From a qualitative point of view, Following are the points:
-The test plans must be in writing as soon as the specifications are ready.
-If test plans are written at the beginning of the life cycle, then the errors in the specifications are found before the start of coding.
-The costs of correcting this error are much smaller than the correction of the program after it is written.
-The disadvantage of this logic is that the test plans must be amended when a change in the requirements. If you take an overall view, the benefits of writing test plans as soon as possible, far outweigh the associated costs.