Why Create a Test Plan?

A sound approach is to start writing the test plan as soon as you know you wUl be testing. Then, as the project proceeds, continue to refine it, get feedback buy-in, and so forth. Of course, there is a limit to flexibility, so prior to the test you need to set a reasonable deadline after which the test plan may not

change. Let that date also serve as the point at which the product can no longer change until after the test. You may find that the test plan is the only concrete milestone at that point in time in the development cycle and, as such, serves an important function.

Once you reach the cutoff date, do all that you can to freeze the design of the product you will be testing. Additional revisions may invalidate the test design you have chosen, the questions you ask, even the way you collect data.
If you are pressured to revise the test after the cutoff date, make sure that everyone understands the risks involved. The test may be invalidated, and the product may not work properly with changes made so close to the test date.