Critique the following methodologies and suggest more appropriate alternatives:
a. A supermarket is interested in determining its image. Cashiers drop a short questionnaire into the grocery bag of each customer prior to bagging the groceries.
b. To assess the extent of its trade area, a shopping mall stations interviewers in the parking lot every Monday and Friday evening. After people park their cars, interviewers walk up to them and ask them for their zip codes.
c. To assess the potential for new horror movies starring alien robots, a major studio invites people to call a 900 number and vote yes if they would like to see such movies or no if they would not. Each caller is billed a $2 charge.