ACT® · Science: Evaluating Scientific Arguments and Models with Evidence (8-13 items)
Identifying Key Assumptions in a Model (EMI 402)
An assumption is the unstated claim a model needs to be true for its logic to work. You find it by asking what would make the whole argument collapse if it turned out false.
Every model in a passage is built like a small argument: a conclusion sitting on top of evidence, held up by at least one block the scientist never states out loud. That hidden block is the assumption. Scientists do not write it down because it seems obvious to them, but the whole argument needs it to be true. Your job is to find the block that is doing invisible work.
The fastest way in is to ask: what would have to be false for this conclusion to fall apart? Take the evidence given, picture it as a floor, and ask what unstated fact connects that floor to the conclusion on top. If a scientist says mixed fossils in one bed mean one flood event, the floor is 'the species are mixed together' and the missing beam is 'those species lived at the same time.' Without that beam, a mixture could just mean the fossils got reworked from different eras, and the flood conclusion never gets built.
Do not confuse an assumption with a weakness. Every model needs assumptions; that is just how arguments are built, and having one is not a flaw. An assumption becomes a weakness only once evidence starts to challenge it directly. This connects to the next skill in the unit: once you can spot the assumption, you can test what happens to the conclusion if new evidence shows that assumption was wrong. Sometimes the conclusion collapses entirely; sometimes it just gets downgraded, like turning an exact age into an upper limit.
Key terms
- Assumption
- An unstated claim a model needs to be true for its evidence to support its conclusion.
- Weakness
- An assumption that evidence in the passage has started to contradict or undermine.
- Collapse test
- Asking what would happen to the conclusion if a given assumption turned out to be false.
- Upper bound
- A revised limit showing the true value could be no higher than the number stated, once an assumption is found faulty.
Watch it explained
- How it worksHow do you find the unstated block a model needs by asking what would collapse if it were false?
- How it worksWalk me through drawing an argument as blocks under a conclusion and spotting the gap where one is missing.
- Common mistakeWhy isn't an assumption the same thing as a weakness in a scientific model?
- Going furtherWhat happens to a model's conclusion when one of its hidden assumptions turns out to be false?
Check yourself
End of topic.