130 Chapter 6 Statistical analysis The sample size was calculated based on the Stoma QoL score of 56.6 as retrieved as baseline from a previous study and the hypothesis that the Stoma QoL of the StomaAPPtimize group would increase to 61.6. Using a sample size calculation with 90% power, a 2-sided alpha of 0.05, and a standard deviation of 10,7, we estimated that 98 participants per study group are needed. A loss to follow-up rate of 10% was also estimated. Therefore, the total target sample size was set at 208 participants ((2 × 98) / 0.9 = 208). Participants who did not receive an ileostomy or colostomy during surgery were excluded and substituted with new inclusions. Data was analysed according to the intention-to-treat protocol. Statistical analyses of differences between the two groups were performed using IBM SPSS for Windows version 28.0. Baseline characteristics were summarized using descriptive statistics and compared between the intervention and control groups. Continuous data were reported as mean and standard deviation in case of normal distribution and as median and 95% confidence intervals in case of non-normal distribution. The normality of data distribution was analysed by visually inspecting the histograms. Categorical variables were presented as frequencies and percentages. Independent t-tests, Mann-Whitney U tests, Chi-squared tests, and Fisher's exact tests were used to assess differences between groups as appropriate. Multivariate linear regression with stepwise backward selection was used to account for the potential confounding and stratifying factors. A two-tailed p-value ≤ 0.05 was considered statistically significant. Patient history was categorized as: none, minimal or extensive, with minimal history defined as one or two diseases generally not affecting or debilitating current quality of life (e.g., hypertension, appendectomy), and extensive history defined as having chronic diseases or several abdominal surgeries affecting or debilitating current quality of life. Patient Reported Outcome Measurements (PROMs) were included in the analysis if the patient completed at minimum 80% of the PROM related questionnaires per domain. Missing data were corrected using the participants’ mean outcome of the (domain of the) PROM. For missing values, a cut-off value of 20% was applied.
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