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Approximately 40% of mobile phone use studies published in scholarly communication journals base their findings on self-report data about how frequently respondents use their mobile phones. Using a subset of a larger representative sample we examine the validity of this type of self-report data by comparing it to server log data. The self-report data correlate only moderately with the server log data, indicating low criterion validity. The categorical self-report measure asking respondents to estimate “how often” they use their mobile phones fared better than the continuous self-report measure asking them to estimate their mobile phone activity “yesterday.” A multivariate exploratory analysis further suggests that it may be difficult to identify under- and overreporting using demographic variables alone.