Validation of short term recall of mobile phone use for the Interphone study.

AIM To validate short term recall of mobile phone use within Interphone an international collaborative case control study of tumours of the brain acoustic nerve and salivary glands related to mobile telephone use METHODS Mobile phone use of 672 volunteers in 11 countries was recorded by operators or through the use of software modified phones and compared to use recalled six months later using the Interphone study questionnaire Agreement between recalled and actual phone use was analysed using both categorical and continuous measures of number and duration of phone calls RESULTS Correlations between recalled and actual phone use were moderate to high ranging from 0 5 to 0 8 across countries and of the same order for number and duration of calls The kappa statistic demonstrated fair to moderate agreement for both number and duration of calls weighted kappa ranging from 0 20 to 0 60 across countries On average subjects underestimated the number of calls per month geometric mean ratio of recalled to actual 0 92 95 CI 0 85 to 0 99 whereas duration of calls was overestimated geometric mean ratio 1 42 95 CI 1 29 to 1 56 The ratio of recalled to actual use increased with level of use showing underestimation in light users and overestimation in heavy users There was substantial heterogeneity in this ratio between countries Inter individual variation was also large and increased with level of use CONCLUSIONS Volunteer subjects recalled their recent phone use with moderate systematic error and substantial random error This large random error can be expected to reduce the power of the Interphone study to detect an increase in risk of brain acoustic nerve and parotid gland tumours with increasing mobile phone use if one exists