Cross-device identification is the effort to recognise that visits from a phone, a computer and a tablet belong to the same person. Without it, one customer appears as several different ones, which overstates the number of users and distorts the path to purchase – an advert that brought in the first visit on mobile gets no credit for an order completed on a computer. It can only be solved reliably for logged-in users, when sessions are merged by a proprietary identifier that must not contain personal data. For those not logged in, what remains is an estimate with limited accuracy. The practical conclusion is to treat user numbers as approximate and to base important decisions on experiments, not on precise attribution of a conversion to a channel.
See also: Modelled conversions, Conversion window, Incrementality.
