In the Methods section of the paper, we read:

"We calculated sample size on the basis of a pilot study done in February to June 2004 and involving 543 patients, which showed an overall infection rate of 5.7%. On the basis of a projected infection rate of 5%, we decided that an increase in incidence of infection of 5% would be clinically significant."

Hence the view of these authors was that there would be no important difference between the treatments if the proportion experiencing infection rose from 5% for covered sutures to 10% for uncovered sutures. They intended to cary out "a one sided equivalence test of proportions".

These authors wanted to know whether allowing wounds to get wet
would be detrimental to the patient, in particular whether it would
increase the risk of infection by more than 5 percentage points.
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 4: What null hypothesis and alternative hypotheses were being tested here? How would would a significant result be interpreted?

They want to know whether the difference between the proportions experiencing infection, uncovered minus covered, is less than 0.05 or 5 percentage points.

The null hypothesis is that the difference between the proportions experiencing infection, uncovered minus covered, is greater than or equal to 0.05 or 5 percentage points.

The null hypothesis is that the difference between the proportions experiencing infection, uncovered minus covered, is less than 0.05 or 5 percentage points.

A significant difference would be interpreted as evidence that the difference is less than 0.05 or 5 percentage points.

Back to Exercise: Can sutures get wet?

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Last updated: 31 July, 2006.