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http://funyakofunyao.click/2019/03/10/r-for-data-science%e3%81%ae%e4%be%8b%e9%a1%8c%e3%82%92%e8%a7%a3%e3%81%8f-chapter-15-%e3%83%95%e3%82%a1%e3%82%af%e3%82%bf/
R for Data Scienceの例題を解く- Chapter 15 ファクタ