Reliability
Types of reliability
- Stability: test-retest. You give a test, let time pass, and
give the
*same* test again. Not some other form of it.
- Equivalence: parallel forms. You give two distinct forms (form
A and
form B) of a test. Both forms are supposed to be representative samples
of the same domain.
- Stability and equivalence: You give form A in September and
form B in
May.
- Internal consistency: how well do the individual items
correlate with
one another (like Coefficient Alpha)? The old way was split-half; now we
use KR20 and KR21 because computers can handle this easily.
- Inter-rater reliability: two observers have different
subjective
judgments of a measurement procedure; how well do they correlate with each
other?
Other important points
- If you have all high ability people grouped together, you lower the
variability, and that lowers the reliability of the test.
- The maximum reliability coefficient - like any correlation - cannot be
greater than |1.0|.
- Correlations are mean free. On two occasions, if you take the same
test, your position remains the same if there is a correlation of 1.0.
That does not mean that your exact score remains the same.
- On Laura's test - read the questions very carefully. 95% means two
sigma.
- As the reliability decreases, the SEM increases, and vice versa. It
is an inverse relationship.
Back to Statistics course
Lorraine Sherry
http://www.cudenver.edu/~lsherry/reliability.html
Updated March 15, 1997