3.10 Exercises

Selected answers are available in Sect. D.3.

Exercise 3.1 In a study on the shear strength of recycled concrete beams (Gonzalez-Fonteboa and Martinez-Abella 2007), beams were divided into three groups. Different loads were then applied to each group, and the shear strength needed to fracture the beams was measured.

Is this a quasi-experiment or a true experiment? Answering these questions may help:

  1. Do researchers allocate treatments to the groups?
  2. Do researchers allocate the who or what to groups?

Exercise 3.2 A study had this aim:

To compare the effectiveness of alternating pressure air mattresses vs. overlays, to prevent pressure ulcers.

Manzano et al. (2013) , p. 2099.

Patients were provided with either alternating pressure air overlays (in 2001) or alternating pressure air mattresses (in 2006). The number of pressure ulcers were recorded.

This study experimental, because the researchers provided the mattresses. Is this a true experiment or quasi-experiment? Explain.

Exercise 3.3 Consider this initial RQ (based on Friedmann and Thomas (1985)), that clearly requires a lot of refining:

Are people with pets healthier?

To answer this RQ:

  1. Describe a useful and practical definition for P, O and C.
  2. Describe an experimental study to answer the RQ.
  3. Describe an observational study to answer the RQ.

Exercise 3.4 Consider this journal extract:

We randomly assigned 811 overweight adults to one of four diets […] The diets consisted of similar foods and met guidelines for cardiovascular health […] The primary outcome was the change in body weight after 2 years in […] comparisons of low fat versus high fat and average protein versus high protein and in the comparison of highest and lowest carbohydrate content.

Sacks et al. (2009), p. 859

  1. Define POCI.
  2. Is this study observational or experimental? Why?
  3. Is this study a quasi-experiment or a true experiment? Why?
  4. What are the units of analysis?
  5. What are the units of observation?
  6. What is the response variable?
  7. What is the explanatory variable?


Friedmann E, Thomas S. Health benefits of pets for families. Marriage & Family Review. Taylor & Francis; 1985;8(3-4):191–203.
Gonzalez-Fonteboa B, Martinez-Abella F. Shear strength of recycled concrete beams. Construction and Building Materials. Elsevier; 2007;21(4):887–93.
Manzano F, Pérez A-M, Colmenero M, Aguilar M-M, Sánchez-Cantalejo E, Reche A-M, et al. Comparison of alternating pressure mattresses and overlays for prevention of pressure ulcers in ventilated intensive care patients: A quasi-experimental study. Journal of Advanced Nursing. 2013;69(9):2099–106.
Sacks FM, Bray GA, Carey VJ, Smith SR, Ryan DH, Anton SD, et al. Comparison of weight-loss diets with different compositions of fat, protein, and carbohydrates. The New England Journal of Medicine. 2009;360(9):859–73.