A functional specification is the set of documentation that describes the requested behavior of an engineering system. The documentation typically describes what is needed by the system user as well as requested properties of inputs and outputs.
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Bridging the Gap with Requirements Definition
Developing a new product or service is tricky. When everything goes well, the product can redefine a market or even create an entirely new one, to the benefit of its manufacturer and its consumers. When the product doesn't click with its audience, though, the costs—development, employee, manufacturing—can be staggering. How do you ensure that your new product doesn't flop? One effective method is to conduct a requirements definition phase before developing a new product.
Olshavsky, Ryan. Cooper Interaction Design (2002). Articles>Usability>Specifications
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Common Mistakes: Functional Specification for Web Development
What are pitfalls that companies should avoid when specifying Web applications for internal or external development?
Buerki, Nicolas. E-Consultancy (2004). Articles>Writing>Specifications>Functional Specifications
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Creating a User Experience Specification 
Creating any system of sufficient complexity requires a diverse team and a dizzying amount of documentation. While these documents do a great job of conveying components of the system, they do not provide an integrated view. This is because each covers different aspects of the system, written by a different author for a different audience. This paper proposes that project teams should create a user experience specification, a document that shows what the system looks like, how it behaves, and how it works. This specification needs to describe the system for all team members, at a useful level of detail, in a form that encourages team members to read it and inviting enough to get them to participate in the design, as well as allow developers to build from.
Oye, Phil and John Payne. STC Proceedings (2004). Design>User Experience>Specifications
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Functional Spec Tutorial: What and Why
Functional specifications (functional specs), in the end, are the blueprint for how you want a particular web project or application to look and work. It details what the finished product will do, how a user will interact with it, and what it will look like. By creating a blueprint of the product first, time and productivity are saved during the development stage because the programmers can program instead of also working out the logic of the user-experience. It will also enable you to manage the expectations of your clients or management, as they will know exactly what to expect.
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A functional specification (or sometimes functional specifications) is a formal document used to describe in detail for software developers a product's intended capabilities, appearance, and interactions with users. The functional specification is a kind of guideline and continuing reference point as the developers write the programming code. (At least one major product development group used a "Write the manual first" approach. Before the product existed, they wrote the user's guide for a word processing system, then declared that the user's guide was the functional specification. The developers were challenged to create a product that matched what the user's guide described.) Typically, the functional specification for an application program with a series of interactive windows and dialogs with a user would show the visual appearance of the user interface and describe each of the possible user input actions and the program response actions. A functional specification may also contain formal descriptions of user tasks, dependencies on other products, and usability criteria. Many companies have a guide for developers that describes what topics any product's functional specification should contain.
Whatis.com. Articles>Writing>Specifications>Functional Specifications
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Functional Specification and Review 
The Functional Specification is created after the Software Requirements Document. It provides more detail on selected items originally described in the Software Requirements Document. Some software development organizations combine these two documents into a single document.
Electric Power Research Institute. Articles>Writing>Specifications>Functional Specifications
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Functional Specification Standard 
In general terms, the functional specification states what the proposed system is to do, whereas design is how the system is to be constructed to meet the functional specification. However in writing it, some consideration of design issues must take place, to ensure a realistic system is specified.
Software Reality. Articles>Writing>Specifications>Functional Specifications
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Functional Specification Template
An outline/template for a project's functional specification.
O'Connor, Rory. Dublin City University (2004). Resources>Writing>Specifications>Functional Specifications
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Functional Specifications Subvert the Hierarchy of Nature
When you use a spec, you give your trust and authority to a piece of paper rather than the people on your team. You codify laws. You strip your 'judges' of the ability to act on intuitive feelings. There’s no fluidity. There’s no ability to respond, change, and evolve.
Fried, Jason. Signal vs. Noise. Articles>Collaboration>Specifications
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Getting Real, Step 1: No Functional Spec
Don't write a functional specifications document. Why? Well, there's nothing functional about a functional specifications document.
Fried, Jason. Signal vs. Noise (2005). Articles>Writing>Specifications>Functional Specifications
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Your requirements document needs to focus on the user’s goals. They should not be marketing’s list of features 'we’ve got to have' because the competition has these features. They should not be a list of things the programmers think ought to be included 'because we can add those things for very little cost.' Feature bloat does not benefit the user.
Ferlazzo, Ellen Lawson. Sprezzatura Systems (2002). Articles>User Centered Design>Specifications>Software
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Although they appear maddeningly incomprehensible at first, W3C specifications are actually great sources of information, once you understand their secrets. Learn how to read the specs.
Eisenberg, J. David. List Apart, A (2001). Design>Web Design>Standards>Specifications
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Identifying Web Site Requirements
The authors emphasize the importance of conducting thorough research on business goals, branding goals, user needs, and technical resources before Web designers undertake a redesign. The article also offers suggestions about how to define, develop, and communicate a client's brand.
