Often software developers create their software product, then scratch their heads and say, "Now, how are we going to test this thing?" Test planning is a major task and must occur in parallel with product development so that test planning and initial (that is, pretesting) development activities are completed in synchrony.
For software system testing, test planners should review the SRS for testability before it is baselined and provide feedback to requirements writers. Serious development of the tests should start soon after baselining requirements. For integrations testing, test planners should review the preliminary design before it is baselined. They should also provide feedback to the project managers and designers concerning (1) sensible allocations of resources to ensure that the "right" components (from a testing point of view) are produced in the right order and (2) modifications to the design to make it inherently easier to test. Serious integration test development should start soon after baselining the preliminary design. For unit testing, unit test plan development can start immediately after the completion of detailed design.
Reference:
Goodenough, J., and Gerhart, S., "Toward a Theory of Test Data Selection," IEEE Transactions on Software Engineering, June 1975.
