Each method has its own associated cost. Telephone interviewing and mailing are more expensive than the others. The Five Don'ts of Questionnaire Design 1. Don't ask too many open-ended questions. Keep them to a maximum of three.
Many respondents skip such questions.
When conducted over the phone, you would just get dead air. Consumer behaviour is in different shades of grey. You use research to find these shades. Don't combine two questions in one. Don't ask factual questions that require respondents to look up records and invoices to report exact amounts.