A third way to determine sample size is to rely on
published tables which provide the sample size for a
given set of criteria. Table 1 and Table 2 present
sample sizes that would be necessary for given
combinations of precision, confidence levels, and
variability. Please note two things. First, these
sample sizes reflect the number of obtained responses,
and not necessarily the number of surveys mailed or
interviews planned (this number is often increased to
compensate for nonresponse). Second, the sample
sizes in Table 2 presume that the attributes being
measured are distributed normally or nearly so. If
this assumption cannot be met, then the entire
population may need to be surveyed