Percentages vs. numbers
A bigger slice of a smaller pie can be less pie.
Fallacy vs. flaw, on this site. No fallacy page covers this; it is a quantitative reasoning error the LSAT tests directly. In Flaw questions the right answer usually says the argument "confuses a percentage with an absolute number" or "fails to consider the size of the group." Do the arithmetic the argument skipped.
This one has no fallacy page. It is an LSAT-specific pattern, which is exactly why it is worth learning.
What it is
This flaw trades on the gap between percentages and absolute numbers. A percentage only means something relative to its base. Change the base and the same percentage means something completely different.
The classic move: "the percentage rose, so the number rose." Not necessarily. If the total shrank, a bigger percentage can be fewer actual cases.
An LSAT-style example
This example is original. It is not a real LSAT question.
The percentage of voters supporting the measure rose from 40 percent to 60 percent, so far more voters support it now.
The flaw: If turnout collapsed, 60 percent of a small electorate can be fewer voters than 40 percent of a large one. The argument never gives the totals, so the conclusion about "far more voters" is unsupported.
How to spot it on test day
- Whenever you see a percent in the evidence and a number-word in the conclusion ("more," "fewer," "most"), slow down.
- Ask: what is the base? Did the base change? The argument almost never tells you.
- Averages and rates work the same way: always ask "out of what?"
The right answer vs. the trap
The right answer
Usually "mistakes a change in percentage for a change in absolute number" or "ignores the size of the population."
The trap
"The argument relies on an unrepresentative sample." Percentages can be perfectly representative and still mislead about totals. Different flaw.
Find your weak flaws
The free diagnostic scores flaw questions separately and shows whether this pattern is costing you points.