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Gait Deviation Index
Please see Schwartz & Rozumalski, 2008 for more information about the instrument: Gait Deviation Index
Supplemental: Cerebral Palsy (CP)
Short Description of Instrument

The Gait Deviation Index (GDI) is a quantitative summary measure of gait impairment obtained from 3-dimensional gait analyses. It considers the individual's gait deviations compared to a control database on 15 different kinematic variables. It was developed to address limitations in the Gillette Gait Index, and is very similar in result to the later developed Gait Profile Score (GPS) (Rasmussen et al., 2015). The GPS uses the root mean square difference on the same variables rather than the scaled distance difference used to calculate GDI. The GDI is a reliable measure and has been validated in several patient populations with gait disorders.

Comments/Special Instructions
Kinematic data from 3-dimensional gait analysis required to calculate. Calculation is open-source (Schwartz and Rozumalski, 2008).
Administration: Standard 3-dimensional gait analysis with processing of lower extremity kinematic data.
Scoring: See Schwartz and Rozumalski, 2008 for calculation.
Type of information, resulting from testing (e.g., standard scores, percentile ranks): The GDI gives a quantitative dimensionless index ranging from 0 to>100. GDI = 100 indicates an individual whose gait is at least as close to the typically developing average as that of a randomly selected typically developing individual. In  other words, a GDI of 100 or higher indicates the absence of gait pathology. Every 10 points that the GDI falls below 100 corresponds to one standard deviation away from the typically developing mean. So, for example, GDIa = 75 means that the overall gait of subject a is 2.5 standard deviations away from the typically developing mean.
The GDI has been found to be repeatable in children with CP (Maassad et al., 2014), related to clinical measures of strength and spasticity in children with CP (Sagawa et al., 2013), and related to Gross Motor Function Classification System level (Malt et al., 2016).
Schwartz MH, Rozumalski A. The Gait Deviation Index: a new comprehensive index of gait pathology. Gait Posture. 2008;28(3):351–357.
Additional references:  
Malt MA, Aarli Å, Bogen B, Fevang JM. Correlation between the Gait Deviation Index and gross motor function (GMFCS level) in children with cerebral palsy. J Child Orthop. 2016;10(3):261–266.
Massaad A, Assi A, Skalli W, Ghanem I. Repeatability and validation of gait deviation index in children: typically developing and cerebral palsy. Gait Posture. 2014;39(1):354–358.  
Rasmussen HM, Nielsen DB, Pedersen NW, Overgaard S, Holsgaard-Larsen A. Gait Deviation Index, Gait Profile Score and Gait Variable Score in children with spastic cerebral palsy: Intra-rater reliability and agreement across two repeated sessions. Gait Posture. 2015;42(2):133–137.
Sagawa Y Jr, Watelain E, De Coulon G, Kaelin A, Gorce P, Armand S. Are clinical measurements linked to the gait deviation index in cerebral palsy patients? Gait Posture. 2013;38(2):276–280.