In recent years, the dual sampling frame approach has become a standard methodology to compensate for the declining coverage of traditional landline telephone surveys. This dual frame approach allows the combination of fixed-line phone samples and mobile phone samples on basis of the combined selection probabilities of each individual respondent in the landline frame and in the mobile phone frame. In the landline frame the number of people in the household is considered in order to determine the selection probability in addition to the number of telephone lines in that household. By contrast, in the mobile phone sampling frame each telephone number is assumed to reach one particular respondent only - mobile phones are considered personal devices. So far, this assumption was justified based on the available findings suggesting that in fact each mobile phone was used by one individual only. However, when taking into account that other persons - like a spouse or partner - could also answer incoming calls on the respondent‟s mobile phone a correctional factor for selection probability within the mobile phone frame would be needed.

In this paper we will question the basis assumption of the dual traditional frame approach according to which mobile telephones are individual devices only (and not household devices). US researchers already indicated that shared mobile phones could turn out to be common for certain household types when asking for it in an adequate way. Also, in one of the pretests to our Experimental Mobile Phone Panel we found supporting evidence. Based on these preliminary findings we developed a questionnaire module concerning mobile phone sharing. We asked for active sharing (the person surveyed answers calls on another person‟s mobile phone) as well as for passive sharing (other people answer calls on the respondent‟s cell phone). If sharing would become a frequent behavioral pattern in households or among other groups of people, it would have serious implications for the selection probabilities of mobile phone surveys. In our paper we will estimate the extent of this behavior among mobile phone telephone users based on data from our 2009 Experimental Mobile Phone Panel. Further on, we will identify social-demographic characteristics of people who are especially prone to sharing.