10 / 12 · Make a fair comparison
Better results, or different people?
See why a difference alone does not tell you what caused it.
Builds on An estimate is not an exact address
Go to practice ↓A question to keep in mind
One class uses a new exercise app and scores higher. Does that prove the app caused it?
The classes might already differ in prior knowledge, study time, or teaching. A factor that differs between the groups can offer another explanation for their scores.
Random assignment gives the comparison a better starting point: chance decides who uses each method. Keep the tasks, time, and scoring comparable too. Random assignment helps with causal comparison; random sampling is about which wider group you can describe.
Work through an example
| Method | Passed | Total |
|---|---|---|
| New | 16 | 20 |
| Old | 12 | 20 |
- Before collecting results, assign the 40 volunteers randomly to two groups of 20.
- With comparable tasks and scoring, observe 16 passes versus 12.
- The observed rates are 80% and 60%. Random assignment helps interpretation, but uncertainty and the study population still matter.
Your turn
0 / 3Which design better isolates the effect of two study methods?
Read solution · does not award completion
Randomly assign volunteers, then use comparable tasks, time, and scoring.
A fair comparison needs more than two numbers. How groups were formed and measured matters.
In the table, calculate the two pass rates and their difference.
Read solution · does not award completion
- New method rate: 80 %
- Old method rate: 60 %
- Difference in rates: 20 points
16/20 = 80%, 12/20 = 60%, a 20-percentage-point observed difference.
Which process is about selecting people, and which is about assigning methods?
Read solution · does not award completion
- Draw names at random from all registered learners. → Random sampling: selecting people
- Use a random allocation to decide which volunteers use each method. → Random assignment: allocating methods
Selecting participants concerns who the results may describe. Assigning treatments concerns the comparison between methods. A study may do one, both, or neither.
Bring it back to your own work
Design a small comparison of two study routines. State who joins, how routines are assigned, and one outcome measured the same way for everyone.