Software development is a highly complex process of creating and maintaining a product that would solve its users’ problems with ease and finesse. In addition to market research, requirement analysis, and other business-related procedures, software development mainly involves a lot of writing the source code and maintaining it in good health. Software development is impossible without code changes, making code quality a real struggle—especially when you’re faced with a tough deadline—as every minor change can have an unexpected, even unseen impact on the rest of your codebase.

In quality assurance, these bugs are called software regressions, defects that appear in the course of development and can cause your entire software solution or its separate features to stop functioning as intended.

There are three types of regressions:

• Local regressions take place when a change to a piece of code introduces a new bug and causes a given component to stop working correctly.
• Remote regressions happen when a change in one part of the code causes an error, or completely breaks functionality, in another part of the software that had no issues before the change.
• Unmasked regressions occur when a change to a piece of code reveals an already existing bug that previously did not affect the software in any way. This type of regression is the most dangerous: a ticking bomb, unexpected, and thus the hardest to find.

Whether your software is early in development or you’re introducing an update to a released solution, regression testing comes in to ensure that any changes you make to your code do not cause you any pitfalls.

What is regression testing in QA?

Regression testing is in charge of identifying and preventing the aforementioned kinds of software regression. It makes sure that the previously developed and tested software still performs and behaves as intended after undergoing functional or non-functional changes. The term regression, in fact, means “a return to a former or less developed state.” The tests are performed to guarantee the new code hasn’t introduced new bugs or triggered any previously undiscovered ones that would cause your software to unintentionally regress.

Regression testing techniques

Depending on the complexity of your software solution, the size of your QA team, and the resources you’re blessed with, regression testing is performed using any combination of the following techniques:

• Retest all involves rerunning all of your existing tests on the new codebase, retesting your entire software solution to reveal the regressions. This technique is used when the software undergoes significant functional and non-functional changes. QA teams usually perform it at the final stages of product delivery or before major releases. This practice is most suitable for small and very well designed projects while in a large project it can be very lengthy and resource-intensive. It is also quite infeasible and time-consuming to perform such a massive total of tests manually, thus, this technique usually implies automated testing using various tools and test-driven development practices.

• Regression test selection is a less resource-intensive alternative to retest all. It takes a good strategy and a risk-based approach to perform regression test selection and yield good results. Using this technique, the QA team doesn’t write a whole new regression test suite but rather revises all the existing test suites. The team takes the affected by recent changes part of code and selects the most relevant test cases that have repeatedly found bugs. This technique takes a lot less time and effort to perform, can be effectively performed manually and helps the QA team discard obsolete test cases, which makes it a great choice for iterative development.

  • Test case prioritization is a technique that focuses on scheduling a limited set of test cases in a way that the test cases deemed most critical are executed ahead of the ones that have a smaller impact. It is used to increase a test suite’s rate of fault detection. There are two types of test case prioritization: general prioritization (targets all subsequent versions) and version-specific prioritization (targets a particular version).

• The Hybrid technique is a combination of regression test selection and test case prioritization. Using this technique, QA teams first rerun the test cases of highest priority and then run all the remaining tests from the selected part of the test suite to make sure no obscure bug makes it through in-between versions.

Regression testing in common methodologies

When choosing a suitable regression testing strategy for your project, one of the most important factors to consider is the product development methodology you’re using.

Regression testing in Waterfall

In the waterfall model, each stage of the software development life cycle must be completed before moving on to the next one. Therefore, testing as such begins only after the product is fully developed, which means the processes of introducing changes to the code and debugging it can turn very tedious, especially when the requirements have been floating in the course of development. It’s crucial to make sure all the functional and non-functional requirements are clear and testable before the development starts.

When it is finally time for testing, the team creates and runs the test suites and hands the reports to developers for fixing. After the bugs have supposedly been fixed, the team runs regression testing against the fixed bugs and the adjacent areas to make sure the fixes didn’t generate new bugs. If they did, however, the team categorizes them by severity and sends the reports to developers for another round of fixing. When the software has been stabilized with all the critical errors fixed, the testing team carries out the final full regression testing to take care of the remaining minor bugs. In the waterfall methodology, there is no way around the full regression testing unless you want to risk turning the delivery into hell for both your development team and users.

Regression testing in Agile

Software development projects using the Agile methodology are carried out through iterations, sprints of 1-6 weeks. Each sprint delivers a piece of software with a specific range of functions. Such an approach provides numerous benefits for product improvement while avoiding rework, if you find the right balance between iterative development and testing, of course. Regression testing plays a key role in putting the existing and updated functionality in order, so it is important to know when to do regression testing in Agile. Regression testing is performed at the end of every sprint to make sure your software is stable and sustainable.

