An ecologist investigated how soil moisture content changes with distance from a riverbank, and how this affects the distribution of wild marsh orchids (Dactylorhiza incarnata).
The table below shows the results of the investigation:
| Distance from riverbank (m) | Mean soil moisture content (%) | Mean density of marsh orchids (plants/m2\text{plants/m}^2plants/m2) |
|---|---|---|
| 2.0 | 82 | 18.6 |
| 4.0 | 65 | 13.1 |
| 6.0 | 48 | 8.4 |
| 8.0 | 33 | 4.2 |
| 10.0 | 19 | 1.1 |
| 12.0 | 8 | 0.0 |
State one variable the researcher should have controlled to make sure the soil moisture measurements could be compared.
