The reliability of survey instruments is usually assessed over items and occasions. Internal consistency estimates such as Cronbach's alpha tell us how well multiple items assess the same underlying construct or dimension. A high Cronbach's alpha means that if a person scored high on one item they also tended to score high on the other items. When you have only a single item that measures your content, reliability is usually tested over time. The item is given on two different occasions and the responses are correlated. A high correlation means that the same or similar responses were given both times and the instrument or question is relatively stable. When choosing existing measures for a survey, look at how reliability was established and the level of reliability.
It is important to establish the construct validity of survey measures. We often do this by correlating instruments with similar measures. Construct validity can also be established by predicting a specific behavior or criterion. Construct validity for existing measures is often established by being successfully used in a wide range of studies. Examining the results of the studies can tell us the various types of factors or behaviors associated with the construct assessed by this measure.
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