2.3 Statistical analyses
The annual incidence and prevalence of CRSwNP were assessed during 1
January 2012 – 31 December 2019 by dividing the number of incident and
prevalent cases by the total Finnish population. Patients who fulfilled
the criteria for CRSwNP during 1 January 2000 – 31 December 2011 were
included in the prevalence assessment. Incident patients were defined as
patients with a first record fulfilling the criteria for CRSwNP during 1
January 2012 – 31 December 2019. The presence of a monotonic time trend
in prevalence and incidence was assessed using the Mann-Kendall test.
Patient demographics, clinical characteristics and disease burden were
tabulated at baseline, overall and by subgroups, and described using
summary statistics (mean with standard deviation, median with
interquartile range, or frequency as the total number of patients and
proportion (%) of all patients). The Charlson comorbidity index was
calculated using ICD-10 codes as implemented in the comorbidity R
package 21,22. Chi-squared test was used for assessing
differences in systemic corticosteroid (SCS) use between the asthma
subgroups during baseline.
Time to first and repeated ESS were assessed using Kaplan-Meier fits,
where time from index to surgery was defined as an event and the end of
follow-up or death as a censoring event. In the Kaplan-Meier fit of
repeated ESS, only patients with at least one ESS were included. The
Kaplan-Meier fits were visualized including the 95% Confidence
Intervals (CIs), and median survival was reported, if reached. A mean
cumulative count of ESS (with 95% CIs) was calculated over time overall
and per subgroup using a Mean Cumulative Function (MCF)23.
All statistical analyses were run using R version 4.0.3 on RStudio
Server version 1.4.1103 24, under Microsoft Windows
Server 2016 Standard.