4 Exploring the mediating role of alexithymia in BPP 123 Eight items were initially included: item 4 (“I engaged in binge-eating”), 5 (“I induced vomiting”), 6 (“I displayed high-risk behavior by knowingly driving too fast, running around on the roofs of high buildings, balancing on bridges, etc.”), 7 (“I got drunk”), 8 (“I took drugs”), 9 (“I took medication that had not been prescribed or if had been prescribed, I took more than the prescribed dose”), 10 (“I had uncontrolled outbursts of anger and/or I got physical towards others”), 11 (“I had uncontrollable sexual encounters of which I was later ashamed or which made me angry”). Three items were not included as these inquired on direct SIB and suicide. Psychometric properties of the scale were established by determining internal consistency and checking loadings on a 1-factor exploratory principal component analysis, interitem correlations, and outcomes of an alpha-if-deleted test. For the 8item scale, internal consistency was questionable (Cronbach’s = .58). Item 4 was removed, as its correlations with other items proved to be (very) low to slightly negative and its removal significantly increased internal consistency of the scale. The resulting 7-item indirect SIB scale had adequate internal consistency (Cronbach’s = .70). Items of the BSL-23 Supplement scale are scored on a 5-point ordinal scale including ‘not at all’ (0), ‘once’ (1), ‘twice’ (2), ‘on a daily basis’ (3), ‘multiple times a day’ (4). As all original measures of SIB showed non-normal distribution of data (also see under ‘Statistical Analysis’), data transformation to reduce unevenness in the distribution was done by rescoring the 5 categories of the BSL-supplement items to 3: no SIB (0), low frequency SIB (once or twice in the last week; 1), high frequency (daily to more than daily SIB; 2). The level of indirect SIB per person was determined by the maximum score obtained on any of the 7 items. 3.2 Statistical analysis IBM SPSS Statistics 25 was used for all statistical analyses. Descriptive statistics were determined for all main variables. Normality of data was examined by visual inspection of the Q-Q plots, histograms and boxplots, use of the Kolmogorov-Smirnov (K-S) and Shapiro Wilk test (SW) and skewness and kurtosis statistics. Data of alexithymia (both TAS-20 and TSIA) and depressive symptoms were normally distributed, but BPP, measures of SIB, Dissociative experiences were not. Therefore, Spearman’s rank correlations were used to quantify the strengths of the relationships between variables. The final data set contained no missing data, as digital administration of tests required participants to respond to all items in order to proceed.
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