In Agile development, regression testing can be performed with both full and partial regression coverage, where partial is conducted at every iteration and full regression testing before major releases and deployment. The regression testing suite is under constant maintenance in Agile projects. Testing teams continuously add relevant test cases and get rid of the obsolete ones. Frequent testing makes dealing with defects faster and more efficient, helping developers focus their efforts on improving the software. For regression testing, the Agile approach is a great way to reduce time and effort without hurting the final quality of the software.

How you perform regression testing

• Locate the changes in the source code and detect all the areas, components, and modules that might have been affected; analyze the impact of the changes on the existing functional and non-functional features; determine if the minimum conditions were met for the execution of regression testing.

• Based on the analysis, compile a regression test suite made of the test cases relevant to the changes and the affected areas; categorize them into small groups with concise descriptions to make it easier for your team to identify and prioritize certain types of tests.

• Prioritize the tests based on the severity of the impact (critical, high, medium, or low) the changes had on your functional and non-functional features. This will help you optimize your testing workflows to focus on dealing with the most problematic cases first, ones that hinder further development.

• Plan and maintain a strict schedule for continuous regression testing throughout your software development life cycle. Find the right timing to perform the testing in a way it doesn’t cause any setbacks in development.

When you perform regression testing

Ideally, regression testing should be performed whenever your codebase has been modified or altered in any way as well as to verify any previously discovered issues marked as fixed. The more often the better: frequent partial regression testing will help your developers fix the reported defects on time, and your project to avoid any long-term pitfalls and technical debt caused by poor code quality. However, even though an occasional project might have the resources to perform the tests after the slightest changes have been introduced to the codebase, for most projects designing and maintaining such a multiplicity of regression tests may simply be infeasible. Therefore, it is important to understand when you need to start regression testing.

The most common reason to run regression tests is the introduction of new functionality. It is hard for developers to follow every thread in the code when modifying it, and there’s always a risk of compatibility issues with the existing code. Regression testing can save developers a lot of time with timely detection of bugs that would otherwise cause the project a lot of pain in the long run.

Sometimes, however, sudden shifts in business strategy and requirements can lead to complete revision of the existing functionality, which requires developers to adjust, reshape, or even discard some of the features. Heavy interference with the source code can potentially cause a lot of damage to the remaining functionality, which makes regression testing an absolute must in such cases.

Attempts at fixing one bug can at times turn into even more bugs appearing in codebase areas you expect them the least. Debugging suggests making a lot of big and small changes to the source code, checking bug statuses, and going through the same process over again. This, in turn, points out the importance of following up the debugging stage with regression testing to guarantee it didn’t cause more issues than it fixed; to make sure everything works as intended after debugging. Last but not least, regression testing has proven to be an effective means of ensuring seamless and bug-free integration with external systems.

Manual vs Automated

Where some particular issues may require the utmost precision and time efficiency of automated regression testing, some defects would be unable to fix without the human critical thinking and creativity applied in manual regression testing. Hence, the key to cost-effective, productive regression testing is finding the right balance between the two approaches that deal with different issues.

Sometimes working on test automation is not worth the effort, especially when we’re talking small projects and simple software solutions built in 2-3 months. In this case, QA engineers can easily perform manual regression testing, using such bug-tracking tools as Jira and Code Quality for Jira. Manual regression testing is a pretty basic approach to testing software regardless of the product development methodology you’re using, and it always precedes automation as it provides more efficiency in the early stages of development.

Nevertheless, as the project scales and grows with features, the scope of regression testing increases as well, which means manual testing becomes significantly less efficient and more time-consuming. At some point, it simply becomes infeasible to manage the regression test cases manually and the team switches to automated regression testing. Once built, your automated regression suite can be easily reused—catching every defect with a press of a button—to the last bug eliminated, which is way cheaper and more reliable than putting your whole QA team through doing it manually. Another big plus is that it enables you to increase the frequency of your test runs and get immediate feedback. When it comes to QA teams, the reason they favor automated regression testing is that testers do not enjoy the process of running the same tests over and over again. Such repetitiveness inevitably affects the tester’s focus and performance. In contrast, automating the regression testing enables them to redirect their brainpower on tasks that require critical thinking and creativity.

Why you perform regression testing

No matter how scrupulous your development team is in the process, some defects are still certain to make it through. Thus, regression testing should never be left to luck as it is one of the key factors of your product’s bug-free, long-term success. Today, regression testing has become a prerequisite part of continuous development and delivery practices.

Although creating efficient regression test suites may take some time and money, by investing in it, you can be sure your software is free from any pitfalls or potential quality-related risks. With a sound strategy, the right combination of regression testing techniques, as well as the proper balance between manual and automated approaches, the testing method will pay off with a stable, functional software solution that has better chances of generating good ROI than a bug-infested mess your solution would be without it. So when you’re asking yourself whether your project needs regression testing, in most cases the answer is “yes.”