The sample of firm-years includes all firms listed on the Center for Research in
Security Prices(CRSP) data base at the beginning of each year and having a share
code of 10 or 11. This gives a sample of 158,194 firm-years for which the market
value of equity can be calculated as the number of shares outstanding multiplied by
the stock price.
原文如上，firm-years是什么意思，怎么得到的？  跪求各位大神了！ 拜谢！