
Analyzing Software Measurement Data ◾ 265
use both size and eort in a duration prediction equation, we may predict
a longer time period than is really required, because we are (in a sense)
double counting; the duration suggested by the size measures may also
be represented in our eort value. We must take care to ensure that the
variables contributing to our equation are as independent of one another
as possible.
In general, if we want to investigate the relationship among several attri-
butes, we want to be sure that subsets of related attributes do not present
a misleading picture. e principal component analysis generates a linear
tr