Changing together doesn't make it the cause
- The idea
- Predict
- See it
- Why
- Check
The idea
“If something changed at the same time as the result, it must have caused it.”
Warmer air and longer days arrive together each spring. When two things rise at once, the one you already suspect looks guilty.
The short version
The mistake: If something changed at the same time as the result, it must have caused it.
What's actually true: A cause still matches the result when it changes on its own. Things that only change together may share a cause.
- A correlation means two things change together. It's a clue, but not proof. If two possible causes always rise together, the pattern fits both.
- In this plant, day length controls flowering. Many plants sense night length with light-sensing proteins such as phytochrome. This is photoperiodism.
- Watch for a third factor too. If one thing drives two others, those two change together even though neither causes the other.
On the exam: Before you blame a factor, find a case where it changed on its own. The likely cause still matches.