Information Engineering

Test Design Principles

Effective test management requires both a framework and process. The framework requires some established design principles, such as:

  • Positive and Negative Testing
  • Simplicity and Repeatability
  • Maximum Coverage with Minimum Effort
  • Risk-Based Testing
  • Owning the Outcome
  • Granularity

Positive and Negative Testing

Many design factors specify what should happen and when. Sometimes these are Boolean in nature, such as “when you press this button, this happens”. There is an implied negative in this statement, which is “when you don’t press it, this doesn’t happen”. Both states should be tested.

More complex design factors introduce a range of valid values. Each range has implied validity, defined by format and boundaries. Formats can be generic, such as dates, or specific, such as UK telephone numbers, or the maximum length of a text field, whilst boundaries relate to value or set-based validation. Each of these may result in three or four specific tests designed to verify the validation routine and the applications response to failure.

Providing a comprehensive suite of tests with known outcomes reduces the necessary level of experience required of the tester, and therefore dependence upon key individuals.

Simplicity and Repeatability

The value of simple tests is that they can be executed quickly, with minimal specialist knowledge and with easily verifiable outcomes. For this reason the start point of each test “pack” should be clearly defined, including the state and content of any supporting database. The benefits are that the tests are repeatable with minimal effort and elapsed time, reducing the overall cost. Repeatable tests are critical to successful testing, and simplicity reduces the cost of repeatability.

Maximum Coverage with Minimum Effort

The most rigorous testing would involve exercising every step in every Business Process at least once for every possible set of data. Clearly this is impractical, so we must find a way of maximising the value of the testing effort.
With the modular programming techniques offered by modern development environments, specific routines and modules are re-used in many places. By constructing a matrix of which routines and modules are used in which business processes it may possible to select a relatively small number of business processes which exercise all routines and modules.

Risk-Based Testing

Having reduced the testing effort to a minimum, there may still be too much to do in the time/budget available. Each test component must therefore be subjected to a critical risk assessment where

  • Risk = Probability x Impact

Probability is difficult to define and predict because it usually involves the complex interaction of many factors. Reducing the impact, or cost, of failure is therefore critical to reducing risk.

The cost of fixing problems grows exponentially the later they are detected in the product lifecycle. These costs come in three forms:

  • Financial – costs such as application rework, providing short-term alternative facilities and contractual penalties, such as SLAs
  • Opportunity – missing delivery deadlines (which may affect your competitive position), market cycles and external deadlines such as legislation or B2B commitments
  • Credibility – missing customer expectations, which have usually been set well in advance of delivery

Some of these costs may be general whilst others are usage-based. Understanding who uses or will use which features can provide a weighting factor in determining the risk-cost.

The tests to be performed can then be selected based on comparing risk-cost against budget.

Owning the Outcome

Once the test plan is agreed some tests will inevitably be left out, each having a risk-cost. Someone needs to own this residual risk-cost, potentially amplified by the number of customers using the application. This can be mitigated by having a clear understand of which customers use which features of the application


Some application components lend themselves to a smaller number of test scripts each comprising a large number of test cases in sequence. The problem this poses is that once a test case fails, the remaining test cases are potentially invalid. For this reason a larger number of tests smaller in scope should be defined.

Certain tests have prerequisites which are provided by earlier tests. These prerequisites are often defined in the context of the state of the underlying data. Where possible halt the overall process after the prerequisite has been satisfied and take a backup of the database. This will provide the entry point for repeating the dependant test.