[Wiki-research-l] How to get the exact date when an article get a quality promotion?

Hi, I'm doing research on Wikipedia article quality, and I take advantage of WikiProject Assessments. But I can only get the latest quality level of an article. I wonder how to get the quality of each revision, or how to get the exact date when an article get a quality promotion, for example, from A-class to FA-class. I really need your help! Thanks! Zhang Shiyue -- Zhang Shiyue *Tel*: +86 18801167900 *E-mail*: byryuer(a)gmail.com, yuer3677(a)163.com State Key Laboratory of Networking and Switching Technology No.10 Xitucheng Road, Haidian District Beijing University of Posts and Telecommunications Beijing, China.