As a follow up in the testing definition series it was my intention to continue with covering Test Types. Initial investigation showed what I had already feared. Such a post would become a Herculean task and probable my longest post ever. So I will continue with that particular endeavor sometime later tackling it one step at a time. This post for starters covers one of the most common but also one of the most peculiar types of testing

“Regression Testing”

Regression Testing is so common as a testing type that the majority of books about software testing, and agile for that matter, that I know, mention regression testing. Almost as common however is that most of them either or both do not tell what regression testing is or do not tell how one should actually go about and do regression testing. To be fair an exception to the latter is that quite a few, particularly the ones with an agile demeanor, tell that regression testing is done by having automated tests but that is hardly anymore informative is it.

Before I go further into regression testing as being peculiar first inline with the previous posts a list of regression testing definitions:

• Checking that what has been corrected still works. (Bertrand Meyer; Seven Principles of Software Testing 2008)
• Regression testing involves reuse of the same tests, so you can retest (with these) after change. (Cem Kaner, James Bach, Bret Pettichord; Lessons learned in Software Testing 2002)
• Regression testing is done to make sure that a fix does what it’s supposed to do (Cem Kaner, Jack Falk, Hung Quoc Nguyen; Testing Computer Software 2006)
• Regression testing is the probably selective retesting of an application or system that has been modified to insure that no previously working components, functions, or features fail as a result of the repairs. (John E. Bentley; Software Testing Fundamentals Concepts, Roles, and Terminology 2005)
• Retesting to detect faults introduced by modification (ISO/IEC/IEEE 24765:2010)
• Saving test cases and running them again after changes to other components of the program (Glenford J. Myers; The art of software testing 2nd Edition 2004)
• Selective retesting of a system or component to verify that modifications have not caused unintended effects and that the system or component still complies with its specified requirements (ISO/IEC/IEEE 24765:2010)
• Testing following modifications to a test item or to its operational environment, to identify whether regression failures occur (ISO/IEC/IEEE 29119-1:2013)
• Testing if what was tested before still works (Egbert Bouman; SmarTEST 2008)
• Testing of a previously tested program following modification to ensure that defects have not been introduced or uncovered in unchanged areas of the software, as a result of the changes made. It is performed when the software or its environment is changed. (Standard glossary of terms used in Software Testing Version 2.2, 2012)
• Testing required to determine that a change to a system component has not adversely affected functionality, reliability or performance and has not introduced additional defects (ISO/IEC 90003:2014)
• Tests to make sure that the change didn’t disturb anything else. Test the overall integrity of the program. (Cem Kaner, Jack Falk, Hung Quoc Nguyen; Testing Computer Software 2006)

Looking at the above definitions the general idea about regression testing seems to be:

“To ensure that except for the parts of the areas* that were intentionally changed no other parts of these areas or other areas of the software are impacted by those changes and that these still function and behave as before”.
(*Area is used here as a general expression for function, feature, component, or any other dimensional divisions of the subject under test that is used)

The peculiar thing now is that however useful and logical such a definition is it only provides the intention of this type, or should I say activity, of testing. Regression testing could still encompass any other testing type in practice.

To know what to do you first need to establish which areas are knowingly affected by the changes and then which areas have the most likelihood of being unknowingly affected by the change. Next to that there probably are areas in your software where you do not want to take the risk of them being affected by the changes. In his presentation at EuroSTAR in 2005 Peter Zimmerer addresses the consequences of this in his test design poster by pointing out that the wider you throw out your net for regression effects the larger the effort will be:

• Parts which have been changed – 1
• Parts which are influenced by the change – 2
• Risky, high priority, critical parts – 3
• Parts which are often used – 4
• All – 5

Once you have identified the areas you want to regression test you still need to figure out how to test those areas for the potential impact of the change. The general idea to solve this, in theory at least, seems to be to rerun previous tests that cover these areas. As this might mean running numerous tests for lengthy periods of time many books and articles propose to run automated tests. This will however only work if there are automated tests to use for testing these areas to begin with. And even if there are you still need to evaluate the results of any failed test and there is no clear indication of how long that may take.

How do you know that these existing tests do test for the impact of the change? After all they were not designed to do so. For all you know they might or might not fail due to changes to the area that is tested by them. Either result could therefore be right or wrong in light of the changes. The test itself could be influenced by an impact of the change on the test (positive or negative) that was not considered or identified yet.

All in all regression testing is easily considered to be necessary, not so easy to determine, difficult to evaluate on success and considerably more work then many people think. Even so next to writing new tests it probably is the best to solution to check if changes bring about unwanted functionality or behavior in your software. My suggestion to you is to at least change the test data so that these existing tests have a better chance of finding new bugs.