A Good User's Guide Means Fewer Support Calls and Lower Support Costs
http://www.ingentaconnect.com/content/stc/tc/1995/00000042/00000001/art00005
  access restricted (by the publisher) to members/subscribers/customers only
  peer-reviewed

Spencer, Cathy J. and Diana Kilbourn Yates
Technical Communication Online
1995

Abstract:

Good user documentation means fewer client support calls and lower support costs at GE Information Services in Rockville, Maryland.