A group of students investigated the efficacy of a new biodegradable, vinegar-based herbicide spray on dandelion seedlings growing on a school playing field. They marked out a test area of the field measuring 40 m×20 m40\text{ m} \times 20\text{ m}40 m×20 m and used 0.5 m×0.5 m0.5\text{ m} \times 0.5\text{ m}0.5 m×0.5 m square quadrats.
Their experimental design was as follows:
Suggest a method the students should have used to position each quadrat.
Give the biological/scientific reason for using the method suggested in part (a).
Explain why the students sprayed Plot Y with water instead of the weedkiller.
The students' results are shown in the table below. Calculate the missing mean value, WWW.
| Quadrat | Count At Start: Plot X (Weedkiller) | Count At Start: Plot Y (Water) | Count After 4 Weeks: Plot X (Weedkiller) | Count After 4 Weeks: Plot Y (Water) |
|---|---|---|---|---|
| 1 | 22 | 14 | 5 | 13 |
| 2 | 27 | 19 | 8 | 18 |
| 3 | 24 | 15 | 4 | 15 |
| 4 | 26 | 22 | 7 | 17 |
| 5 | 26 | 20 | 6 | 17 |
| Mean | 25 | 18 | 6 | W |
Calculate the percentage decrease in the mean number of dandelion seedlings on Plot 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 their methodology to obtain more valid results, and give a reason for your answer.