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.
Answer
The main problem of linking Compustat with IBES is not the fact that Compustat's cusip is 9 character, whereas IBES is 8-character. The main issue is that Compustat Cusip is header (most recent), whereas IBES Cusip is historical (as of date).
Therefore matching through Cusips is likely to be correct for many cases but not all. The standard way of doing the matching is indeed as you say to through CRSP.
There are many scripts out there that can do the matching for you. One potential script that will match it for you in less than a minute:
https://gist.github.com/JoostImpink/0e5a8ae738cc8ef14baf
which makes use of the WRDS macro iclink to merge CRSP and IBES:
https://wrds-web.wharton.upenn.edu/wrds/research/macros/sas_macros/iclink.cfm
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