Sample size estimation
NPS, sinonasal polyposis
The sample size was estimated for the primary outcome measure of this study (NPS), modeling the interaction term “group x time” with alpha = 0.05 and 1-ß = 0.8. It was based on Harissa et al. (2006) who investigated changes in the nasal specific subscores of the RSOM in patients with sinonasal polyposis, who received either oral prednisolone (50 mg, N = 20) or a placebo (N = 20) over a period of 14 days, with no other treatment being given. In this double-blind RCT, strong decreases in symptoms from pre to post were reported for the intervention group (64%, p < 0.001, with d = 1.4), while there was no significant decrease in the control group (11%, p > 0.05, with d = 0.35). While no significant difference between groups was found before treatment started (p > 0.05, with r = 0.04), a substantial difference was found after 14 days (p < 0.001, with d = 1.71). With no interaction term being reported, we concluded, based on the reported means/SDs and t-Tests, that the effect size of the observed interaction (time x group) would have been at least medium-sized. Therefore, to detect at least medium-sized effects, 12 participants per group would have been required.Based on the assessed study population of 53 participants per group, we were able to detect effect sizes of at least Sample size calculation was conducted in G*Power (3.1.9.4).