Loyalty-Card Purchase Data

For cases, purchase data are backdated from cancer diagnosis; for controls, they are backdated from study consent. Tesco data access uses OpenID Connect authentication to reduce manual identity verification and participant burden. CLOCS-2 requests up to six years of backdated loyalty-card purchase data when available. Purchase data include purchase-level records, dates, store location by postcode, and retailer-supplied product categories. Prescription pharmacy data are excluded because prescriptions are not part of loyalty-card purchase histories. The study captures over-the-counter medicine purchases and food purchases as possible indicators of symptom management or dietary change.