Peningkatan kunjungan di situs web Online Public Access Catalog (OPAC) Perpustakaan Universitas Indonesia perlu disertai dengan peningkatan kualitas pelayanan, salah satunya melalui penyusunan sistem rekomendasi buku dalam situs web OPAC. Untuk menyusun sistem tersebut, penggalian pola asosiasi antar buku dilakukan dengan menggunakan salah satu metode data mining yaitu association rules. Dua algoritma digunakan yakni algoritma Apriori dan Apriori Inverse. Untuk menguji dan menyeleksi pola yang dihasilkan, objective interestingness measure dilakukan dan menghasilkan sembilan belas pola asosiasi sebagai luaran (output) akhir penelitian.

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An increasing use of Online Public Access Catalog (OPAC) Universitas Indonesia Library's website should be followed by a service quality improvement, such as organizing a book recommendation system. To build mentioned system, user behaviors in terms of loaned books were discovered by using one of data mining techniques namely association rules. In this research, two algorithms were used which are Apriori and Apriori Inverse. After all association rules were gathered, objective interestingness measure were conducted to evaluate the quality of association rules and resulted nineteen association rules as final output.