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Another possibility is to do what a place I interviewed at did: come up with
a scenario that matches the client's industry or needs, and have the writer
creates a sample (in some cases, within certain time limits).
For example: if the prospective client is a software company, you could give
the writer the functional specs for a small sample of the client's system or
a demo copy of the client's software. Then the writer could use that to
create a documentation sample (and an outline?). Or even asking the writer
to document how to, for instance, create and apply a template in Word, would
give the client some idea of how the writer deals with documenting programs.
I suggest this so that the sample will have some relevance to the client.
Many clients may not realize that being able to describe the steps involved
in tying your shoes can easily translate to being able to describe the steps
involved in setting up a complex data management system. Also, I fear that
both writers and clients alike may sometimes look askance at samples based
on documenting processes and objects in everyday life, like shoelaces or
microwaves.