Functional Behavior Assessment (FBA) and behavior interventions have been effective in the management of challenging behavior among children with developmental disabilities including autism spectrum disorders. Research suggests the need for valid measurement instruments for verifying, calibrating and scoring competence in FBA and behavior interventions. The validation for the Ability in Behaviour Assessment and Interventions for Teachers (ABAIT) adopted the Delphi method for developing consensus followed by the application of Rasch Measurement Model (RMM). RMM among 292 special educators reported appropriate infit (0.84–1.11), outfit (0.94–1.05), and item separation reliability (0.99), though some items reported low point-biserial correlation. The ABAIT was developed with expert consensus and was found to have a suitable fit with RMM.