An agricultural researcher designed an experiment to investigate the effect of a new organic fertiliser on the growth height of tomato plants.
In the first stage, the researcher grew 50 tomato plants of variety A A\,A in a temperature-controlled indoor greenhouse with automated drip irrigation using standard soil.
In the second stage, they grew 50 tomato plants of variety B B\,B in an outdoor field plot during a wet, overcast spring using soil treated with the organic fertiliser.
Suggest one way in which the researcher could modify this experiment to improve its internal validity. Justify your answer.
689 exam-style questions on AQA A Level Psychology Scientific processes. Each one has a worked solution and a mark scheme showing where the marks go.