Suggested answer to exercise: Can sutures get wet?, 7

In the Results section, Heal et al. (2006) report that:

"The intervention group had an infection rate of 8.4% compared with 8.9% in the control group. The one sided 95% confidence interval of the difference of the two proportions was infinity to 0.028, so the non-inferiority side was lower than 0.05, the maximum allowable difference. We therefore concluded that the intervention group was not inferior to the control group with respect to the resulting infection rates (P < 0.05)."

Question 7: What alternative analysis could these authors have done?

Suggested answer 7

They could have presented the ordinary two-sided 95% confidence interval, which for the difference between proportions infected, uncovered minus control, -0.042 to +0.033, or -4.2 to +3.3 percentage points.

An alternative would be to give the risk ratio for infection, uncovered over covered, which I estimate to be 0.95, 95% confidence interval 0.61 to 1.47.

I think that either of these approaches this would have been more meaningful to many BMJ readers.

They could dispense with the P value altogether; it does not add anything useful.

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Last updated: 31 July, 2006.

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