Another limitation is
that this approach cannot be applied to estimating the effort of the
test-only project in which the software to be tested exists but the
testing team does not have access to the software source code or
requirements in order to measure the overall software size in
source lines of code (SLOC) or function points. Finally, the
software size in these two most popular size metrics may not be a
reliable indicator of the amount of testing that needs to be
performed by the testing team. While the productivity of software
testing is computed as the amount of testing over the effort spent
for testing, it may not be accurately measured using these size
metrics.