Some ecology students investigated the effect of an organic vinegar-based herbicide spray on dandelion plants (Taraxacum officinale) growing on a coastal dune grassland measuring 40 metres by 25 metres. They used 0.5 m×0.5 m0.5\text{ m} \times 0.5\text{ m}0.5 m×0.5 m quadrats.
Here is their experimental method:
Suggest a method the students should have used to place each quadrat.
Give the reason for the method you suggested in part (a).
Explain why the students sprayed Area Y with water instead of the vinegar-based herbicide spray.
The data shows dandelion counts in the table below. Calculate the mean value, YYY.
| Quadrat | Count At Start: Area X (Herbicide) | Count At Start: Area Y (Water) | Count After 4 Weeks: Area X (Herbicide) | Count After 4 Weeks: Area Y (Water) |
|---|---|---|---|---|
| 1 | 27 | 14 | 5 | 16 |
| 2 | 22 | 17 | 7 | 21 |
| 3 | 31 | 20 | 4 | 17 |
| 4 | 18 | 13 | 6 | 15 |
| 5 | 27 | 16 | 8 | 21 |
| Mean | 25 | 16 | 6 | Y |
Calculate the percentage decrease in the mean number of dandelion plants on Area X after 4 weeks. Use the following equation:
percentage decrease=mean at start−mean after 4 weeksmean at start×100 \text{percentage decrease} = \frac{\text{mean at start} - \text{mean after 4 weeks}}{\text{mean at start}} \times 100 percentage decrease=mean at startmean at start−mean after 4 weeks×100Suggest one improvement the students could make to the method to obtain more valid results, and give a reason for your answer.