The Widespread Pain Index quantifies the extent of bodily pain. The WPI is required to identify individuals that meet 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.

Lifelines repeatedly assessed the presence of fibromyalgia by self-report. However, recent studies indicate that this method misses ~77% of fibromyalgia diagnoses¹⁾. The WPI improves the identification of participants that meet the diagnostic criteria for fibromyalgia by assessing the presence of individual symptoms rather than the disease itself.
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 instrument 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 ²⁾. In Lifelines, this information can be derived from other variables (i.e. fatigue (general), symptoms (SCL-90), and various cognition-related variables).

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⁾.