Reliability measurements indicate
the degree to which scores of a clinical
test are free from measurement
errors," and although conceptually
straightforward, the application of
this notion can be complex."'^ Reliability
can be expressed as relative
reliability or as absolute reliability. If
a measurement has high relative reliability,
this indicates that repeated
measurements will reveal consistent
positioning or ranking of individuals'
scores within a group. ' ' If a measurement
has high absolute reliability,
this indicates that, upon repeated
measurement, scores show little variability."
Relative reliability is measured
with correlation coefficients.
The intraclass correlation coefficient
(ICC) evaluates correlation based
upon variance estimates from analysis
of variance'-"*; the more common
the variance between sets of measurements,
the higher the ICC. '^ The
ICC is an appropriate statistic for examining
test-retest reliability." As a
general guideline, an ICC above .75
is considered to demonstrate good
reliability; for clinical measures, it is
suggested that reliability should exceed
.90 to ensure reasonable
validity.''