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.
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The other members of the Interphone Study Group include: Canada: Professor Daniel Krewski, Dr Mary McBride, Dr Marie-Elise Parent, and Professor Jack Siemiatycki; Japan: Dr Naohito Yamaguchi; UK: Professor Anthony Swerdlow.
Funding sources: we acknowledge funding from the European Union Fifth Framework Program, “Quality of Life and Management of living Resources” (contract QLK4-CT-1999-01563), the International Union against Cancer (UICC), and national funding sources. The UICC received funds for this purpose from the Mobile Manufacturers’ Forum and GSM Association. Provision of funds to the Interphone study investigators via the UICC was governed by agreements that guaranteed Interphone’s complete scientific independence. These agreements are publicly available at http://www.iarc.fr/pageroot/UNITS/RCA4.html. Funding sources for the national validation studies included: Australia: National Health and Medical Research Council (EME Grant 219129), Bruce Armstrong is supported by a programme grant from the University of Sydney Medical Foundation; Finland: Emil Aaltonen Foundation, TEKES National Technology Agency, and Academy of Finland; France: French Association for Research on Cancer (ARC); Germany: Ministry for Environment, Agriculture and Consumer protection of North Rhine Westphalia; New Zealand: Health Research Council of New Zealand, New Zealand Cancer Society, Waikato Medical Research Foundation. UK: Department of Health, contract ref RRX51.
Competing interests: as stated above, the UICC received funding for Interphone from the Mobile Manufacturers’ Forum and the GSM Association, and Interphone’s complete scientific independence from these funders was guaranteed. The University of Leeds (authors: SJH, RCP, PAMcK) and the University of Lyon (author MH) received some financial support for the main Interphone study (not the validation study), from, respectively, the UK Network Operators (O2, Orange, T-Mobile, Vodafone, ‘3’) and French Network Operators (Orange, SFR, Bouygues) under legal signed contractual agreements which ensure complete independence for the scientific investigators. These funders had no involvement in the study design, data collection, statistical analysis or interpretation of the data, report writing, or decision to submit this paper. Individual authors have no competing interests to declare.
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