In the linked cohort study (doi:10.1136/bmj.d6387), Frei and colleagues found no evidence that the risk of brain tumours was raised in 358 403 Danish mobile phone subscribers.1 This was also true when the cohort was restricted to people who had been subscribing for more than 10 years, when gliomas and meningiomas were analysed separately, and when tumours in the anatomical region closest to the handset were analysed.

The study has two important methodological advantages over most other studies. Firstly, it was based on a computerised cohort that was followed passively in registries, so it avoided the need to contact people. Consequently the problem of non-response and selection bias—which has been of considerable concern in studies with other designs—was eliminated. Secondly, it used digitised subscriber data obtained from the operators rather than retrospective questionnaire or interview information obtained from users. This circumvented the recall bias that is present in other studies.

One weakness, however, was that having a mobile …