1. Introduction to Testing Design Strategies – The Smarter Tester

2. Software components have defects, no matter how well our defect prevention activities are implemented. Developers cannot prevent/eliminate all defects during development.
Therefore, software must be tested before it is delivered to users. It is the responsibility of the testers to design tests
That
reveal defects, and
can be used to evaluate software performance, usabilty, and reliability

3. To achieve these goals, testers must select a finite number of test cases, often from a very large execution domain. Unfortunately, testing is usually performed under budget and time constraints. Testers often are subject to enormous pressures from management and marketing because testing is not well planned, and expectations are unrealistic.

4. The smart tester must plan for testing, select the test cases, and monitor the process to insure that the resources and time allocated for the job are utilized effectively.

5. Novice testers, taking their responsibilities seriously, might try to test a module or component using all possible inputs and exercise all possible software structures. Using this approach, they reason, will enable them to detect all defects. However an informed and educated tester knows that is not a realistic or economically feasible goal. Another approach might be for the tester to select test inputs at random, hoping that these tests will reveal critical defects.

6. TEST CASE DESIGN STRATEGIES
The ability to develop effective test cases is important to an organization evolving toward a higher-quality testing process.
It has many positive consequences. For example, if test cases are effective there is
a greater probability of detecting defects
a more efficient use of organizational resources
a higher probability for test reuse
closer adherence to testing and project schedules and budgets, and,
the possibility for delivery of a higher-quality software product. What are the approaches a tester should use to design effective test cases?

7. There are two basic strategies that can be used to design test cases
There are two basic strategies that can be used to design test cases. Black box (sometimes called functional or specification) and White box (sometimes called clear or glass box) test strategies.  

8. WHITE BOX TESTING
The white box approach focuses on the inner structure of the software to be tested. To design test cases using this strategy the tester must have knowledge of that structure.
The code, or a suitable pseudo code like representation must be available.

9. For example, test cases are often designed to exercise all statements or true/false branches that occur in a module or member function.
Since designing, executing, and analyzing the results of white of box testing is very time consuming, this strategy is usually applied to smaller-sized pieces of software such as a module or member function.

10. White box testing methods are especially useful for revealing
design and code-based control
logic and sequence defects
initialization defects, and
data flow defects.

11. Neither approach by itself is guaranteed to reveal all defects
Neither approach by itself is guaranteed to reveal all defects. The approaches complement each other; each may be useful for revealing certain types of defects.
With a suite of test cases designed using both strategies the tester increases the chances of revealing the many different type of defects in the software under test.
The tester will also have an effective set of reusable test cases for regression testing (re-test after changes), and for testing new releases of the software.