请翻译成英文 长期监控用户质量： a. 首单质量：首单价、品类结构、用户分层结构等 b. 复购质量：复购率、复购频次、复购ARPU、用户分层复购表现等 c. ROI：T+14/30 ROI等

Long-term monitoring of user quality: a. First order quality: first order price, category structure, user segmentation structure, etc. b. Repeat purchase quality: repeat purchase rate, repeat purchase frequency, repeat purchase ARPU, user segmentation repeat purchase performance, etc. c. ROI: T+14/30 ROI, etc.