Although critical thinking has been widely accepted as an educational goal for decades, clear evidence for the development of students' critical thinking is sparse. Critical thinking has been conceptualised as skill, knowledge, and disposition respectively in psychology, philosophy and critical theory, but which applies to education is unclear. In this paper, we argue that treating critical thinking as skill is most appropriate in education. However, there are still three competing theories of critical thinking as skill: as a single unitary skill, as a set of disparate sub-skills, and as a composite skill with several sub-skills. We evaluated the validity of these theories through conducting Rasch and correlation analyses on the results of 385 undergraduate students in Thailand on the Multi-Purpose Assessment of Critical Thinking (MPACT) test. The findings suggest that critical thinking is a composite skill with several sub-skills with implications for the teaching and testing of critical thinking.