Topic 2The effects of continuous positive airway pressure (CPAP) for obstructive sleep apnoea (OSA) on insulin resistance are not clear. Forty-two men with known type 2 diabetes and newly diagnosed OSA were randomised to receive therapeutic or placebo CPAP for 3 months. Baseline tests were performed and repeated after 3 months. The table below is adapted from their results (West, Nicoll, Wallace, Matthews, & Stadling, 2007).

a. The baseline HbA1c for the two groups is very similar. Explain why this would be important.

b. Provide an interpretation of the results associated with the change in HbA1c using the confidence interval provided.

c. The study was powered not to miss a difference of 0.8 in HbA1c (assuming a within subject SD of 0.8)19 at a significance level of 5% and with a power of 90%, which required 20 subjects in each treatment group. The authors failed to reject the null hypothesis (p 5 0.7). Should the study have been designed with a larger sample to detect a smaller difference?

Confidence Interval Group Baseline HbA1c HbA1c Change for Change Therapeutic CPAP (n = 20) 8.5 (1.8) -0.02 (1.5) -0.6 to 0.9 Placebo CPAP (n = 22) 8.4 (1.9) 0.1 (0.7)