D.9 Answers: Interpretation
Answers to exercises in Sect. 9.7.
Answer to Exercise 9.1: Population: ‘USC students on-campus.’ External validity refers to whether the results apply to other members of this population, not to people outside this population (such as members of the general public).
Answer to Exercise 9.2: 1. P: Aircraft passengers aged 18 and over. O: Unclear; something about ‘composite of death or major traumatic injury.’ C: Between wearing a parachute and wearing a backpack. I: Yes: Having participants wear the parachute or backpack. 2. Experimental: The researchers decide if the participants use a parachute or backpack. 3. Explanatory: ‘whether or not a parachute is worn.’ Response: harder to understand; is it ‘whether or not the participant dies or sustains a major injury?’ 4. These results won’t apply in the real world; not ecologically valid. In the real world, parachutes are used at high altitude, for example. 5. The study is not very useful! 6. Speaking loosely: That jumping from a small plane that is on the ground, parachutes are equally effective as backpacks in keeping people safe.
Answer to Exercise 9.3: Because the sample is not a random sample, the researchers are (rightly) noting that the results may not generalise to all hospitals. Because the data was only collected at night, perhaps the data is not ecologically valid.