The ???smallest real difference??? as a measure of sensitivity to change: a critical analysis

Abstract

If assessment instruments are used to measure efficacy or effectiveness, for example of rehabilitation programmes, they have to be sensitive (or responsive) to change. However, up to now there has been no consensus on what coefficient should be used for this end. Various effect sizes and criterion measures are widely acknowledged; tests of inferential statistics are also still used. The different coefficients may well provide different rank ordering of competitive instruments. Recently, the so-called 'smallest real difference' (SRD) was proposed as a measure of sensitivity to change. In the original formulation, the SRD was defined as the 95% confidence limit of the standard error of measurement (SEM) of the difference scores. Conceptually this is equivalent to what is known as the 'reliable change index' in psychotherapy research. The absolute values of the SEM/SRD indicate measurement error. In our view, this is merely complementary to the reliability concept and not a measure of sensitivity to change. Instead, we suggest using the percentage of patients reaching the SRD criterion to compare the sensitivity to change of competitive instruments. In contrast to other sensitivity-to-change indices, such an approach takes the different reliabilities of competitive assessment instruments explicitly into account.

Measuring community integration after spinal cord injury: validation of the Sydney psychosocial reintegration scale and community integration measure

“…Because we expected improvement in community reintegration due to natural recovery and/or potential additional benefit from the intervention, one-tailed tests of significance was used, and thus z MD = 1.64. The percentage of participants meeting the MD was calculated to compare the three instruments directly in terms of sensitivity [24,28].…”

Challenges for defining minimal clinically important difference (MCID) after spinal cord injury

“…A valid MCID cannot be less than the MDD and preferably should exceed the value of the observed MDD. 19,21,22 The few continuous SCI outcome scales, such as timed or distancemeasured assessments of walking, can be used as possible clinical end points, but this is limited to a subset of study participants who have some independent ambulatory capability. The measures include the 10-m walk, the 2-or 6-min walk or the timed up and go.…”

Relationship Between Motor Recovery and Independence After Sensorimotor-Complete Cervical Spinal Cord Injury

“…The objective of this study was not to determine MCID. However, the MCID for neurological recovery has been repeatedly suggested as being at least equal to the smallest real difference (SRD) [21][22][23] : A change of 2 motor points has been previously reported as the SRD for UEMS. 24 Based on a reliability coefficient of 0.95, an estimate of the SRD at 48 weeks of those 74 individuals examined in this study is maximally 3 motor points.…”