This document has been compiled from number of testing related sources and provides an introduction to software testing. It is intended for those who are new to software testing and as a reminder of some finer points for experienced software testers. This document covers the basic definitions of testing, validation and verification; the levels of testing from unit testing through to acceptance testing; the relationship with functional and design specifications; and test documentation.
There are many published definitions of software testing, however, all of these definitions boil down to essentially the same thing: software testing is the process of executing software in a controlled manner, in order to answer the question "Does the software behave as specified?"
Software testing is often used in association with the term’s verification and validation. Verification is the checking or testing of items, including software, for conformance and consistency with an associated specification. Software testing is just one kind of verification, which also uses techniques such as reviews, analysis, inspections and walkthroughs. Validation is the process of checking that what has been specified is what the user actually wanted.
The term bug is often used to refer to a problem or fault in a computer. There are software bugs and hardware bugs. The term originated in the United States, at the time when computers were built out of valves, when a series of previously inexplicable faults were eventually traced to moths flying about inside the computer.
Software testing should not be confused with debugging. Debugging is the process of analysing and locating bugs when software does not behave as expected. Although the identification of some bugs will be obvious from playing with the software, a methodical approach to software testing is a much more thorough means of identifying bugs. Debugging is therefore an activity that supports testing, but cannot replace testing. However, no amount of testing can be guaranteed to discover all bugs.
Other activities that are often associated with software testing are static analysis and dynamic analysis. Static analysis investigates the source code of software, looking for problems and gathering metrics without actually executing the code. Dynamic analysis looks at the behaviour of software while it is executing, to provide information such as execution traces, timing profiles, and test coverage information.
The key component of the above definitions is the word specified. Validation and verification activities, such as software testing, cannot be meaningful unless there is a specification for the software. Software could be a single module or unit of code, or an entire system. Depending on the size of the development and the development methods, specification of software can range from a single document to a complex hierarchy of documents.
A hierarchy of software specifications will typically contain three or more levels of software specification documents.
With such a hierarchy of specifications, it is possible to test software at various stages of the development, for conformance with each specification. The levels of testing that correspond to the hierarchy of software specifications listed above are:
A further level of testing is also concerned with functionality is:
Once each level of software specification has been written, the next step is to design the tests. An important point here is that the tests should be designed before the software is implemented. If not then it would be too tempting to test the software against what it is observed to do (which is not really testing at all), rather than against what it is specified to do.
Within each level of testing, once the tests have been applied, test results are evaluated. If a problem is encountered, then either the test are revised and applied again, or the software is fixed and the tests applied again. This is repeated until no problems are encountered, at which point development can proceed to the next level of testing.
Testing does not end following the conclusion of acceptance testing. Software has to be maintained to fix problems that show up during use and to accommodate new functionality. Software tests have to be repeated, modified and extended. The effort to revise and repeat tests consequently forms a major part of the overall cost of developing and maintaining software. The term regression testing is used to refer to the repetition of earlier successful tests in order to make sure that changes to the software have not introduced side effects.
The design of tests is subject to the same basic engineering principles as the design of software. Good design consists of a number of stages that progressively elaborate the design of tests from an initial high level strategy to detailed test procedures. These stages are test strategy, test planning, test case design, and test procedure design.
The design of tests has to be driven by the specification of the software. At the highest level this means that tests will be designed to verify that the software faithfully implements the requirements of the Functional Specification. At lower levels tests will be designed to verify that items of software implement all design decisions made in the Design Specification and Detailed Design Specifications. As with any design process, each stage of the test design process should be subject to informal and formal review.
The ease with which tests can be designed is highly dependent on the design of the software. It is important to consider testability as a key (but usually undocumented) requirement for any software development.
The first stage is the formulation of a test strategy. A test strategy is a statement of the overall approach to testing, identifying what levels of testing are to be applied and the methods, techniques and tools to be used. A test strategy should ideally be organisation wide, being applicable to all of organisations’ software development.
