Paola - You may be looking more in the direction of sampling your customer base (what's a good number of surveys to send out at a time, how often and to whom). We have set up a program sampling 20 customers a month. They are chosen at random and once questioned, they are excluded from the selection base. Our questions are numerically scored by the customer from 1 to 10. Additional spaces are provided for comments. Once returned, the surveys are entered into a spreadsheet and average numerical values are charted for each month on each of the questions. They are then reviewed in total and compared with the National Customer Satisfaction Index. All returned surveys are analyzed by management.
Be sure the survey is sent to someone who can give you valid and objective answers. Good luck.
Dave