Answer
All the secondary factors, other than the factor to be tested are important, same in the control and treatment plots before and after the installation of the nets. Any change occurred is due to the exclusion of birds not due to the involvement of any other factor.
Work Step by Step
The given data was collected before installing the nets. This will exclude the effect of other factors on the percentage of berries with borers in the treatment plots with nets. In any scientific research, researchers always confirm that the factors do not differ between the control and treatment plots to draw a significant conclusion from the study.
This will make sure that the difference obtained after the study will only show the change caused because of the exclusion of the birds. In the first graph, it can be clearly seen that there is not much difference between the treatment and control plots. After the installation of nets, there is a huge difference.
Any change, if observed, should not be due to an error. In the given case, the change observed should be due to exclusion of birds only not due to some other physical factors, such as shade, abundance of beetles, or water availability.
