What are the causes that introduce defects in software?

"Defect-Free Software does not exist."

Wietse Venema

This post discusses about what all probable reasons which may cause the defects in the software. Below, the list of most probable reasons that introduce defects in software.

• Miscommunication or No Communication - Software specification or requirements are not understood by all members of the software development project team due to miscommunication or no communication among the all members of the software development project team.
• Software Complexity - Software are getting more complex due to an increase in complexity in requirements. This complexity leads to more defects creeping into the software. Proliferation of many kinds of software with diverse technology backbones also leads to complexity of software. Multitiered softwares, client-server and distributed software, data communications, enormous relational databases, and the sheer size of software have all contributed to the exponential growth in software complexity.
• Programming Errors - If design specifications are not understood by programmers or if programmers make mistakes in programming structures or implement any design wrongly by mistake, then defects will be introduced in the software.
• Changing Requirements - This is one of the most important sources of defects in software. A change in requirements upsets the whole project, from design to build to testing to deployment. In extreme situations, it may lead to the throwing away of all already designed and built software. In other cases, it may lead to significant change required in existing design and build. Sometimes a small change required in one module of software may lead to significant changes required in other dependent modules.
• Overloading of Resources - Due to intangibility and the abstract nature of any software, it is very difficult to make a good schedule for software development projects. This results in bad scheduling, which causes many software professionals to be overloaded. This results in more introductions of defects in the software. Most of the time software professionals find it difficult to meet deadlines, and this results in hasty and less careful work, resulting in more defects in the software.
• Less Skillful Resources - The software profession needs very skilled and experienced professionals. Many times less skilled professionals are recruited due to lack of skilled professionals in the market.
• Unprofessional Attitude - Many times professionals on the software development project have attitude problems. They take their assignments lightly, play office politics, or try to shun their assigned work and off-load it to other members of the software development project team. These tactics not only create problems in meeting deadlines for the software development project but also result in introduction of more defects in the software.
• Poor Documentation - In the case of smaller software development projects where some form of extreme programming or agile programming is followed, less documentation is acceptable because the software development project team is located at one place, and due to smaller software, complexity is less. But in the case of bigger software development projects where the team may be located at different sites and where software requirements are complex, good documentation is very important. Otherwise, the team runs a great risk of introducing more defects. Another aspect of poor documentation is difficulty in maintaining the software after it goes into production.
• Development Tools - In today’s fast-paced business environment, software development project teams use many tools to increase their productivity. Some of these tools include visual tools, class libraries, compilers, scripting tools, Rapid Application Development (RAD), integrated development studios, integration tools, and so on. Many times these tools introduce their own defects, and sometimes, due to their poor documentation, help in adding defects.
• Lack of Version Control - If the functionality was tested in previous builds and after next few build it is seeing that regression errors occurs so it is very difficult to identify the regression errors. So in such a case version controlling makes a key role. Even if a version control system is in place, errors might still slip into the final builds if the programmers fail to make sure that the most recent version of each module are linked when a new version is being built to be tested.
• Poor Software Testing Skill - In many organizations poor software testing can be executed, this would not like to see by any tester but let’s face it. There can be insufficient knowledge of software testing skills (lack of skilled software testing) which leads to defects in the system. Also if tester is not paying attention in testing activity and doing testing without giving importance to it, so it results poor quality of the software and major bugs remains in the software. Moreover, in this era of agile software development poor unit tests (e.g. in TDD) may result in poor coding and hence escalate the risk of errors.
• Last Minute Changes in the Requirements - Requirements changes in the last minute can be dangerous which results instability of software. To implement the changes which are introduced in the last hour requirement changes will definitely introduce errors in the software and also existing working feature may stop working.

This post contains general information only and is based on the experiences and reseach in the specialist litterature about Software Quality Assurance (SQA).