Lesson planning is a core part of teachers' professional competence. Written lesson plans play a significant role in science teacher education as a preparation for demonstration lessons during the final teacher certification exam. However, the few existing scoring rubrics on lesson plans are not particularly theoretically sound and are barely tested for the validity of score interpretations. In response to the demand for transparent and applicable criteria, we developed the rubric to assess science lesson plans (RALP) to assess science teachers' lesson plan quality. We employed a mixed-methods approach: First, we present multiple sources of validity evidence (based on test content, internal structure, relations to other variables, and consequences of testing) as mainly quantitative indicators for the quality of the RALP. Based upon that, we applied the RALP to lesson plans written by preservice and trainee science teachers (N = 100) and provided a qualitative analysis of six cases to illustrate common patterns in these lesson plans. Results indicate that teacher educators consider the RALP criteria (N = 24) relevant and objectively applicable. Correlation analyses of the scores and two teacher educators' holistic quality assessment of all lesson plans provide convincing evidence that the RALP can discriminate lesson plan quality levels. Moreover, comparisons between preservice science teachers and trainee science teachers reveal that trainee teachers score significantly higher than preservice teachers, indicating that the RALP is sensitive to differences in teaching and planning experience. The application and in-depth analysis of three criteria of the RALP illustrate these differences in levels of planning quality. We discuss possible applications of the RALP in science teacher education and research in science teaching.
