Item scoring is the same for the GMFM-88 and the GMFM-66. There is a scoring system with each item scored as 0, 1, 2, 3, or “not tested”. A scoring key of 0 – does not initiate, 1 – initiates, 2 – partially completes, and 3 – completed, is used; however parameters such as distance, time, support provided, accuracy, counts, and tasks will determine specific item scores. Items that a child refuses to attempt despite reason they may be able to perform at least partially or items not administered are scored as ‘not tested’. It is important to use the specific starting positions and detailed scoring guidelines outlined in the Gross Motor Function Measure (GMFM-66 & GMFM-88) User’s manual 2nd Edition (2013) for accurate and reliable testing. It can be ordered through Wiley Publishing (MacKeith Press). Cost information is available from the publisher.
Item scores are summed to calculate raw and percent scores for each of the five GMFM-88 dimensions. Dimension percent scores are averaged to obtain an overall total score. Not tested items in the GMFM-88 are scored as ‘0’.
Scoring the GMFM-66 requires the use of a computer program called the Gross Motor Ability Estimator (GMAE). Individual item scores are entered and a mathematical algorithm calculates an interval level total score. The total score is an estimate of the child’s gross motor function. The GMAE provides a standard error and a 95% confidence interval (CI) around the obtained GMFM-66 total score. For example, a child might obtain a total score of 42, with a 95% CI of 39 to 44. This means the estimate of gross motor function on the day of testing was 42 with 95% confidence that the child’s true score was somewhere between 39 and 44. The GMAE will consider not tested items as missing information and estimate the child’s score on the not tested items based on the child’s pattern of responses to other items.
The GMAE-2, GMAE-3, and GMFM App
There have been three versions of the GMAE. The GMAE-2 was the second version. It continues to be downloadable from the CanChild website but may not compatible with some computer systems.
The GMAE-2 imputed a raw score based on the items tested and then converted this to a GMFM-66 score. The updated GMAE-3 algorithm computes the GMFM-66 scores directly from the item calibrations (item difficulty scores) that a child is tested on. The difference between the GMAE-2 score and the GMAE-3 score will generally be quite small. The GMAE-3 algorithm reflects a slightly more accurate measure of motor function than the previous algorithm for children with higher and lower functioning and should be the preferred measure moving forward. It is possible to convert GMAE-2 to GMAE-3 scores. The GMAE-3 is available only in the GMFM App.
The GMFM App is an application version of the GMFM-88 and GMFM-66 software which provides the following:
- Ability to enter item scores for the GMFM-88, GMFM-66, GMFM Item Sets, and GMFM Basal & Ceiling
- Calculations of GMFM-88 dimension and total scores
- Calculations of GMFM-66 scores calculated using the GMAE-3, the most current version of the scoring algorithm.
- Data displays including
- Spider charts of GMFM-88 scores
- Individual items scores and GMFM-66 scores with 95% Confidence Intervals on Item maps
- GMFM-66 scores on Motor Growth Curves to illustrate patterns of motor development classified on the Gross Motor Function Classification System levels
- Access to on-line tutorial
- Multi-User and Single-User versions for both PC and Mac computers plus an Android mobile version. (link to store coming soon)
Interpretation of Change in GMFM Scores
Due to the variation among children with cerebral palsy, the magnitude of change that is considered clinically important for an individual child will vary, and will depend on the judgments made by the child, family, and therapist. In the original validation work with the GMFM-88, parents and therapists identified a gain of approximately five and seven percentage points respectively, as being a “medium” positive change.
Data from the GMAE scoring program can be used to determine if change has occurred that is greater than measurement error. If the 95% confidence intervals (CI’s) between the first and second test occasion overlap, the difference in the scores may be due to measurement error. If they do not overlap, the difference can be interpreted as true change. The overall magnitude of change in GMFM-66 score that is considered “clinically important” has not been determined scientifically.
The Gross Motor Function Measure User’s Manual has an appendix of average GMFM-88 and GMFM-66 change scores for children of varying ages and GMFCS levels over six and twelve month intervals at children’s rehabilitation centres in Ontario, Canada.
Motor Growth Curves
A set of five Motor Growth curves was developed from longitudinal GMFM-66 data from a sample of over 650 children with cerebral palsy from Ontario. These curves describe the patterns of motor development of children in each Gross Motor Function Classification System (GMFCS) level and are similar to the growth charts that are used to follow the height and weight of children as they grow. By plotting GMFM-66 scores by GMFCS level, estimations regarding future motor capabilities for children with cerebral palsy can be illustrated. Because GMFCS level and age are the major determining factors of gross motor function in children with cerebral palsy, knowledge of these two factors can enable one to plot a child’s GMFM-66 score over time, and to see how close the measured GMFM-66 score is to the ‘expected’ (average) motor growth curve value for other children of the same GMFCS level. Information can assist in predicting future motor capabilities and setting realistic gross motor goals. It is important to remember that the ‘line of best fit’ on the motor growth curves is not the actual ‘true’ value of every child in that GMFCS level because there is variation in patterns of gross motor function among children within each GMFCS level.
Item maps illustrate GMFM item difficulty estimates. They can be displayed by item number order or item difficulty order. They provide information on what a child has accomplished, which items are close to the child’s ability level, and which are emerging skills.
GMFM-66 Tabulated Reference Percentiles
Tabulated reference percentiles are available for use with the GMFM-66 to assess children with cerebral palsy. The percentiles are presented by GMFCS levels. They are derived from a longitudinal sample of 1940 GMFM-66 assessments of 650 children with cerebral palsy, 2-12 years of age. This stratified (by age and GMFCS levels) random sample was collected from among all children with cerebral palsy who were receiving a variety of developmental therapies and services at publically funded children’s rehabilitation centres in Ontario, Canada, between 1996 and 2001.
The development and appropriate use of these percentiles is described in Hanna SE, Bartlett DJ, Rivard LM, Russell DJ. (2008) Reference curves for the Gross Motor Function Measure: Percentiles for clinical description and tracking over time among children with cerebral palsy. Physical Therapy 88(5) 596-607. Doi 10.2522/ptj.20070314