1. Reliability means the extent to which a method may be repeated and give the same results; validity is the extent to which a method produces a true picture of the situation being researched.
  2. Because they can be used with large numbers of participants, to provide a typical cross-section of the population under study.
  3. Confidentiality; informed consent; effects on participants; vulnerability; deception.
  4. Time and/or money available; subject matter; skills required; personal characteristics; source of funding; opportunity.