Goal attainment scaling as an outcome measure in rare disease trials: a conceptual proposal for validation

C M W Gaasterland,M C Jansen Van Der Weide,J H Van Der Lee,K C B Roes

doi:10.1186/s12874-019-0866-x

Abstract

BackgroundGoal Attainment Scaling (GAS) is an instrument that is intended to evaluate the effect of an intervention by assessing change in daily life activities on an individual basis. However, GAS has not been validated adequately in an RCT setting. In this paper we propose a conceptual validation plan of GAS in the setting of rare disease drug trials, and describe a hypothetical trial where GAS could be validated.MethodsWe have used the COnsensus-based Standards for the selection of health Measurement INstruments (COSMIN) taxonomy to deduce which measurement properties of GAS can be evaluated, and how. As individual GAS scores cannot be interpreted outside the context of a RCT, the validation of GAS needs to be done on trial as well as on individual level.ResultsThe procedure of GAS consists of three steps. For the step of goal selection (step 1) and definition of levels of attainment (step 2), face validity may be assessed by clinical experts. For the evaluation of the goal attainment (step 3), the inter and intra rater reliability can be evaluated on an individual level. Construct validity may be evaluated by comparison with change scores on other instruments measuring in the same domain as particular goals, if available, and by testing hypotheses about differences between groups. A difference in mean GAS scores between a group who received an efficacious intervention and a control group is an indication of well-chosen goals, and corroborates construct validity of GAS on trial level. Responsiveness of GAS cannot be evaluated due to the nature of the construct being assessed.ConclusionGAS may be useful as an instrument to assess functional change as an outcome measure in heterogeneous chronic rare diseases, but it can only be interpreted and validated when used in RCTs with blinded outcome assessment. This proposed theoretical validation plan can be used as a starting point to validate GAS in specific conditions.

Highlights

Goal Attainment Scaling (GAS) is an instrument that is intended to evaluate the effect of an intervention by assessing change in daily life activities on an individual basis
We propose a conceptual validation plan to evaluate the measurement properties of GAS both on individual level and on a trial level, with the ultimate aim to qualify it as a validated instrument for use in drug trials for regulatory approval, meaning in the setting of a Randomized Controlled Trial (RCT)
Because it is of vital importance that goals are selected that are relevant for the intervention, we suggest that a content validity check is performed in every trial where GAS is used, for example by physicians who are able to judge what the effect may be of the intervention on the patient

Summary

Introduction

Goal Attainment Scaling (GAS) is an instrument that is intended to evaluate the effect of an intervention by assessing change in daily life activities on an individual basis. Goal Attainment Scaling (GAS) [1] is an instrument that is intended for standardized evaluation of the effect of an intervention based on individualized goals. It was originally developed by Kiresuk and Sherman in 1968 to evaluate mental health services. The setting of the goals and the definition of the goal attainment levels is decided by the patient or (when the patient is unable to do this) family and their (trained) treating physician. GAS is mainly used as an assessment instrument in (pediatric) rehabilitation [2,3,4], geriatrics research [5, 6], and psychosocial interventions [7]

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Conclusion