Reliability, sensitivity to change, and responsiveness of the Peabody Developmental Motor Scales--Second Edition for children with cerebral palsy.(Research Report)(Clinical report)

During the past 20 years, physical therapists have had considerable interest in the development and evaluation of health status outcome measures. (1) Outcome measures are used by researchers and clinicians to assess changes in patients' abilities before and after health care to promote the accountability of health care services. (2) Outcome measures must have the psychometric properties of reliability and responsiveness. (3-5) The low intrasubject variation in stable subjects reflects the test-retest reliability of a measure. (3) Only measures with high test-retest reliability can detect real change and reduce the bias caused by measurement error. The responsiveness of a measure is defined as the ability to assess clinically important change over time. (6) Thus, evidence supporting the test-retest reliability and responsiveness of an outcome measure must be established before its use in research or clinical settings.

Cerebral palsy (CP) describes a group of disorders of the development of movement and posture, causing activity limitation, that are attributed to nonprogressive disturbances that occurred in the developing fetal or infant brain. (7) To evaluate the effectiveness of treatment for the motor domain, clinicians need a motor evaluative tool. The Gross Motor Function Measure (GMFM) (8) and the Peabody Developmental Motor Scales (PDMS) (9) are the 2 most well-known motor instruments for children with CP. However, the GMFM measures the gross motor domain only. (8) For measurement of the fine motor domain, the GMFM is inadequate as an evaluative tool.

A previous responsiveness study with the gross motor (GM) composite of the PDMS (PDMS-GM) for infants with CP showed that the PDMS-GM had limitations when used as an evaluative measure for infants with CP. (10) The PDMS has been revised to the Peabody Developmental Motor Scales-Second Edition (PDMS-2), with new norms, revised testing materials, more precise scoring criteria, and more information on norm samples. (11) Each item of the PDMS-2 was evaluated with both conventional item analyses and modern differential item functioning analyses to select the appropriate items. New normative data on the PDMS-2 were collected through 1997 and 1998 for a sample of 2,003 children residing in the United States and Canada; not only children without disabilities but also 10% of children with various types of disabilities were included in the sample. There are also more reliability and validity data for the PDMS-2 than for the PDMS. (11) Therefore, the PDMS-2 is potentially appropriate for investigating the progress of the gross and fine motor domains for children with CP because it assesses both GM and fine motor (FM) composites and incorporates both quantitative and qualitative rating criteria.

The concurrent validity studies of the standard scores on the PDMS-2 showed high correlations with the PDMS or the Mullen Scales of Early Learning: AGS Edition in the GM or the FM composite (r = .80-.91) for children for whom detailed information on health conditions was not available. (11) For children with developmental delays, although the developmental quotients (DO) of the PDMS-2 were significantly correlated with the Bayley Scales of Infant Development-Second Edition, the classification agreement between these 2 tests was poor. (12) The construct validity of the PDMS-2 was established by confirmatory factor analyses, and the results showed that the GM and the FM composites are 2 separate constructs within general movement. Another construct validity study of the PDMS-2 demonstrated high correlations between age and subtest raw scores. (11) One recent study (13) showed that the overall diagnostic accuracy of the PDMS-2 was high, with an area under the receiver operating characteristic curve of 0.98 for children with motor disabilities. These results indicate that clinicians could diagnose motor disabilities correctly 98% of the time with the test results of the PDMS-2. (14) One of the purposes of the PDMS-2 is to evaluate a child's progress after intervention. (11) However, norm-referenced motor assessments should not be used as evaluative measures until they are validated to have acceptable responsiveness for children with motor dysfunction. (4) Because the responsiveness of the PDMS-2 for children with CP is still unknown, one purpose of this study was to investigate the responsiveness of the PDMS-2 for children with CP.

The reliability of PDMS-2 scores was investigated by the test developers. In their study, 3 types of error variance--internal consistency, test-retest reliability, and interscorer reliability--were investigated. All of the reliability coefficients for 3 composites and 6 subtests of the PDMS-2 (Cronbach [alpha] = .89-.97, test-retest r = .82-.93, and interscorer r = .96-.99) showed that the PDMS-2 is a reliable tool for the assessment of motor development in children. (11) However, only children without disabilities were recruited for that reliability study. Because the reliability levels may vary for different populations, (15) the reliability of PDMS-2 scores for children with CP needs further investigation. The test-retest reliability coefficients are often thought of as stability coefficients; however, they do not reveal how much variability should be expected on the basis of measurement error. (16) Thus, for estimating the confidence intervals of test scores, the standard error of measurement (SEM) of the PDMS-2 for children with CP needs to be calculated. Therefore, the other purpose of this study was to examine the test-retest reliability and SEM of the PDMS-2 for children with CP.

Method

Participants