Documentation is one of those tasks we often dodge, but that will come back to bite you with endless support calls. To work out priorities for documentation, check out forum posts to identify the most frequent issues.
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Gartner analyst Nick Heudecker makes the point well:
Increasing traffic on a user mailing list indicates growing use and adoption. However, if that traffic continues growing, especially after the milestone release, it can indicate poor documentation or quality control. Do the same questions keep coming up on user forums? Your documentation is poor or nonexistent.
This approach pays off both for in-house software and for when you roll out commercial packages. For their former, checking support emails could be useful if you don't have online support forums. For the latter, you have a broader range of sites to search.
Which Database are You Taking to Prom? [Gartner Blogs]