Prospective graduate students are usually required to have attained an undergraduate degree in a related field and high prior grades to gain admission. There is consensus that some relatedness between the students' undergraduate and graduate programs is required for admission. We propose a new measurement for this relatedness using cosine similarity, a method that has been tried and tested in fields such as bibliometric sciences and economic geography. We used this measurement to calculate the relatedness between a student's undergraduate and graduate program, and tested the effect of this measure on study success. Our models show that there is an interaction effect between undergraduate grades and cognitive relatedness on graduate grades. For bachelor students with high cognitive relatedness, the relationship between bachelor grades and master grades is about twice as strong compared to bachelor students with low cognitive relatedness. This is an important finding because it shows that undergraduate grades, the most common admission instrument in higher education, have limited usefulness for students with relatively unrelated undergraduate programs. Admissions officers need to carefully assess their admission instruments for such students and rely less on grades when it comes to the decision to admit students.