Independence refers to the degree to which each test case stands alone. That is, does the success or failure of one test case depend on another, and if so what is the impact of the sequence of execution? This is an issue because it may be necessary or desirable to execute less than all of the test cases within a given execution cycle; if dependencies exist, then planning the order of execution becomes more complex.


Independent data


Independence is most easily accomplished if each test case verifies at least one feature or function by itself and without reference to other tests. This can be a problem where the state of the data is key to the test. For example, a test case that exercises the delete capability of a record in a file should not depend on a previous test case that creates the record; otherwise, if the previous test is not executed, or fails to execute properly, then the later test will also fail because the record will not be available for deletion. In this case, either the beginning state of the database should contain the necessary record, or the test that deletes the record should first add it.


Independent context


Independence is also needed where application context is concerned. For example, one test is expected to commence at a particular location, but it relies on a previous test to navigate through the application to that point.


Result independence


Again, if the first test is not successfully executed, the second test could fail for the wrong reason. Your test framework should give consideration to selecting common entry and exit points to areas of the application. and assuring that related tests begin and end at one of them.


It is also risky for one test case to depend on the successful result of another. For example, a test case that does not expect an error message should provide assurance that, in fact, no message was issued. If one is found, steps should be added to clear the message. Otherwise, the next test case may expect the application to be ready for input when in fact it is in an error status.


If proper attention is paid to independence, the test execution cycle will be greatly simplified. In those cases where total independence is not possible or desirable, then be certain that the dependencies are well documented; the sequence, for example, might be incorporated into the naming conventions for test cases (ADD RECORD 01,ADD RECORD 02, etc.).