Internal Consistency Reliability
Whether a test's items correlate with each other, indicating they measure the same underlying construct. Measured with Cronbach's alpha or similar indices.
Internal consistency reliability (APA Standards, 2014) measures whether all items in a test measure the same thing. High internal consistency (Cronbach's α=0.80+) means if you answer one item a certain way, you'll answer similar items consistently.
Example: A 10-item Extraversion scale should show high correlations between items. If item 1 ("I like parties") and item 5 ("I enjoy socializing") are uncorrelated, that suggests the scale is measuring different constructs or items are poorly written.
Internal consistency is not the same as validity. A scale can be highly internally consistent but not valid. For example, 10 items all asking "Do you like activities?" would have high internal consistency but low validity — they're measuring only one specific aspect, not the full construct.
The goal: internal consistency high enough (α=0.70-0.90) to suggest items cohere, but not so high (α>0.95) that items are redundant and provide no new information.
Source: Cronbach, L. J. (1951). Coefficient alpha and the internal structure of tests.