Some students investigated geotropism in the roots of seedling plants using two different setups over 3 days: * Setup X: 5 seedlings pinned to a stationary vertical board. * Setup Y: 5 seedlings pinned to a board rotating slowly on a motor (clinostat). The results of the root length change are shown in the table below: | Seedling number | Setup X (Stationary) change in length (mm) | Setup Y (Rotating) change in length (mm) | | :---: | :---: | :---: | | 1 | 18 | 14 | | 2 | 19 | 15 | | 3 | 5 | 16 | | 4 | 17 | 13 | | 5 | 21 | 12 |
Explain how the rotating board in Setup Y acts as a control.
Identify the anomalous seedling in Setup X and suggest why including its measurement in the mean calculation reduces the validity of the results.
Calculate a more valid mean change in length for the seedlings in Setup X by omitting the anomalous result. Give your answer to 1 decimal place.