I am new to US stocks and find that US stocks are distinguished by ticker symbols. For example, AAPL is for Apple Inc. hwoever, in CRSP and COMPUSTAT, there are CUSIP, PERMNO and PERMCO to distinguish one stock from another.

In the section "The playing field" of Fama and French (2015), it's said "the sample is all NYSE, AMEX, and NASDAQ stocks on both CRSP and Compustat with share codes 10 or 11". I wonder what does "share codes 10 or 11" mean? For CUSIP or PERMNO? Or something else? It may be a silly question, but I really need help. Thanks.

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    $\begingroup$ The main way to reference stocks in CRSP is via PERMNO, which is assigned by CRSP itself, and which never changes. In Computer Science terms, PERMNO is the "index" to the CRSP database. CUSIPs, Tickers, etc. are assigned by entities outside of CRSP, and are less reliable or less efficient means of getting information out of CRSP. For each PERMNO there is a unique record in the database containing the SHRCD, the returns and many other things, a described in the CRSP documentation. $\endgroup$ – Alex C May 23 '18 at 11:14
  • $\begingroup$ @AlexC Yes, at first I focus on how to link the CRSP data and Compustat data by permno. Now it has been solved, linking by permno. $\endgroup$ – Gregory Xue May 23 '18 at 11:35

You just need to check the definition of the variable. They basically represent Ordinary common shares (see below the exact definition).

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