A plant physiologist investigated the effect of physical stress on the growth of soybean plants. Individually potted seedlings were randomly allocated between four treatment groups of 13 seedlings each:

Treatment group Description

1 Low light (control)

2 Low light with stress

3 Moderate light (control)

4 Moderate light with stress

The stress was shaking of each plant for 20 minutes twice daily. The growth data were in millimetres (mm) of plant height gained over the period of the experiment. The data were analysed to evaluate the evidence for any difference in mean growth between the treatments.

what formal test procedure would I use?