165 Oral dryness and use of dry-mouth interventions the severity of the intra-oral dryness on a 5-point Likert scale ranging from 1—”No dryness” to 5—”Severe dryness” [8, 9]. The third part was the Xerostomia Inventory (XI), consisting of 11 items on a 5-point Likert scale ranging from 1—”Never” to 5—”Very often”. The items concern patients’ oral dryness and mouth feel. Per item, patients indicate how often they experience problems regarding mouth feel and oral dryness. The scores of the 11 items are summed to produce a total XI score that ranges between 11 (no xerostomia) and 55 (extreme xerostomia) [11]. The fourth part consisted of the Bother Index (BI). In the BI, the patient is asked to rate the severity of dry mouth on a scale from 0 to 10 [12–16]. Finally, the questionnaire included a list of potential interventions to relieve the feeling of a dry mouth [17]. These interventions are summed up in Table 2 and divided into two categories: the frequently (> 20%) and less frequently used (< 20%) interventions. The participants could indicate with yes/no which options they apply for the relief of their dry mouth. With the option “using other interventions’’, they could report additional interventions applied not listed in the questionnaire. Because some respondents did not answer all items of the questionnaires, the total number for items may differ. Data analysis The data were statistically analyzed with SPSS, version 26.0 (IBM Corp SPSS statistics, Armonk, NY, USA). The Shapiro–Wilk test was used to assess the normality of the data. As not all variables were normally distributed, the data are presented as medians and their interquartile range (IQR). To clarify relatively small differences, the mean and standard deviation (SD) are also reported. A Friedman test was conducted for the RODI scores of the total study sample, followed by a Wilcoxon signed rank test as a post hoc procedure. The possible relationships between frequently used dry-mouth interventions and the perceived oral dryness (RODI and XI) and patients’ discomfort (BI) were investigated initially by using a univariate analysis, using Mann–Whitney U tests. Only the significant interventions found in the univariate analysis were further explored in the multivariate analysis, the binary logistic regression. The drymouth interventions were considered as dependent variable and the total XI score, BI-score and RODI scores of the nine intra-oral regions were considered as independent variables. To identify the degree of multicollinearity among the independent variables, the variance inflation factor (VIF) was calculated. The VIF for these variables was < 5, which indicates that there is no multicollinearity present among these variables [18, 19], so they do not influence each other. 8
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