If you had a choice in the selection of participants or the environment for your investigation, it is important to specify how the choice was made.
This is necessary because people differ widely in age, education, levels, socioeconomic status, physical abilities, and a range of other demographic features that can influence the results of your investigation.
Similarly, the environment where investigations take place, the time of day, day or week, and time of year data was collected, may all play a part in the results you record.