Blaming an ingredient the data rule out
- The idea
- Predict
- See it
- Why
- Check
The idea
“The ingredient that sounds most likely is the cause, as long as it was in the treatment that worked.”
The suspect is in the mix that worked, and it sounds like it should work. So it's easy to credit it before checking the group that went without it.
The short version
The mistake: The ingredient that sounds most likely is the cause, as long as it was in the treatment that worked.
What's actually true: An ingredient is ruled out as the cause when the group without it gives the same result.
- To test one ingredient, compare two groups that differ only in that ingredient.
- If the group without it gives the same result, that ingredient isn't needed. Something else in the mix does the work.
- Here that's IBA, an auxin. Auxins are plant hormones, and IBA makes roots form at the cut end of a stem.
On the exam: Before you credit an ingredient, find the group without it. If the result didn't change, it isn't the cause.