Internal Consistency

Authors
Affiliations

Nate Yomogida, B.S., SPT

Doctor of Physical Therapy

B.S. in Kinesiology

Chloë Kerstein, B.S., SPT

Doctor of Physical Therapy

B.A. in Neuroscience

Internal consistency refers to the degree to which all of the items on a scale are correlated with one another.

Significance

  • Similarly, if the items of a scale are not related to each other, we would not be sure just what it is that the scale is measuring (assuming that the scale is measuring a homogeneous construct).

When to Assess Internal Consistency

This is important for scales that measure 1 phenomenon:

Example

such as quality of life or depression

Internal consistency is not appropriate for indices that measure things that are not expected to be correlated with each other

Example

The Apgar scale consists of 5 items (heart rate, respiration, muscle tone, reflex response, and skin color). These may all occur together in healthy neonates, but in sick children a low score on one item does not necessarily mean there will be low scores on another item.

Measures

Interpretation

  • You want internal consistency results (i.e. Cronbach’s Alpha (α)) to be high, we do not want it to be too high.
  • An (α) greater than about 0.90 may indicate unnecessary redundancy among the items and means that some of them can be eliminated without jeopardizing the reliability of the scale.
What if my scale has low internal consistency?

If the scale you are using has already been developed and validated, you cannot change much with respect to internal consistency

  • Changing the items makes it impossible to compare to studies on its psychometric validity

References

1.
Streiner DL. Statistics Commentary Series: Commentary #15-Reliability. Journal of Clinical Psychopharmacology. 2016;36(4):305-307. doi:10.1097/JCP.0000000000000517

Citation

For attribution, please cite this work as: