An ultramarathon, as you might guess, is a footrace longer than the 26.2 miles o

Question

An ultramarathon, as you might guess, is a footrace longer than the 26.2 miles of a marathon. Runners commonly develop respiratory infections after an ultramarathon. Will taking 600 milligrams of vitamin C daily reduce these infections? Researchers randomly assigned ultramarathon runners to receive either vitamin C or a placebo. Separately, they also randomly assigned these treatments to a group of nonrunners the same age as the runners. All subjects were watched for 14 days after the big race to see if infections developed. (E. M. Peters et al., "Vitamin C supplementation reduces the incidence of postrace symptoms of upper-respiratory tract infection in ultramarathon runners," American Journal of Clinical Nutrition, 57 (1993), pp. 170-174.)

(a) What type of study is this?

Observational study: prospective

Observational study: retrospective    

Observational study: cross-sectional

Experiment: matched pairs design

Experiment: randomized block design

Experiment: completely randomized design

(b) What are the explanatory variables? Select all that apply.

Gender (male, female)

Vitamin C supplementation (yes, no)

Runner (yes, no)

Respiratory infection (yes, no)

(c) What is the response variable?

Runner (yes, no)

Vitamin C supplementation (yes, no)    

Respiratory infection (yes, no)

Gender (male, female)

Explanation / Answer

a) This is definitely an experiment because here a treatment is being imposed on subjects for getting a response rather than collecting and analysing data without changing the existing conditions( which is done in observational study). Moreover, this is a Randomised Block Design because here we are first dividing the subjects to homogenous blocks (runners and non-runners) before assigning randomly to treatment groups (VitaminC and Placebo).

So, our answer to part (a) is Experiment: randomized block design.

(b) The explanatory variables are Vitamin C supplementation and Runner because these are the factors that are affecting our study.

(c) The response variable is Respiratory infection because that is the focus of our study.

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