Internal consistency is usually measured with Cronbach's alpha, a statistic calculated from the pairwise correlations between items.
[1] A commonly accepted rule of thumb for describing internal consistency is as follows:[2] Very high reliabilities (0.95 or higher) are not necessarily desirable, as this indicates that the items may be redundant.
[3] The goal in designing a reliable instrument is for scores on similar items to be related (internally consistent), but for each to contribute some unique information as well.
An alternative way of thinking about internal consistency is that it is the extent to which all of the items of a test measure the same latent variable.
[5][6][7][8][9][10][11] The hierarchical "coefficient omega" may be a more appropriate index of the extent to which all of the items in a test measure the same latent variable.