Background: Women runners are at high-risk of low bone density and stress fractures. Observational studies have found a

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Background: Women runners are at high-risk of low bone density
and stress fractures. Observational studies have found a
relationship between vitamin D and bone density in runners, but no
randomized trials have tested this hypothesis. Methods: We
performed a randomized placebo-controlled trial to determine
whether vitamin D supplementation can increase bone density in
women. We randomly assigned 100 women runners to take a vitamin D
supplement (1000 IU/day) and 100 women runners to take a placebo
pill daily. We compared bone density before and after
supplementation using a paired t-test. Results: Women in the two
groups were balanced with regards to age, weight, height, BMI,
weekly mileage, and best mile time. After three months of
supplementation, there was a significant increase in bone density
in the vitamin-D supplemented group (p<.05), but not in the
placebo group (p>.05). Conclusions: To increase bone density and
prevent stress fractures, women runners should take a daily vitamin
D supplement. Which of the following statistical pitfalls
(discussed in Unit 4) most undermines the authors’
conclusions?
a. You need to compare effect sizes, not p-values
between two groups.
b. Correlation is not causation
c. Statistical significance does not imply clinical
significance
d. There is a multiple testing issue