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If you want to know what people are thinking, ask them directly. I have no info on what kind of business you're in or how you regularly interact with your customers, but you do indicate that you offer a service. So after customers have used your services, you could mail them an offer for additional work at a discount when they complete your follow-up questionnaire. Or, if you have employees that perform the work, you could follow up by phone to get customer feedback after each job is completed. You can also bring together a group of your customers--offer snacks and beverages, perhaps a monetary incentive--for an informal round table discussion of your services. Another idea is to have a message board on your website and monitor customer input.
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