Summers, Kathryn and Michael Summers. Intercom (2001). Articles>Usability>Specifications
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Errors in requirements specifications translate into poor designs, code that does the wrong thing, and unhappy customers. Requirements documentation should be inspected early and often. Anything you can do to prevent requirements errors from propagating downstream will save you time and money. Karl Wiegers shows you how.
Wiegers, Karl E. StickyMinds (2004). Articles>Documentation>Engineering>Specifications
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Whatever your role in a project, insist on getting the spec right before the code is written. The spec'ing process may take several iterations, so plan accordingly.
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Painless Functional Specifications - Part 1: Why Bother?
Why won't people write specs? People claim that it's because they're saving time by skipping the spec-writing phase. They act as if spec-writing was a luxury reserved for NASA space shuttle engineers, or people who work for giant, established insurance companies. Balderdash.
Spolsky, Joel. Joel on Software (2000). Articles>Writing>Specifications>Software
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Painless Functional Specifications - Part 2: What's a Spec?
When you design a product, inside and out, the most important thing is to nail down the user experience. What are the screens, how do they work, what do they do. Later, you worry about how to get from here to there. There's no use arguing about what programming language to use before you've decided what your product is going to do. In this series, I'm only talking about functional specifications.
Spolsky, Joel. Joel on Software (2000). Articles>Writing>Specifications>Functional Specifications
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Your requirements will assist you in delivering a software solution that meets your users' needs. You can find all sorts of templates and formal processes for requirements of various kinds, and while they are useful, the biggest problem I've found is that most people confuse defining the need with proposing a solution. As soon as a requirements document contains any part of 'how we're solving this', you've crossed the line into presupposing that you already know what the problem is and can stop listening.
Ferlazzo, Ellen Lawson. Sprezzatura Systems (2002). Careers>Consulting>Specifications
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Seeing the World Through Different Specs: Or, How I Came to Love Writing Software Specifications
Much has been said and written about Object-Oriented Programming in the past few years, some of it even worthwhile. While not the panacea on which we've all waited, OOP is, however, changing not only our concept of software design and development, but is subtly re-shaping the way in which we see and know the world. For technical communicators, this epistemological change will radically affect not only the way we craft software specifications, but will permanently re-shape our worldview.
Weathington, Thomas L., Jr. STC Proceedings (1998). Articles>Writing>Specifications>Technical Writing
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Specifications and Standards Resources
A collection of dozens of online resources for writers of standards and specifications.
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UsabilityNet: International Standards
Standards related to usability can be categorised as primarily concerned with: the use of the product (effectiveness, efficiency and satisfaction in a particular context of use); the user interface and interaction; the process used to develop the product; the capability of an organisation to apply user centred design
UsabilityNet. Resources>Usability>International>Specifications
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Writing a Functional Specification
A functional specification can substantially simplify and streamline the process of application development. Intended to describe how a piece of software works, it provides a ready reference for software developers andaligns large and disparate development teams to a single goal. In the process, it provides technical clarity on how the different components of aparticular applications are to be designed, implemented and integrated witheach other, and (if used correctly) significantly reduces the time and costcomponent of any development exercise.
Deepa L. Software Reality (2003). Articles>Writing>Specifications>Functional Specifications
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Writing Effective Requirements Specifications
The Goddard Space Flight Center's (GSFC) Software Assurance Technology Center (SATC) has developed an early life cycle tool for assessing requirements that are specified in natural language. The Automated Requirements Measurement (ARM) tool was used to analyze more than 50 NASA System/Software Requirements Specification (SRS) documents. ARM reports were used to focus human analysis on specific aspects of the documentation practices exhibited by these documents. Several significant weaknesses were identified. This paper identifies the underlying problems that produce these deficiencies and recommends methods that can be used to prevent such problems.
Wilson, William M. NASA (1997). Articles>Writing>Specifications>Engineering
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Writing Software Requirements Specifications
For technical writers who haven't had the experience of designing software requirements specifications (SRSs, also known as software functional specifications or system specifications) templates or even writing SRSs, they might assume that being given the opportunity to do so is either a reward or punishment for something they did (or failed to do) on a previous project. Actually, SRSs are ideal projects for technical writers to be involved with because they lay out the foundation for the development of a new product and for the types of user documentation and media that will be required later in the project development life cycle. It also doesn't hurt that you'd be playing a visible role in contributing to the success of the project.
Le Vie, Donald S., Jr. TECHWR-L (2000). Articles>Writing>Specifications>Technical Writing
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Writing Technical Specifications in the Present
Technical specifications are improved in several ways with one easy procedure - writing them in the present tense. That is, rather than trying to specify constraints on a product that does not yet exist, describe the product as though it already existed.
Kendall, Matthew. Ionocom (2005). Articles>Writing>Specifications>Grammar
