9.7 Exercises

Selected answers are available in Sect. D.9.

Exercise 9.1 A student project at the university where I work had the RQ:

Among USC students on-campus, is the percentage of word retention higher in male students than female students?

When they were discussing external validity, they said:

We cannot say whether or not that the general public have better or worse word retention compared to the students that we will be studying.

Why is the statement not relevant?

Exercise 9.2 Despite their common use, no experimental scientific evidence shows that parachutes are effective (Smith and Pell 2003). To obtain evidence, researchers studied this scenario (Yeh et al. 2018). Part of the Abstract for the paper (slightly edited for clarity) says:

Objective To determine if using a parachute prevents death or major traumatic injury when jumping from an aircraft.

Design Randomized controlled trial.

Setting Private or commercial aircraft between September 2017 and August 2018.

Participants 92 aircraft passengers aged 18 and over were screened for participation. 23 agreed to be enrolled and were randomized.

Intervention Jumping from an aircraft (airplane or helicopter) with a parachute versus an empty backpack (unblinded).

Main outcome measures Composite of death or major traumatic injury (defined by an Injury Severity Score over 15) upon impact with the ground measured immediately after landing.

Results Parachute use did not significantly reduce death or major injury (0% for parachute v 0% for control; \(P>0.9\)).

Conclusions Parachute use did not reduce death or major traumatic injury when jumping from aircraft in the first randomized evaluation of this intervention. However, the trial was only able to enroll participants on small stationary aircraft on the ground, suggesting cautious extrapolation to high altitude jumps […]

Yeh et al. (2018)

Based on this information:

  1. Carefully define POCI.
  2. What type of study is this: observational or experimental?
  3. What are the variables?
  4. Comment on the ecological validity of this study.
  5. Comment on the limitations of the study.
  6. What are the conclusions?

Exercise 9.3 A study of how well hospital patients sleep at night (Delaney et al. 2018) set out to

…to investigate the perceived duration and quality of patient sleep and identify any environmental factors associated with patient-reported poor sleep in hospital.

Delaney et al. (2018) p. 1

In discussing the study, the researchers state:

Patients and nursing staff were recruited for this study. Non-probability convenience sampling was used to recruit patients to participate…

Delaney et al. (2018) p. 2

Later, while discussing the limitations, the researchers state:

While the multiple methods of data collection and inclusion of 15 clinical areas are strengths of this study, the results may not be generalisable to all hospitals or all ward areas […] while most healthy individuals sleep primarily or exclusively at night, it is important to consider that patients requiring hospitalization will likely require some daytime nap periods. This study looks at sleep only in the night-time period 22:00–07:00h, without the context of daytime sleep considered.

Delaney et al. (2018) p. 7

Discuss these issues using the language introduced in this chapter.


Delaney LJ, Currie MJ, Huang H-CC, Lopez V, Van Haren F. “They can rest at home”: An observational study of patients’ quality of sleep in an Australian hospital. BMC Health Services Research. BioMed Central; 2018;18(1):524.
Smith GCS, Pell JP. Parachute use to prevent death and major trauma related to gravitational challenge: Systematic review of randomised controlled trials. BMJ. 2003;327:1459–61.
Yeh RW, Valsdottir LR, Yeh MW, Shen C, Kramer DB, Strom JB, et al. Parachute use to prevent death and major trauma when jumping from aircraft: Randomized controlled trial. BMJ. 2018;363:k5094.