The purpose of this article is to describe the processes followed to develop an authentic, reliable and valid play-based assessment of communication-related behaviours in young children with severe disabilities. The Daily Multiple Measurement Instrument (DMMI) was developed to be used based on an intervention package of play activities. The process of operationalisation is discussed where observable and measurable behaviour indicators were identified. Face and content validity were established by using experts to evaluate the scope and grading of observable behavioural indicators. Construct and convergent validity were investigated through empirical research. Data were collected from five intellectually impaired participants over an eight-week period of pre-intervention measurements, treatment, post intervention measurements, withdrawal, and post-withdrawal measurements. Two other authentic measurement instruments were used to investigate convergent validity.