An optometrist investigated how the near point of vision (the closest distance at which an eye can focus clearly on an object) changes with age.
Their results are shown in the table below:
| Age of volunteers (years) | Near point distance (cm): Volunteer A | Near point distance (cm): Volunteer B | Near point distance (cm): Volunteer C | Mean near point distance (cm) |
|---|---|---|---|---|
| 20 | 10.2 | 9.8 | 10.0 | 10.0 |
| 35 | 14.1 | 15.3 | 14.4 | 14.6 |
| 50 | 38.5 | 42.1 | 39.4 | 40.0 |
| 65 | 78.0 | 84.5 | 81.1 | 81.2 |
Give two improvements that are needed in this investigation before a valid conclusion can be made.
