Conversation:

I have worked in areas where market researchers have been active on the streets. I worked out that whatever they were researching was flawed for several reasons:
1) If they surveyed at lunch time, most people with full-time jobs were too busy to help;
2) If they surveyed at other times, most people on the streets did not have full-time jobs;
3) When I refused to leave my name and address they discarded my information (why waste my time by making this the last question)?

The results were only indicative for a section of the population - the majority of whom do not work full-time and do not mind getting follow-up junk mail!