers Flaws
Estimated Time:1m 30s
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Topics

Percentages vs. Numbers Flaws

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