LSAT Flaw Practice Question: Absence of Evidence
An original flaw question from our free Question of the Day rotation, built around a trap the LSAT reuses constantly: absence of evidence. Try it first, then read the explanation — it teaches the move, not just the answer.
Your job:Name what's wrong with the argument.
Residents of Alder Pond have worried that the pond's recurring algae blooms might harm its native fish. The county has monitored the pond for three seasons, and no study conducted so far has demonstrated that the algae harms the fish. We can conclude, therefore, that the algae poses no danger to the pond's fish population.
The reasoning in the argument is most vulnerable to criticism on the grounds that the argument
Show the answer & full explanation
The correct answer is B
The JBDM line
Just because no study has shown harm doesn't mean there is no harm.
The maybes
- MaybeMaybe three seasons is too short for the damage to show.
- MaybeMaybe the studies measured the wrong things.
- MaybeMaybe harm accumulates slowly across generations of fish.
Why B is right
'Nobody has proven X' is a fact about the research, not a fact about the pond. The studies may be too brief, too narrow, or aimed at the wrong indicators. Treating unproven as disproven is the appeal-to-ignorance flaw, and this answer names it precisely.
Why the others are wrong
- ANo conflict of interest is suggested — the county isn't selling algae.
- CPossible benefits would push toward the conclusion, not expose its flaw.
- DThe argument makes no claim about population stability; that's not the gap it leaps over.
- EThe conclusion stays at Alder Pond — no generalization to other ponds occurs.
Sammy's note
'Nobody's proven it' describes the research, not the world.
What's a JBDM line?
Just Because Doesn't Mean (JBDM) is the Burton Method's foundational framework for attacking LSAT arguments: identify the premise, identify the conclusion, then say "just because [premise] doesn't mean [conclusion]" and generate "maybes" — alternative explanations that expose the gap between them. That gap is what weaken, flaw, strengthen, and assumption questions all test, so every explanation on this page starts with the JBDM line. It's a micro-lesson in the system, not just an answer key.
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