CRSP: ncusip and cusip

The CUSIP that CRSP reports is the so-called ``header CUSIP'', which is the last CUSIP belonging to that PERMNO. A PERMNO may have had several different CUSIPs in the past. These are called the ``names CUSIPs'' (NCUSIPs) by CRSP.

Suppose I have data about stocks that only has historical CUSIPs as identifiers. Examples include holdings data, governance data, and mutual fund voting data. To convert these into PERMNOs, I could use the stocknames file, doing as suggested by Subsection 6.2. Alternatively, I could use the fact that CUSIPS are not reused in the data:

     proc sort data=crsp.stocknames(keep=permno cusip) out=permnocusip(rename=(cusip=ncusip)) nodupkey;
        where cusip is not missing;
        by permno cusip;
     run;

     proc sort data=crsp.stocknames(keep=permno ncusip) out=permnoncusip nodupkey;
        where ncusip is not missing;
        by permno ncusip;
     run;

     proc append base=permnoncusip data=permnocusip;
     run;

     proc sort data=permnoncusip nodupkey;
        by permno ncusip;
     run;

The last file is a permno-ncusip lookup table, which I can merge with my CUSIP data to obtain permnos.

Why did I include CUSIPs as well? One would expect that the NCUSIP history perfectly accounted for even the last CUSIP held - the header CUSIP. But it does not always.