What is internal consistency?

Internal consistency is an assessment of how reliably survey or test items that are designed to measure the same construct actually do so. A construct is an underlying theme, characteristic, or skill such as reading comprehension or customer satisfaction. A high degree of internal consistency indicates that items meant to assess the same construct yield similar scores. There are a variety of internal consistency measures. Usually, they involve determining how highly these items are correlated and how well they predict each other. Cronbach's alpha is one commonly used measure. To use internal consistency measures, items usually should be in a single measurement instrument and administered to a group of people on one occasion in order to avoid confounding variables.

For example, a bank manager wants to assess customer satisfaction. The manager asks customers to respond to the following three statements using a 5-point scale to denote their level of agreement:

I was very satisfied with the service.
I am likely to buy from your company again.
I am likely to recommend your company to others.

If the responses to these items are internally consistent, then there is evidence that the items measure the same construct of customer satisfaction. Conversely, if the items are not well written, customers might interpret them differently and the responses will vary from item to item.