Use a proxy question when you suspect that your customer or employees might try to tell you only what they think others would say is the right answer. A proxy question is a safe substitute to the real question we want to ask.
Many times we have been culturally conditioned to say one type of answer, whether or not that answer reflects our real belief or behavior. In order to get the right answer, you have to ask a roundabout question that will give you the same end-result. This requires you to be in-tune with the culture around you so if you are not a native, you may want to cross-check with someone who can tell you the right way to ask a delicate question. Remember, the purpose of a survey is to discover what your customers or employees are really thinking and doing and not just what they are saying.
A proxy question requires you to use your intuition and to be in tune with the spirit of the times. Be creative in addition to methodical in the crafting of your questions. Like a puzzle, there are different ways you can approach putting together an accurate picture of your customer's thinking. As you become more in tune with your business and customers, your survey questions will become more and more effective.
For example: if you are trying to find out how often your customers eat out at fast food restaurants, you might get misleading answers if you ask the question directly. Many feel guilty about eating at fast food restaurants and so will often be evasive in answering. Instead of asking of asking "For your evening meal, do you prefer eating at a fast food restaurant?" you would want instead to ask something similar to this: "For your evening meal, how many times in the last week did you eat at a fast food restaurant."