This problem has been known for some time. Other factors compromising phone sampling include: - multiple phone lines which increase the probability of selection of those who have them (i.e. those who can afford them are more likely to be oversampled than those who cannot); -telemarketer fatigue - the intrusiveness of telemarketing produces high levels to refusals to participate in any kind of survey; again the probability of refusal is non-randomly distributed, which results in oversampling and undersampling of different groups
Wojtek