Something to compare with
Show on a big screenTwenty seeds are soaked. Seventeen sprout. Does soaking work?
Chance means small differences nobody controls.
- Soaked group: 17 sprouts (made-up numbers).
- Unsoaked group: 11 sprouts, right beside it.
- On this bench, chance moves a group about this much (made up).
- Six more is beyond that. It looks like a real difference.
You cannot say from one group. You need a second group, not soaked.
The group you leave alone is the comparison group. Here the comparison group is the seeds you do not soak. Treat the comparison group like the soaked group in every other way.
The rule
A result on its own has nothing to compare with. So run a second group that is left alone. Compare the two groups. Treat them alike, except for the one thing you test.
Watch out!
Two groups treated alike still differ a little by chance. A gap is the difference between the two groups. A gap only counts if it is bigger than the chance difference.
Slide the plain group, the comparison group. Watch the gap and the verdict, the answer the bench reaches.
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Try the lab to finish this lesson.