**2. Methods**

#### *2.1. Study Design*

This cohort study used questionnaires administered according to ESM. Experience sampling is a structured digital diary technique to appraise subjective experiences in daily life, often applied in patients with psychiatric disorders or somatic illnesses [9]. Patients are repeatedly asked to complete short questionnaires during the day, which allows for the assessment of moment-to-moment changes in both symptoms and mental states, aiming to map daily functioning [10]. This study was approved by the local medical ethical committee (METC-number: 2018-0955).

#### *2.2. Study Population*

The cohort of the present study consisted of chronic pain patients who were referred to the University Hospital Pain Centre of the Maastricht University Medical Centre+ (MUMC+). The patients were recruited from March 2019 until July 2021 while performing their digital intake at the pain center, during which they were asked whether they wanted to be approached for participation in this study. If their answer was positive, patients were contacted by a research nurse for a more extensive explanation about the ESM-procedures. Patients with any type of pain at any location were eligible for participation. To be included, patients had to be 18 years or older and to have experienced pain complaints for at least three months. Additionally, the patient had to be in possession of a smartphone and able to use the ESM application named Psymate. Patients who were interested in participation also received all required information by an information letter, complemented with a consent form. Before the start of the study, all patients who wanted to participate provided informed consent.

#### *2.3. Experience Sampling*

Both outcome (pain intensity), predictor (fear-avoidance) and potential confounders (affect) were measured by repeated ESM assessments. These ESM assessments consisted of 18 questions and were completed through a smartphone application (Psymate). The items in the Psymate application illustrate adequate psychometric properties, and sensitivity to

change over time [11]. Patients were asked to answer the questions 10 times a day, for six consecutive days. The questionnaires were completed in semi-random time blocks of 112.5 min from 7:30 a.m. until 10:30 p.m. during the patients' daily life, whenever patients received a notification ('beep') from the Psymate-app on their smartphone [12,13]. Fear-avoidance was assessed by the statement 'due to fear for (more) pain I did not make unnecessary movements since the last beep', asking the participants about their fearavoidance behavior since the last beep. The items of positive and negative affect come originally from the validated PANAS questionnaire [14–16] and were assessed thoroughly before the application in the ESM. Positive affect was assessed by the following statements 'I feel cheerful', 'I feel relaxed', 'I feel satisfied', and 'I feel enthusiastic', whereas negative affect was measured by the statements 'I feel insecure', 'I feel irritated', 'I feel lonely', 'I feel anxious', 'I feel guilty' and 'I am worrying'. The 10 different items concerning the affective state, as well as the item assessing the level of fear-avoidance, were answered on a 7-point Likert scale, ranging from 1 (not at all) to 7 (very much). The outcome variable 'pain intensity' was assessed by the statement 'I am in pain', and could be answered on an 11-point scale, ranging from 0 (no pain) to 10 (worst pain possible).
