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A student investigated the effect of trampling by walkers on the distribution of a species of grass across a footpath. She placed a belt transect across the path and recorded the percentage cover of the grass in quadrats at 0.5 m intervals.
Name the independent variable in this investigation.
Explain why the student placed the quadrats at regular intervals along the transect rather than randomly.
Suggest two variables the student should record or control to make her conclusion about trampling more valid.