By defining test cases, it becomes not only easy to evaluate tests and help even the novice run through several test cases but it makes the testing accountable. Once all the planned test cases have been tested it can be proved whether the product or functions as it should.
For the example of opening a door with a key, the test case would be 'Test whether the key opens the door'. Instead of using the word 'Action' the word 'Data' will be used. Also included will be whether the test passed or failed and more importantly when it was carried out.
The latter is very important as the test case may be run several times and a way to clearly differentiate each test is required.
If the key did not open the door as expected then the test case could be written as follows:
Now this may seem a very over complicated way of representing how a door is opened by using a key. However in the real world when a product is being tested, representing the information this way, that is in a format that can be easily understood and interpreted by all those people who would be interested in the testing, is very important.