The purpose of this research was to investigate how the degree of implementation of school based assessment schemes for practical work in science can be systematically conceptualised and then measured. The Teacher Assessment Scheme for the advanced level practical chemistry in Hong Kong was used as the context for the present research. To construct a theoretical base for measuring degree of implementation of that scheme, five dimensions of implementation were identified. These five dimensions are related to logistic arrangements, use of assessment activities, the quality of the relationship between assessment, teaching and learning, knowledge of the characteristics of the assessment scheme, and attitude towards school based assessment. A conceptual framework was built by breaking down each dimension into attributes and these into content areas. The research methods used in the present study incorporated some modifications on most previous designs in order to resolve the two methodological issues of data validity and method specific biases. Based on the conceptual framework, three measuring instruments (namely, a student questionnaire, a teacher questionnaire and an observation schedule) were constructed. A total of 1377 students and 63 teachers participated in the survey, and a subsample of teachers was selected for direct observations. The construct validity of student survey data on the degree of implementation was demonstrated through confirmatory factor analysis and multitrait multimethod analysis. A total of ten attributes of implementation were found with the five conceptual dimensions. Triangulation of students' perceptions, teacher self reports and independent observational data resulted in convergent findings for three attributes of implementation and inconsistent findings for the other seven attributes. Plausible explanations for the convergent and inconsistent findings are proposed, considering possibilities such as student bias, teacher bias, situational factors and accuracy of direct observation data. This study contributes to curriculum implementation research because new knowledge is available to systematically measure the degree of implementation of school based assessment schemes for practical work in science. The process of conducting the present research (eg. Identification of the dimensions of the construct degree of implementation. Development of the conceptual framework, data collection, data analysis) represents a new approach to measurement of curriculum implementation and may be applied to measure the degree of implementation of other curriculum innovations.