The average (mean) of a data set is a very useful indicator of a
system's behavior. Your example does point out its weakness though.
That is why another number, called the Standard Deviation, is usually
also reported. This value gives an indication of how far from the mean
you might expect a given data point to deviate from. Together, the
mean and the standard deviation tell quite a lot about a set of
numbers.
Peter Covitz
Department of Biological Sciences
Stanford University
