Suggested answer to exercise: Estimation, standard error and confidence intervals, 7

Question 7: Confidence intervals are presented for each group separately. Suggest a more informative confidence interval. To which population would this relate?

Suggested answer

The confidence intervals provide estimates of the mean change in bone density over two years. Since we are here interested in any effect of exercise on this, a confidence interval for the difference between the two groups would be more useful than the two separate intervals provided. Using the data given the difference in mean change is 3.7 with an approximate 95% confidence interval 0.9 to 6.5. This would still relate to the population of volunteers but since the women were randomized to two groups, we might reasonably expect the sample difference to be representative of all women.

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Last updated: 27 July, 2009.

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