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.
Practise AQA A Level Psychology Scientific processes with exam-style questions for A Level Psychology. 689 questions, matched to the AQA A Level Psychology (7182) specification and written in Paper 1, Paper 2 and Paper 3 style. Every question includes a full worked solution and mark scheme, so you can see where marks are awarded rather than just whether you got the answer right.
