As integration testing is conducted, the tester should identify
critical modules. A critical module has one or more of the following
characteristics:
addresses several software requirements,
has a high level of control (resides relatively high in the
program structure),
is complex or error prone (cyclomatic complexity may be
used as an indicator), or
has definite performance requirements.
Critical modules should be tested as early as is possible.
In addition, regression tests should focus on critical module
function.