This prose by Alberto Savoia answers precisely that question (in a nicely entertaining manner at that!):
http://www.artima.com/forums/flat.jsp?forum=106&thread=204677
Testivus On Test Coverage
Early one morning, a programmer asked
the great master:“I am ready to write some unit tests. What code coverage should I aim
for?”The great master replied:
“Don’t worry about coverage, just write some good tests.”
The programmer smiled, bowed, and
left.…
Later that day, a second programmer
asked the same question.The great master pointed at a pot of
boiling water and said:“How many grains of rice should I put in that pot?”
The programmer, looking puzzled,
replied:“How can I possibly tell you? It depends on how many people you need to
feed, how hungry they are, what other
food you are serving, how much rice
you have available, and so on.”“Exactly,” said the great master.
The second programmer smiled, bowed,
and left.…
Toward the end of the day, a third
programmer came and asked the same
question about code coverage.“Eighty percent and no less!” Replied the master in a stern voice,
pounding his fist on the table.The third programmer smiled, bowed,
and left.…
After this last reply, a young
apprentice approached the great
master:“Great master, today I overheard you answer the same question about
code coverage with three different
answers. Why?”The great master stood up from his
chair:“Come get some fresh tea with me and let’s talk about it.”
After they filled their cups with
smoking hot green tea, the great
master began to answer:“The first programmer is new and just getting started with testing.
Right now he has a lot of code and no
tests. He has a long way to go;
focusing on code coverage at this time
would be depressing and quite useless.
He’s better off just getting used to
writing and running some tests. He can
worry about coverage later.”“The second programmer, on the other hand, is quite experience both
at programming and testing. When I
replied by asking her how many grains
of rice I should put in a pot, I
helped her realize that the amount of
testing necessary depends on a number
of factors, and she knows those
factors better than I do – it’s her
code after all. There is no single,
simple, answer, and she’s smart enough
to handle the truth and work with
that.”“I see,” said the young apprentice,
“but if there is no single simple
answer, then why did you answer the
third programmer ‘Eighty percent and
no less’?”The great master laughed so hard and
loud that his belly, evidence that he
drank more than just green tea,
flopped up and down.“The third programmer wants only simple answers – even when there are
no simple answers … and then does not
follow them anyway.”The young apprentice and the grizzled
great master finished drinking their
tea in contemplative silence.