A student investigated how the recovery time of the human pupil after exposure to a bright light change with age.
Their results are shown in the table below:
| Age of volunteers (years) | Recovery time (s): Person 1 | Recovery time (s): Person 2 | Recovery time (s): Person 3 | Mean recovery time (s) |
|---|---|---|---|---|
| 15 | 1.8 | 1.6 | 1.7 | 1.7 |
| 25 | 2.3 | 2.5 | 2.4 | 2.4 |
| 35 | 3.1 | 2.9 | 3.0 | 3.0 |
| 45 | 3.9 | 4.1 | 4.0 | 4.0 |
Give two improvements that are needed in this investigation before a valid conclusion can be made.
