I am trying to link Thomson Reuter's I/B/E/S dataset with Compustat. Both I obtained via WRDS. The only halfway useful info I could find was on a two year old forum post, which suggests to go through a third database (CRSP) via a link table.
My question is, why wouldn't we just use the 6-digit CUSIP to map the two datasets?
It can be constructed from, both, the 8-digit "old" CUSIP of I/B/E/S as well as the "new" 9-digit CUSIP on Compustat. As this website (as well as the wikipedia article) explain, the first 6 digits identify a company, the subsequent 2 digits a specific issue of a security, and the 9th digit is a checksum. Since Compustat is firm-specific, it shouldn't matter for most forecasts which security we're looking at.
Moreover, most forecasted measures, such as ROA or turnover, also seem firm-specific, not security-specific to me. I'm not fully sure for EPS forecasts, but usually we wouldn't see multiple simultaneous issues at the same time either if I'm not mistaken.