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.
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.