GAS (Goal Attainment Scaling)

What the GAS measures

Goal Attainment Scaling (GAS) was developed by Thomas Kiresuk and Robert Sherman in 1968 at the Hennepin County Mental Health Service in Minneapolis and published in Community Mental Health Journal. Unlike standardized questionnaires that assess a fixed set of symptoms or behaviours, GAS is built around individualized goals negotiated collaboratively between the clinician and the patient. This patient-centered design makes it applicable across virtually any clinical population or treatment modality.

For each goal, the clinician and patient prospectively define five possible levels of outcome, anchored by behaviourally specific descriptors. The scale runs from −2 (much less than expected) through −1 (less than expected), 0 (expected outcome), +1 (more than expected), and +2 (much more than expected). Crucially, the expected level (0) is defined before treatment begins, representing a realistic and mutually agreed goal rather than the most optimistic or pessimistic possibility.

Typically 3–5 goals are set per patient. At a pre-specified follow-up point, each goal is rated on the 5-point scale and a composite T-score is calculated using a standardized formula. A T-score of 50 means the patient achieved exactly the expected level of outcome across all goals, performing above expectations yields a T-score above 50, and performing below expectations yields a T-score below 50. GAS is widely used in rehabilitation medicine, pediatric occupational treatment, neurological recovery, and person-centered mental health programs.

Rate each goal on the 5-point GAS scale. Equal weights assumed. Correlation coefficient (ρ) = 0.3.

Reference calculation only. GAS requires individual goals set collaboratively with the clinician before treatment begins.

How to administer the GAS

Goals must be set prospectively using the SMART framework and anchored to behaviourally specific descriptors before treatment begins.

Step 1, Identify Priority Goals

The clinician and patient collaboratively identify 3–5 goals that are meaningful to the patient and relevant to the treatment focus. Goals should reflect functional activities, participation, or behavioural changes rather than abstract aspirations. The patient's values and priorities guide goal selection.

Example areas: returning to work, managing daily routines independently, improving sleep hygiene, reducing avoidance behaviour, or resuming a specific social activity.

Step 2, Define the Expected Outcome (Level 0)

For each goal, the clinician and patient define what a realistic level of achievement looks like at the target follow-up date. This is the Level 0 ("expected outcome") anchor. It should be achievable with the planned treatment but not trivially easy. This prospective definition is critical, Level 0 must be set before treatment begins.

The expected level reflects clinical judgment about what is realistically achievable given the patient's current status and the planned intervention.

Step 3, Anchor All 5 Scale Levels

After defining Level 0, the team creates specific, observable behavioural descriptors for Levels −2, −1, +1, and +2. These descriptions should be mutually exclusive and measurable. Vague language should be replaced with observable behaviours (e.g., "walks 200m without rest" rather than "improved walking").

All five anchors must be defined before treatment so the rating is not influenced by hindsight or knowledge of actual outcomes.

Step 4, Rate Goals at Follow-Up and Calculate T-Score

At the agreed follow-up point (e.g., 4, 8, or 12 weeks), each goal is independently rated on the −2 to +2 scale using the pre-defined anchors. The T-score formula is applied to convert the goal ratings into a composite score. A T-score of 50 means expected outcomes were achieved. Scores above 50 indicate better-than-expected performance.

Goals may optionally be weighted (e.g., a higher-priority goal receiving a weight of 2) to reflect their relative importance to the patient.

GAS scoring and interpretation

The 5-point scale is applied to each individualized goal. The T-score formula aggregates outcomes across all goals into a single composite score.

T-Score Formula

T = 50 + 10 × Σ(wᵢ × xᵢ) / √(1 − ρ)Σwᵢ² + ρ(Σwᵢ)²

Where wᵢ = weight of goal i (equal weights = 1 each), xᵢ = score for goal i (−2 to +2), and ρ = 0.3 (assumed inter-goal correlation). The formula yields a T-score with mean 50 and standard deviation 10 when goals are met as expected.

When all goals are rated 0 (expected outcome achieved), the T-score equals exactly 50 regardless of the number of goals or weights. The T-score increases above 50 when performance exceeds expectations and falls below 50 when performance is below expectations.

Psychometric properties

GAS is a mathematical technique for quantifying goal achievement rather than a fixed questionnaire, so its measurement properties depend on how well each goal scale is written. Using Kiresuk's T-score, the aggregate is normally distributed about a mean of 50 when goals are achieved precisely, with a standard deviation of around 10 when they are over- or underachieved (Turner-Stokes, 2009). A literature review in rehabilitation notes that several GAS variants exist and urges caution with the T-score; because GAS ratings are ordinal, medians and rank tests are appropriate for analysis (Krasny-Pacini et al., 2013).

Limitations

• Depends on goal quality. Valid results need all five levels described in detail before treatment; vague or post-hoc anchors bias the score (Krasny-Pacini et al., 2013).
• No normative bands. The T-score shows performance relative to each person's own expected outcome, not against published norms, so scores are not severity categories.
• Ordinal data. Treating the T-score as interval data calls for caution (Krasny-Pacini et al., 2013).
• Not a diagnostic or symptom measure. GAS complements, rather than replaces, diagnosis and standardized symptom measures.

Measurement-Based Care in HiBoop

HiBoop does not offer the GAS; this page is reference material. HiBoop supports measurement-based care with the scales it offers, such as the PHQ-9 and GAD-7; see the assessment library.