The purpose of this study is to design a learning trajectory (develop mathematics reasoning skills) for junior high school statistics material to be used in eighth-grade classrooms. This design research study comprises three stages: preliminary design, experimental design and Retrospective Analysis. The subjects in this study were eighth-grade learners in a public junior-high school in Wonogiri City. Data collection methods included observation, interview and tests. Based on the results of the retrospective analysis, a learning trajectory for statistics material with the traditional Congklak game in application combines both informal and formal activities. In informal activities, through the Congklak game experiments conducted. These helped to form, mode, medium, examination, quartiles and semi-interquartile range interpretation concepts. In formal activities, based on the experimental results learners decided how to define mean (average), mode (the number of times a value appears in given data), medium and quartiles. The statistical results show that there are differences in mathematics reasoning skills between teaching with a learning trajectory assisted by Congklak and the direct method. The marginal mean for learning path is 71.57, while it is 62.66, and this outside the margin of error. This suggests that by application of a realistic mathematics approach using learning path driven Congklak traditional game of so can improve learners' mathematics reasoning ability.

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