Exercise: Reliability of balance performance measures in chronic stroke patients, answer 5

Question 5: Why are the ICCs for BBS and PASS very similar, but the SEM and SRD are quite different?

Suggested answer:

The ICC does not depend on the units in which the measurement is recorded. If we multiplied all the PASS scores by 10, the ICC would be unchanged. BBS and PASS are two similar scales, recorded in the same way, and so we should not be surprised that they have similar properties.

The SEM and SRD are absolute measures of measurement error, reported in the same units as the orignal measurement. If we multiplied all the PASS scores by 10, the SEM and SRD would also be multiplied by 10.

The BBS has 14 items scored 0 to 4, range 0 to 56. The PASS has 12 items scored 0 to 3, range 0 to 36. Hence BBS will produce bigger and more variable numbers than the PASS. As a result, the SEM for the BBS is larger than that for the PASS.

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