The effectiveness of speech and language therapy for developmental phonological disorders has typically been measured in terms of the diminution of the application of phonological processes. Often substantial amounts of intervention and time may be needed to show up such changes. Shorter and more clinically realistic intervention regimes can result in small but significant changes in speech production which represent positive developments in the phonological system. In order to measure progress in this sort of therapy a sensitive instrument of measurement is required. In this paper we describe a research measurement tool, the probe scoring system, which was designed to detect small changes in the child's phonological system following therapy and contrast it to some of the instruments selected for use in other efficacy studies. The potential for wider clinical application is also considered.