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Robert Plamondon suggest a field study that would answer the question, but
there are a couple of techniques that could also help with less time and less
If the tech support people do indeed keep call records, get a copy of the
calls for the last year or so and have an impartial panel do a content
analysis to identify calls that could have been handled by documentation.
Calculate the expense of handling those calls by tech support and compare
that to the cost of printing documentation.
Simply set up a lab study wherein two groups of users are asked to perform
identical tasks, one group with documentation and one without. Count the
number of requests for help from each group and extrapolate each help request
to a comparable phone call. You don't need a fancy lab, just an office where
the test can be done without interruption.
If you have any human factors engineers in your organization, either of these
would be a snap to set up. If not, write me for pointers.