Software
Software development and maintenance

Software companies aim to deliver a high-quality product on time and under budget. A challenge is that software development effort estimation is often inaccurate. Software development begins by conceiving the project, evaluating its feasibility, analyzing the business requirements, and making a software design. Most software projects speed up their development by reusing or incorporating existing software, either in the form of commercial off-the-shelf (COTS) or open-source software. Software quality assurance is typically a combination of manual code review by other engineers and automated software testing. Due to time constraints, testing cannot cover all aspects of the software's intended functionality, so developers often focus on the most critical functionality. Formal methods are used in some safety-critical systems to prove the correctness of code, while user acceptance testing helps to ensure that the product meets customer expectations. There are a variety of software development methodologies, which vary from completing all steps in order to concurrent and iterative models. Software development is driven by requirements taken from prospective users, as opposed to maintenance, which is driven by events such as a change request.

Frequently, software is released in an incomplete state when the development team runs out of time or funding. Despite testing and quality assurance, virtually all software contains bugs where the system does not work as intended. Post-release software maintenance is necessary to remediate these bugs when they are found and keep the software working as the environment changes over time. New features are often added after the release. Over time, the level of maintenance becomes increasingly restricted before being cut off entirely when the product is withdrawn from the market. As software ages, it becomes known as legacy software and can remain in use for decades, even if there is no one left who knows how to fix it. Over the lifetime of the product, software maintenance is estimated to comprise 75 percent or more of the total development cost.

Completing a software project involves various forms of expertise, not just in software programmers but also testing, documentation writing, project management, graphic design, user experience, user support, marketing, and fundraising.
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