Women in the study have been asked twice about
mobile phone use. In a survey conducted between
1999 and 2005 (to which about 65% of women recruited
in 1996–2001 replied), women were asked:
‘About how often do you use a mobile phone?’, and
given three options to respond: ‘never’, ‘less than once
a day’, ‘every day’; and ‘For how long have you used
one?’ (participants were asked to provide total years
of use). The responses to these questions provided
baseline exposure data for analyses. In 2009, study
participants were asked ‘How much do you talk on
a mobile phone?’ (average minutes per week) and
‘How long have you used a mobile phone?’ (in
years). This information is currently available for a
random sample of 31 110 women who had also responded
to the questions on mobile phone use at
baseline, and although it was not used to define exposure
status for analyses,