OC, depressive and insomnia symptoms in adult women with and without depression 51 To assess insomnia symptom severity, participants filled-out the Women’s Health Insomnia Rating Scale (WHI-IRS) by Levine et al. (2003) at every assessment wave. The WHI-IRS consists of five items on a scale of 0 to 4, with a sum score ranging from 0 to 20. Items inquire about difficulty falling asleep, waking up during the night, early awakening, getting back to sleep after wakefulness and feeling rested after sleep. The WHI-IRS has an acceptable Cronbach’s alpha of 0.78 (Levine et al., 2003). 2.4. Covariates Sociodemographic variables in this study included age, highest education, country of birth, partner status and body mass index (BMI). Participants’ level of education was defined as the highest number of years of education that the participant reported over the whole study duration. This was used instead of the number of years of education at each assessment because the number of years of education and participants’ age were highly codependent, violating statistical assumptions. Since contraceptive choices in the Netherlands significantly differ between groups of different ethnicities (Marra et al., 2020), we adjusted for participants’ birth country in the analyses. Birth country was grouped based on whether the participant was born in the Netherlands or outside the Netherlands. Partner status was reported at every assessment wave by asking about marriage and partner status (e.g., “Are you married or do you have a person you consider your long-term partner?”). Age, education level and birth country were incorporated into the multivariate adjusted model, partner status and BMI were included only if these differed between the OC and NC measurements, meaning they could be confounding effect estimates. As an additional covariate, the course of participants’ depression diagnoses (obtained through CIDI interviews) before and throughout the study was determined to categorize observations into three groups. Observations were categorized in either the never depressed control group, the remitted group or the depressed group. Observations in participants who had no MDD or dysthymia diagnoses in their lifetime were labeled as never depressed, observations in participants with a previous but no current diagnosis of MDD or dysthymia were labeled as remitted, observations in participants with a current (within the last 6 months) MDD or dysthymia diagnosis were labeled as depressed. This method of labeling meant that a participant who was recruited without a history of depression could contribute her first
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