D.8 Answers: Designing observational studies

Answers to exercises in Sect. 8.9.

Answer to Exercise 8.1: 1. Since this is an observational study, we cannot allocate stduents to receive bottled or tap water (because then the study would be an experimental study). In an experiment we could randomly allocate students to receive either bottled or tap water and have them rate the taste (or even randomly allocate students to receive bottled or tap water first, then swap to the other type of water, and each student would then provide two ratings). 2. The students would not be aware of which water they would be drinking. 3. Neither the students nor the researchers who give the students the water would know which type of water the students are drinking. 4. We can’t really set up a control here. 5. Any of the random sampling methods are possible, and are preferred. In practice, perhaps use a convenience sample, but try to get a sample as representative as possible (Sect. 5.9).
Answer to Exercise 8.2: Yes. Consider a study of the effect of smoking: non-smokers are the control. However, in an observational study, cases cannot be allocated to be controls.
Answer to Exercise 8.3: No.  People can know they are being observed.
Answer to Exercise 8.4: The descriptions indicates that patients probably knew they were involved, so the Hawthorne effect should be considered when interpreting the results.