Study design and sample

Adults aged ≥40 years who were resident in England with knee pain, with or without a recorded knee OA diagnosis, with daily access to the internet, and ability to complete questionnaires in English were invited to take part in a 13-week web-based case-crossover study^15,16. Exclusions included inflammatory arthropathies (including gout), fibromyalgia, joint replacement in the flaring knee or knee surgery in the last 3 months. Participants were identified via three methods: (1) Fifteen general practice (GP) registers (ten, West Midlands; five, South East). Patients with a relevant Read-coded consultation for knee OA or knee OA-related joint symptoms in the last 2 years were identified and invited via mailed invitation and one reminder. (2) Offline community advertisement. Study posters, flyers and business cards were displayed in GPs, pharmacies, hospitals and public libraries across England, where permission was granted. (3) Online social media advertisement. Using Facebook, adverts were targeted at adults ≥40 years. For methods 2 and 3, advertisements directed people to the study registration page where eligibility against the criteria detailed above was self-declared. Ethical approval was obtained from Yorkshire & The Humber-Leeds East Research Ethics Committee (REC reference number: 18/YH/0075). All participants provided informed electronic-consent.