Identity Resolution

Identity resolution (or identity stitching) is the problem of recognizing that the anonymous visitor who read three blog posts last week, the person who signed up on mobile, and the account that upgraded on desktop are all the same human. Analytics tools assign an anonymous ID from a cookie or device; the moment a user logs in, you `identify()` them and the system must merge the prior anonymous activity into the known profile so their journey isn't split across three ghosts. For builders this is what makes end-to-end attribution and full-funnel analysis possible — without it, the marketing touchpoints before signup are orphaned from the revenue they produced. CDPs and warehouse-native tools handle the merge with deterministic matching (shared login, email, user ID) and sometimes probabilistic matching (device, IP, behavior). Practical note: deterministic is accurate; probabilistic guesses and can wrongly fuse two people, so prefer it cautiously. Plan your ID strategy early — retrofitting identity onto months of un-stitched events is painful and often lossy.