The answer to this question should be: <span>Participants should be selected into the treatment group randomly.
In an experiment, the participant is given an intervention and it hoped to change a parameter. In every experiment, you must ask the participant permission so they must be aware of the experiment. Since this is an experiment, the researcher is allowed to do something that changes the participant behavior as long as it is intentional for the research. The experiment questionnaire should be given to the participant, not the population. By putting the participant randomly, you can minimize confounding factor from the participant.</span>
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