What Do We Manage? A Survey of the Management Portfolios of Large Technical Communication Groups
http://www.ingentaconnect.com/content/stc/tc/2004/00000051/00000001/art00006
  access restricted (by the publisher) to members/subscribers/customers only
  peer-reviewed

Carliner, Saul
Technical Communication Online
2004

Abstract:

Finds that user's guides, reference manuals, and help account for most products, and about half are print. Reports that no widely used method or metric of assessing effectiveness exists