The Widespread Pain Index quantifies the extent of bodily pain. The WPI is required to identify individuals meeting the diagnostic criteria of fibromyalgia in epidemiological studies (section: Diseases & symptoms).
Note that pain was also assessed in Lifelines participants as a symptom of several specified medical conditions, for example here.

The WPI assesses bodily pain on a 0-19 scale by asking participants if they have had pain or tenderness in 19 different body regions over the past week, with each painful or tender region scoring 1 point.
The WPI originated as a part of the Fibromyalgia Survey Questionnaire (FSQ) that sets diagnostic criteria for fibromyalgia. The full FSQ also contains a symptom severity score (SSS) for the following symptoms: fatigue, waking unrefreshed, cognitive symptoms and somatic symptoms ¹⁾. In Lifelines, this information can be derived from other variables (i.e. fatigue (general), symptoms (SCL-90), and various cognition-related variables).
The WPI was implemented in assessment 2A as an addition to the self-report fibromyalgia question for fibromyalgia at baseline, as recent studies indicate that this methodology misses 77% of the fibromyalgia diagnoses ²⁾. Implementation of the WPI facilitates the identification of Lifelines participants that meet the diagnostic criteria of fibromyalgia.

A recent German study showed that the reliability, convergent and discriminant validity of the FSQ were good, however, the The WPI was not evaluated separately³⁾.