Hands up, or heads?

About this video

The school with the reading app tries once more. This time the teacher asks who would like to use it. Twenty children put their hands up and get the app; the rest are the control group. At the end of the term the app group has improved much more. Has the app worked this time? Think about who put their hands up. Children who love reading are more likely to want a reading app, and they are also the ones who read most at home, so they were likely to improve more anyway. The two groups were different before the test started, and the test can't tell whether the difference at the end came from the app or from the kind of children who chose it.

The fix is surprising: don't let anyone choose, not the children and not the teacher, who might pick the keenest without meaning to. Toss a coin for each child instead: heads the app, tails the control group. A coin doesn't know who loves reading, so the keen readers end up spread across both groups, about half in each, and so do the tall and the short, the tired and the lively, and every other difference, even the ones nobody thought of. Choosing by chance like this is called random assignment.

With random groups, the groups are alike at the start in every way except the app, so if the app group improves more, the app is the likely reason. Scientists use the same idea for a new crop, a new teaching method or a new medicine. This video goes with the Toss a Coin to Choose lesson of Science Level 1 on Knaph.

Transcript

1. 0:00Who would like the reading app? Hands go up. They get the app.
2. 0:05But who put their hands up? The children who already love reading.
3. 0:11So don't let anyone choose. Toss a coin for each child.
4. 0:17The coin doesn't know who loves reading. The keen readers split about half and half.
5. 0:23And so does every other difference. Even the ones nobody thought of.
6. 0:29Let chance choose, and the groups start out alike.