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PHYS THER
Vol. 88, No. 7, July 2008, p. 891
DOI: 10.2522/ptj.2008.88.7.890
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Letters and Responses

Author Response


Using a type 2,1 intraclass correlation coefficient (ICC) rather than a type 3,1 ICC changed the ICCs for 13 of the 24 tests less than one hundredth of a point. An ICC(2,1) increased the reliability coefficients for the Berg Balance Scale and the Sharpened Romberg Test with eyes open, reducing the minimal detectable change (MDC) scores by 1 point each. ICC(2,1) decreased the remaining ICCs by one hundredth of a point, which increased the MDC scores of 6 tests by 1 point; 2 showed no change, and the Six-Minute Walk Test (6MWT) increased to 86 meters. Dr Stratford was sent the gait speed data to utilize his suggested ICC(2,2) formula for tests that incorporated averaged scores. This ICC formula is not available in the SPSS software we utilized. The analysis did not change the ICC values or the MDCs for the gait speed tests. Our article states that gait speed is the strongest gait outcome variable in the population with parkinsonism, and Stratford's analysis supports this.

We understand Stratford's suggestion on ICCs that test-retest reliability should always use ICC(2,k) formula. However, the article by Shrout and Fleiss1 did not suggest an ICC formula for test-retest reliability, and changing the ICC formula had little effect on our study. Considering the same rater performed the same test each session, the formula for intrarater reliability ICC(3,k) was used. We appreciate Stratford's correction that arose from our report of the 6MWT being the only test to demonstrate a small learning effect.

The incorrect use of the ICC formula can affect test-retest reliability when a systematic error occurs. The Table reports ICC(3,k), ICC(2,k), and minimal detectable change values using a 95% confidence interval (MDC95) for all the tests. Eleven MDC95 values had no change, 6 decreased, and 7 increased utilizing ICC(2,k) rather than ICC(3,k).


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  		Table. Intraclass Correlation Coefficients (ICC) for Test-Retest Reliability and Minimal Detectable Change Scores Utilizing a 95% Confidence Interval (MDC95) for Functional Tests, a Quality-of-Life Measure, and Disease Severity Rating Scale in People With Parkinsonisma

 

Teresa M Steffen and Megan Seney

TM Steffen, PT, PhD, is Professor in Physical Therapy at Concordia University Wisconsin, Mequon, WI.


  Footnotes
 
This letter was posted as a Rapid Response on June 3, 2008, at www.ptjournal.org.

Reference

  1. Shrout PE, Fleiss JL. Intraclass correlation: uses in assessing rater reliability. Psychol Bull. 1979;86:420–428.[CrossRef][Web of Science][Medline]

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This Article
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Google Scholar
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PubMed
Right arrow Articles by Steffen, T. M
Right arrow Articles by Seney, M.
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