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.''
Reliability measurements indicatethe degree to which scores of a clinicaltest are free from measurementerrors," and although conceptuallystraightforward, the application ofthis notion can be complex."'^ Reliabilitycan be expressed as relativereliability or as absolute reliability. Ifa measurement has high relative reliability,this indicates that repeatedmeasurements will reveal consistentpositioning or ranking of individuals'scores within a group. ' ' If a measurementhas high absolute reliability,this indicates that, upon repeatedmeasurement, scores show little variability."Relative reliability is measuredwith correlation coefficients.The intraclass correlation coefficient(ICC) evaluates correlation basedupon variance estimates from analysisof variance'-"*; the more commonthe variance between sets of measurements,the higher the ICC. '^ TheICC is an appropriate statistic for examiningtest-retest reliability." As ageneral guideline, an ICC above .75is considered to demonstrate goodreliability; for clinical measures, it issuggested that reliability should exceed.90 to ensure reasonablevalidity.''
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