The Widespread Pain Index quantifies the extent of bodily pain on a 0-19 scale by asking Lifelines 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 (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 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).
This instrument is required for the diagnosis of fibromyalgia in epidemiological studies. At baseline, the presence of fibromyalgia was assessed by self-report (i.e. asking participants whether they suffer from fibromyalgia), but recent studies indicate that this methodology misses 77% of the fibromyalgia diagnoses ²⁾. Implementation of the WPI improves the identification of 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⁾.