Critique the following methodologies and suggest more appropriate alternatives:
a. A supermarket was interested in determining its image. It dropped a short questionnaire into the grocery bag of each customer before putting in the groceries.
b. To assess the extent of its trade area, a shopping mall stationed interviewers in the parking lot every Monday and Friday evening. Interviewers walked up to persons after they had parked their cars and asked them for their zip codes.
c. To assess the popularity of a new movie, a major studio invited people to call a 900 number and vote yes, they would see it again, or no, they would not. Each caller was billed a $2 charge.