Symptoms experienced by Lifelines participants were assessed using the Symptom Checklist (SCL-90), a multidimensional self-report instrument (section: Diseases & symptoms) that was found to be suitable for use in large-scale studies including elderly people¹⁾²⁾.
Note that the assessment of symptoms in specific medical fields are listed in the associated subsections (see here for a list).
The SCL-90 for somatic symptoms was also incorporated in the Covid-19 additional questionnaires.

The full SCL contains the following dimensions:

• somatic complaints (12 items) (see note for item SCL90SOM11).
• anxiety (10 items)
• depression (16 items)
• agorafobia (7 items)
• distrust and interpersonal sensitivity (18 items)
• anger/hostility (6 items)
• insufficience of thinking and acting (9 items)
• sleep problems (3 items)

Within the general assessments of Lifelines, only the dimension of somatic symptoms was used (in conjunction with symptoms associated with a specific medical field). This dimension reflects symptoms of the autonome and musculoskeletal systems, which correlate with a general physical feeling of dysfunctioning.
The 26 items within the dimensions of anxiety and depression were (together with somatic symptoms) assessed in the context of an additional questionnaire, EARQ.

Respondents were asked to what extent they were hampered by 12 different somatic symptoms, 10 different anxiety-related symptoms, and/or 16 depression-related symptoms in the last 7 days on a five point scale, from “not at all” to “extremely”.

In the following publication, Lifelines SCL-90 data was investigated:

• Acevedo-Mesa, A. et al. (2020): Improving the measurement of Functional Somatic Symptoms with item response theory. Assessment 1073191120947153.

*SCL90SOM11 was wrongly translated into Dutch to “fealing very weak in your body”. This variable was replaced by the correctly translated SCLSOM11A for assessments 2A and NEXT.