Cross-Device Tracking
Cross-Device Tracking is the practice of linking several devices – smartphone, laptop and tablet, for example – to one and the same person so that their entire interaction history can be analysed across device boundaries. In GA4 this linking now rests on what Google calls the reporting identity: first on a User-ID you assign yourself at login, otherwise on the device ID and – depending on the setting – on modelled data. Google Signals has not contributed to the reporting identity since February 2024 and now supplies only remarketing audiences plus age and interest data.
In practice
In practice, Cross-Device Tracking matters most in sectors with long research phases, such as high-priced products, where people often research on a phone and buy later on a desktop. Without cross-device matching, such a purchase would be counted incorrectly as a new user with no prior touchpoint, which distorts attribution models. Reliable cross-device analysis today requires your own consistent login system that assigns the same User-ID across every device; Google Signals on its own has not done this since the change in February 2024. Where data is stored on or read from the device for this, consent under section 165 para. 3 TKG 2021 has to be obtained.