摘要：Critical thinking skills are the 21st-century life skills that are needed by students. However, in elementary schools, there are no instruments that are truly effective and efficient to measure critical thinking skills. This research aims to develop an open-ended question assessment instrument to measure students’ critical-thinking skills, to test its validity, reliability, and practicality. The research method used is Research and Development with the ADDIE model by Borg and Gall (2003) includes the stage: analyze, design, development, implementation, and evaluation. Data collection techniques used were test, questionnaire, and documentation methods. In this research, there are several points to be discussed namely: 1) research and information collection, 2) design, and 3) developing prototype, and 4) preliminary field test. The results of this research are in the form of question-items validity test that are valid and reliable criteria of 8 open-ended questions. Difficulty levels include difficult, moderate, and easy. The discrimination power is quite good. The students’ and teachers’ responses to the test instrument are categorized as high. The conclusion of this research is the open-ended question instrument for evaluating students' critical-thinking skills is valid, reliable, and practical. The open-ended question instrument is an effective assessment instrument to measure students' critical-thinking skills by taking into several important aspects such as learning materials in the curriculum, characteristics of students' cognitive development, and the functioning of the instrument and good grammar.