Developing a test strategy that efficiently meets the needs of an organisation is critical to the success of software development within the organisation. The application of a test strategy to a software development project should be detailed in the project’s software quality plan.
The next stage of test design, which is the first stage within a software development project, is the development of a test plan. A test plan states what the items to be tested are, at what level they will be tested, what sequence they are to be tested in, how the test strategy will be applied to the testing of each item, and describes the test environment.
A test plan may be project wide, or may in fact be a hierarchy of plans relating to the various levels of specification and testing:
The objective of each test plan is to provide a plan for verification, by testing the software, the software produced fulfils the functional or design statements of the appropriate software specification. In the case of acceptance testing and system testing, this means the Functional Specification.
Once the test plan for a level of testing has been written, the next stage of test design is to specify a set of test cases or test paths for each item to be tested at that level. A number of test cases will be identified for each item to be tested at each level of testing. Each test case will specify how the implementation of a particular requirement or design decision is to be tested and the criteria for success of the test.
The test cases may be documented with the test plan, as a section of a software specification, or in a separate document called a test specification or test description.
It is important to design test cases for both positive testing and negative testing. Positive testing checks that the software does what it should. Negative testing checks that the software doesn't do what it shouldn't.
The process of designing test cases, including executing them as thought experiments, will often identify bugs before the software has even been built. It is not uncommon to find more bugs when designing tests than when executing tests.
The final stage of test design is to implement a set of test cases as a test procedure, specifying the exact process to be followed to conduct each of the test cases. This is a fairly straightforward process, which can be likened to designing units of code from higher level functional descriptions.
For each item to be tested, at each level of testing, a test procedure will specify the process to be followed in conducting the appropriate test cases. A test procedure cannot leave out steps or make assumptions. The level of detail must be such that the test procedure is deterministic and repeatable.
Test procedures should always be separate items, because they contain a great deal of detail that is irrelevant to software specifications.
When tests are executed, the outputs of each test execution should be recorded in a test results file. These results are then assessed against criteria in the test specification to determine the overall outcome of a test.
Each test execution should also be noted in a test log. The test log will contain records of when each test has been executed, the outcome of each test execution, and may also include key observations made during test execution. Often a test log is not maintained for lower levels of testing (unit test and software integration test).
Test reports may be produced at various points during the testing process. A test report will summarise the results of testing and document any analysis. An acceptance test report often forms a contractual document within which acceptance of software is agreed.
Software can be tested at various stages of the development and with various degrees of rigour. Like any development activity, testing consumes effort and effort costs money. Developers should plan for between 30% and 70% of a project’s effort to be expended on verification and validation activities, including software testing.
From an economics point of view, the level of testing appropriate to a particular organisation and software application will depend on the potential consequences of undetected bugs. Such consequences can range from a minor inconvenience of having to find a work-round for a bug to multiple deaths. Often overlooked by software developers (but not by customers), is the long term damage to the credibility of an organisation which delivers software to users with bugs in it, and the resulting negative impact on future business. Conversely, a reputation for reliable software will help an organisation to obtain future business.
Efficiency and quality are best served by testing software as early in the life cycle as practical, with full regression testing whenever changes are made. The later a bug is found, the higher the cost of fixing it, so it is sound economics to identify and fix bugs as early as possible. Designing tests will help to identify bugs, even before the tests are executed, so designing tests as early as practical in a software development is a useful means of reducing the cost of identifying and correcting bugs.
In practice the design of each level of software testing will be developed through a number of layers, each adding more detail to the tests. Each level of tests should be designed before the implementation reaches a point which could influence the design of tests in such a way as to be detrimental to the objectivity of the tests. Remember: software should be tested against what it is specified to do, not against what it actually observed to do.
The effectiveness of testing effort can be maximised by selection of an appropriate testing strategy, good management of the testing process, and appropriate use of tools to support the testing process. The net result will be an increase in quality and a decrease in costs, both of which can only be beneficial to a software developers business.
The following list provides some rules to follow as an aid to effective and beneficial software testing.
This page last updated: 12 June, 2001