Assessing the Accuracy and Consistency of Formative Assessment Instruments in Physics: A Validity and Reliability Study

This study examines the validity and reliability of a formative assessment instrument designed to measure students’ conceptual understanding in physics. Using a psychometric validation approach, data were collected from 220 secondary school students who had studied wave-related topics. The instrument consisted of 12 essay items. Construct validity was tested using exploratory factor analysis (EFA) and confirmatory factor analysis (CFA), while reliability was assessed using McDonald's omega and Cronbach's alpha. The results show that the model meets factor analysis assumptions and demonstrates acceptable fit. Reliability coefficients exceed 0.90, indicating strong internal consistency and supporting the instrument's use in formative physics assessment.