Methods
Since 1 April 1968, all Danish residents have been registered
in the central population register. At birth they are assigned a
unique personal identification number that is used in all national
registers, ensuring accurate linkage of information among these
registers.9 This system enables researchers to conduct purely
register based cohort studies for exposure data available from
respective registries, as the follow-up of vital status, migration,
and many health outcomes, in particular cancer, can be done by
computerised linkage on an individual level with an exact
calculation of person years at risk. Such a design has been used
to follow adult people with a mobile phone contract (subscribers)
for risk of disease compared with the rest of the Danish adult
population; in other words, the whole Danish adult population
was subdivided into subscribers and non-subscribers of mobile
phones and followed up for incidence of cancer and other
diseases.7 8