A general rule of thumb is that solid scientific instruments should have a Cronbach’s Alpha of at least .7.
There are exceptions to this rule in the case of brief measurements when breadth of content is of primary interest in recapturing a longer scale (see example here).
We report the development and validation of a continuous measure of general patient condition that is independent of diagnosis, and that can be used for medical-surgical as well as critical care patients.
A survey of Electronic Medical Record data identified common, frequently collected non-static candidate variables as the basis for a general, continuously updated patient condition score.
In this example, the overall reliability statistic is .732.
The analysis also elucidates the efficacy of each individual item by reporting information such as corrected item-total correlation and Cronbach’s Alpha if an item were deleted.
Reliability is the degree to which an instrument consistently measures a construct -- both across items (e.g., internal consistency, split-half reliability) and time points (e.g., test-retest reliability).