Cronbach's alpha, also called coefficient alpha, is a statistic used to measure the internal consistency of a collection of test or questionnaire items. For 
items, let 
be the score recorded for item 
and let 
be the composite total score. Writing 
and 
for the corresponding variances,
Cronbach's alpha is
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The coefficient increases when the items have larger positive covariances relative to their individual variances. Its value depends both on the relationships among the items and on the number of items, so a high value alone does not establish that all items measure a single underlying quantity.
