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Interference can occur because of the existence of two or more information/knowledge, which is mixed. Interference is due to the similarity of information/knowledge that is close together. One of the material in SMP that allows interference is a proportion. This study aims to develop a proportion test instrument to identify students thinking interference in solving problems. The question instrument contained three questions, namely direct proportion and inverse proportion and two types of proportions in one question. The development process involves experts both in the field of mathematics and in the field of mathematics education so that instruments are valid questions. Also, the development process involved 32 seventh grade junior high school students in one of the junior high schools in Yogyakarta. The result is that the instruments developed are declared valid and can be used to identify students thinking interference in solving comparison problems. This is reinforced by the results of trials that of 32 junior high school students, six students were wrong in solving comparative problems of value, 17 students were wrong in the turnover ratio, and 14 students were wrong if both types of comparisons were present together. Of the students who experienced this error, there were more than 40% who indicated interference thinking.