First look at the more official explanation:
Test Plan Testing plan, the document describes the scope of testing activities to be performed, methods, resources and schedule; the whole information system application software assembly and testing and validation testing right.
It identifies test items, test characteristics and testing tasks, who perform a task, all possible risks. The smooth implementation of test plans can be an effective prevention program risk, protection scheme.
So why write a test plan?
(1) Test Manager to facilitate better coordination to control the progress of the project and related resources
(2) test the students can clearly know the specific scope of the project and stages, and each stage their own things to do
(3) allow other people to know task scheduling tests, such as the development, operation and maintenance, to facilitate their work arrangements, such as joint deployment environment reconcile
So what time to write the test plan it?
(1) needs analysis phase of sorting out the overall project test plan
(2) After the iterative determination to write detailed iteration test plan
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(Source: ed ed school test software testing)