Sample
This study used adult general population data from a cross-cultural study in seven European countries (21–23) to obtain normative values for the K10 scale. We outsourced the panel data collection to a market research institute, SurveyEngine (Berlin, Germany, https://surveyengine.com/), which contacts panel members who have registered voluntarily and agreed to participate in similar studies. The countries were assessed in consecutive projects; therefore, the online surveys were sent out between September 2015 and December 2018. We set quotas for sex and the predefined age groups (18–29, 30–39, 40–49, 50–59, 60–69, and 70 + years) to obtain a raw approximation of the proportion of the general population in these age and sex groups based on United Nations statistics (24).
Socio-demographic and health data
The questionnaire included a data form that collected the participants’ basic socio-demographic and health data, including their age, sex, educational level, marital status and living situation. We also asked the participants if they had been hospitalised during the previous 12 months and if they suffer from health conditions. For the latter, we provided a list of major chronic disorders, including mental health disorder, with a binary response format (i.e., no/yes) for each of these conditions.
The K10 and K6 scales
The K10 comprises 10 items exploring the non-specific psychological distress experienced in the last 4 weeks (10). In addition, the K6 scale uses the first six items from the K10 scale. Both questionnaire versions can be used to indicate distress in populations or individuals. All items are scored on a 5-point Likert scale (1 = ‘none of the time’ to 5 = ‘all of the time’). All items assess the participants’ psychological distress with questions focusing on anxiety and depression, such as, ‘In the last 4 weeks, how often did you feel nervous?’
A total score can be calculated by adding all item scores, with high scores indicating high levels of distress. Following the original scoring instructions (10), the score range for the K10 is 0 to 40 points, while the score range for the K6 short-form is 0–24 points.
Statistical analysis
Sample characteristics are given as means, standard deviations, and absolute and relative frequencies. While the data collection already approximated the age and sex distribution in the individual countries, we applied additional weights using raking to more precisely match the national age and sex distributions (24).
We described the weighted normative data for the K10 and K6 scales using means and standard deviations (SDs) and percentiles (10th, 25th, 50th, 75th and 90th) separately for the total sample and country-, age- and sex-specific groups.
To allow for more precise normative values in specific groups of individuals, we also developed a regression model to predict their K10 and K6 scores using the following independent variables: sex, age group, educational level, somatic chronic conditions, mental chronic conditions, and country. All predictors that were statistically significant in the univariate analysis (p < 0.05) were included in the multivariable model, except for mental health, which we excluded from the multivariate analysis to avoid over-adjustment.
To evaluate the diagnostic accuracy of the K10 scale in predicting self-reported mental health disorders (as reported in the initial questionnaire data), we used receiver operating characteristic (ROC) analysis to calculate the area under the curve (AUC) as a measure of diagnostic accuracy and determined the possible cut-off values separately for each country.