Maartje Boer

CHAPTER 6 162 been found to have solid structural and criterion validity and good reliability among adolescents (Boer, Stevens, Finkenauer, Koning, et al., 2021). Due to the dichotomous nature of the items, internal consistency was calculated using the tetrachoric correlation matrix (Gadermann et al., 2012). This yielded an ordinal alpha that varied between 0.83 and 0.85 at the different waves. SMU Intensity Four items assessed respondents’ SMU intensity (Boer, Stevens, et al., 2020), which measured the frequency of different social media activities. The first three items examined ‘How many times per day do you view social network sites’, ‘How many times per week do you ‘like’ messages, photos, or videos of others on social network sites’, and ‘How many times per week do you respond to messages, photos, or videos of others on social network sites’ (1 never or less than once to 7 more than 40 times ). The questionnaire provided examples of social network sites (‘for example Facebook, Twitter, Instagram, Google+, or Pinterest, but not WhatsApp or SnapChat’). The fourth item examined ‘How many times per day do you send a message, photo or video via your smartphone, for example a WhatsApp, Chat, SnapChat, or SMS’ (1 less than once to 7 more than 80 times ). Cronbach’s alpha varied between 0.78 and 0.84 across waves. The original scale consisted of two additional items: ‘How many times per week do you post a message, photo, or video on social network sites’ and ‘How many times per day do you check your smartphone to see whether you have received a message, photo, or video, for example a WhatsApp, Chat, SnapChat, or SMS?’. The first itemof these two was excluded because its factor loading was low ( < 0.5). The second item was excluded because it showed relatively high overlap with the other item on smartphone use ( r = 0.7), and therefore removing the item yielded substantial model fit improvement (Boer, Stevens, et al., 2020). Depressive Symptoms Respondents reported on their depressive symptoms using the 6-item Depressive Mood List (Kandel & Davies, 1982). They indicated, for example, how often in the past year they were ‘Feeling too tired to do things’, ‘Feeling unhappy, sad, or depressed’, ‘Having trouble going to sleep or staying asleep’,