The time-shift flaw
Old evidence, new conclusion.
Fallacy vs. flaw, on this site. No fallacy page covers this as a named error, though it is related to hasty generalization. On the LSAT it shows up in Flaw questions as "the argument applies past findings to the present without establishing that conditions are unchanged." Watch the dates in every argument.
This one has no fallacy page. It is an LSAT-specific pattern, which is exactly why it is worth learning.
What it is
The time-shift flaw uses evidence from one time period to support a conclusion about a different time period, without showing that nothing relevant changed in between. What was true then is assumed to be true now.
It also runs forward: past trends projected into the future as if trends continue by themselves. They do not.
An LSAT-style example
This example is original. It is not a real LSAT question.
A 1998 study found that residents near the airport reported high stress. The airport has since tripled in size, so current residents must be even more stressed.
The flaw: Everything changed between 1998 and now: flight paths, noise insulation, the residents themselves. The old study cannot carry a conclusion about today without a bridge.
How to spot it on test day
- Check the dates on the evidence and the date in the conclusion. Different times? Flag it.
- Watch trend projections: "has been rising, so it will keep rising." Past direction is not a law.
- The fix the argument never provides: evidence that conditions stayed the same.
The right answer vs. the trap
The right answer
Usually "presumes without warrant that past conditions still hold" or "relies on outdated evidence."
The trap
"The argument generalizes from too few cases." Time-shift is about when, not how many. One perfect study from 1998 still cannot prove a claim about today.
Find your weak flaws
The free diagnostic scores flaw questions separately and shows whether this pattern is costing you points.