As dogs age, diminished joint and hip health may lead to joint pain and thus red

Question

As dogs age, diminished joint and hip health may lead to joint pain and thus reduce a dog’s activity level. Such a reduction in activity can lead to other health concerns such as weight gain and lethargy due to lack of exercise. A study is to be conducted to see which of two dietary supplements, glucosamine or chondroitin, is more effective in promoting joint and hip health and reducing the onset of canine osteoarthritis. Researchers will randomly select a total of 300 dogs from ten different large veterinary practices around the country. All of the dogs are more than 6 years old, and their owners have given consent to participate in the study. Changes in joint and hip health will be evaluated after 6 months of treatment.

a. What would be an advantage to adding a control group in the design of this study?

b. Assuming a control group is added to the other two groups in the study, explain how you would assign the 300 dogs to these three groups for a completely randomized design. Outline your experimental design. (See text and PowerPoint notes for sample diagram.)

c. Rather than using completely randomized design, one group of researches proposes blocking on clinics and another group of researchers proposes blocking on breed of dog. Decide which one of these two variables to use as a blocking variable. State the reason for your decision.

Explanation / Answer

a)

Adding a control group would allow us to see how the dogs’ joint and hip health changes naturally over time. It would allow us to compare the effect of the treatments to the natural (untreated) joint and hip health. If we include a control we can better assess the effect of the treatment, and reduce the possibility of confounding results or of a placebo effect.

b)

c)

Blocks = groups of experimental units that have something in common that affects the results of the experiment.

Due to the definition above, I would decide to block based on which variable has the most effect on joint and hip health.

For example, if I believe that different breeds have different natural joint and hip health, then I should block the dogs based on breed. Then I can eliminate the lurking variable of breed.

However, if I believe that different veterinary clinics treat joint and hip health differently (thus different vet clinics would have different natural joint and hip health of the animals they treat), then I should block based on clinics.  

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