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.