Answer:
to provide common conditions of research
Explanation:
In simple words, the common conditions of research must be provided when that particular condition can be a distorting factor to the results obtained. In the given case, the two subjects groups were tested on two different times. There is a high chance that the morning group would have performed better than the afternoon ones.
Hence, it will be seen as a failure of the experimenter to test the groups in two different times.
