Operator-recorded mobile phone use

Operator-recorded duration of mobile phone use (RECORD) was collected for all participants who provided consent and had a subscription under their own name. Operator-recorded data were used only when available for all mobile phones that were reported (up to 2 phones in Denmark, the Netherlands, Sweden, Finland, and up to 3 in the United Kingdom) and for at least 3 full months at the time the baseline questionnaire was administered. Participants with mobile phones that were also used by others were excluded. Operator-recorded data were available for 21% of participants in Denmark, 76% in Finland, 40% in the Netherlands, 63% in Sweden, and 76% in the United Kingdom.

Providers collected information on number and duration separately for outgoing and incoming calls. Across countries, outgoing calls made up 55%-72% of the total number. Where the discrepancy between incoming and outgoing calls was large, this could suggest under-recording of incoming calls, possibly because calls between subscribers from the same provider were not always charged and may have gone unrecorded in some billing systems. For this reason, to evaluate the best RC method, we based our simulations on outgoing call duration only. Data for the 3 months at baseline were used to calculate average duration of calls (minutes per week).