If you are talking to other providers of questionnaires, look for statements in their materials or on their site about the reliability of the questionnaires they are selling. Look in their "Frequently Asked Questions". If you can actually find someone to ask, listen carefully to their answers. Here are some things to look for and questions you can ask:
If you buy an unreliable questionnaire, it's worse than buying an unreliable car. At least with a car, you know when it is not running. A questionnaire, on the other hand, will always produce numerical results, even if they're meaningless. You could be making business decisions based on survey results that don't mean anything. Only a test of reliability can tell you if you should trust the results.
Non-technically speaking, a reliable questionnaire is one that that would give the same results if you used it repeatedly with the same group. That may sound funny because most organizations don't administer a questionnaire to the same group twice. But if they did, they would learn how reliable their questionnaire is, because a reliable survey will give the same results on Tuesday as it did the previous Monday. Instead of doing it twice, statisticians have devised tests of reliability for questionnaires. These tests let us know whether the results are meaningful.
Reliability is a property of the measuring instrument. If you are like many people, you probably get on your bathroom scale in the morning, look at the weight displayed, then step off, and do it again. You have learned that what is displayed by a bathroom scale the first time is not always exactly the same as the second, but it is usually very close.
What if one morning you weighed yourself, then a second time, and the second weight displayed was 5 lbs. heavier than the first? You would probably step off, then weigh yourself a third time. What if it was now 4 lbs. lighter than the first? Would you still be concerned about your weight? Or would you be more concerned about finding out what's "wrong" with the scale? What's wrong is that your scale has become unreliable. You can see unreliability by repeatedly measuring the same thing. And when you know the scale is unreliable, you don't even try to measure your weight, you concentrate on fixing the scale first.
Only when you know your questionnaire is reliable, can you begin to discuss validity.
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