Primary - Self-Completed Questionnaires:
Practical advantages: Relatively cheap, quick to gather data, easy to access, and can study a large population.
Ethical advantages: Ensure informed consent, anonymity, and no harm to participants.
Theoretical advantages: Reliable, high chance of representation, and can easily obtain quantifiable data.
Disadvantages: Uncertainty of the correct person completing the questionnaire, potential cost, and low response rate.