The success of your next project depends on whether you have accurate and detailed requirements, which means you also need functional and non-functional requirements.
But interestingly, do you know why software development projects fail?
Forbes says unclear requirements and a lack of detailed planning are two common reasons development projects fail.
Gathering the right requirements for a project’s success requires you or your project management team to clearly understand and detail the differences between functional and nonfunctional requirements.
We have developed a new requirement gathering tool to help quicken the process and ensure project success with prompted entities so no requirements get left out.
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We could help you define the differences once you know more about us.
Alternatively, discover what every key difference is between the two requirement types.
Why Should You Use Requiment for Functional and Non-Functional Requirements Gathering?
The benefits of using Requiment include doing requirements gathering with little to no knowledge. Our demo videos help you through a guided process. Some features we offer with Requiment include:
- Download output reports
- Create wireframes without experience
- Update easily any requirements document
- Use task generation to allocate faster tasks
We also have a complete guide to requirements gathering in 2024.
However, knowing the differences between functional and non-functional requirements also guides you.
Important Requirements Types for Analysis and Development
But first, non-functional and functional requirements aren’t the same as business requirements or user requirements, but they derive from the other requirements types.
- Business requirements come first in the gathering process. They represent high-level business needs, including gaining market share, increasing revenue, and improving customer satisfaction.
- User requirement concentrates on objectives users can accomplish with the system and derive from user stories, use cases, and scenarios. It’s the second part of requirements gathering.
- Product requirements come last in the gathering process. They represent how the system functions to satisfy users and the business. They comprise functional and non-functional requirements.
Non-functional or functional requirements are often captured on product requirements documents (PRDs) or as a separate functional requirements document (FRD) or non-functional requirements document (NFRD). However, they contain the details of the end of requirements processing.
What Is Functional and Non-Functional Requirements?
Functional vs not-functional requirements define two requirements to gather before a development team can design a software system.
However, functional requirements describe a system, its components, and the functions it must perform. On the other hand, non-functional requirements describe software systems’ quality attributes.
Understanding comprehensive differences will help the software engineering department with clearly defined requirements to accurately design system components and intended behaviour.
What Is a Functional Requirement?
A functional requirement defines a system’s or product’s interactive and usable elements. A functional requirement describes functions or features developers design to enable users to complete desired actions to reach a specific objective.
Software engineers describe functional requirements as a system function or component, where the functions define the intended behaviour between inputs and outputs.
In other words, you can define functional requirements as the system’s basic behaviours.
What Is a Non-Functional Requirement?
A non-functional requirement defines the specifications that describe a system’s constraints or operational abilities.
It also describes criteria used to judge a system’s operations and doesn’t refer to the behaviours or functions within that system.
Software engineers and requirements analysts consider nonfunctional requirements (NFRs) as the limits and capabilities of system behaviour.
A non-functional requirement also acts as a system quality that designs solutions based on constraints and abilities.
Key Differences Between Functional and Non-Functional Requirements
Functional and Non-Functional Requirements Types
Any software system has functional and non-functional requirement types. The types define the software feature beneath non-functional or functional requirements.
Functional Requirement Types
Ultimately, functional requirements should include a detailed description of the following functions:
- Algorithms – Data formulations and manipulations that guide what must occur in the system.
- Archiving – The ability to archive data for long-term storage.
- Audit Tracking – The process of tracking critical data for non-functional requirements.
- Authentification Function – A tool that collects information users must share for authentication.
- Authentification Level – Specifications for how systems determine authentication levels for CRUD users.
- Backup & Recovery – Backup and recovery instructions for a system crash.
- Business Rules – Business rules a system must follow.
- Certification Requirements – Embedded security certifications and external interfaces certification requirements within a system.
- Database – Formats and elements that define the necessary data to store in a system.
- Data Processing – Data entry, validation, storage, and retrieval features.
- External Interfaces – The functions of a system’s external interface outside of the main system.
- Historical Data – Defined data storage to accommodate dynamic data growth.
- Legal or Regulatory Requirements – Governing laws and regulations every system and company must follow.
- Searching & Reporting Requirements – The requirements detailing how users search and retrieve data.
- Transactions – Transaction adjustment, business rules transaction corrections, and cancellation specifications.
- User Interface & User Experience (UI/UX) – User-friendly design elements of the system’s interface and experience.
Non-Functional Requirement Types
Non-functional requirement types are also called the “ity” requirements. Ultimately, non-functional requirements should include a detailed description of the following features:
- Availability/Reliability – What is the normal usage critical failure time, and do users need 24/7 access?
- Capacity – The system storage requirements for today and the future, and how will it scale?
- Compatibility – The minimum hardware requirements and which operating systems/versions on which devices must it support?
- Maintainability/Manageability – The time it takes to repair components and the ease of administrator system management.
- Performance – A quality attribute that oversees the system’s responsiveness to multiple user interactions to avoid a negative user experience.
- Recoverability/Serviceability – The time it takes the system to recover from critical failures and the time between critical system events.
- Scalability – Another quality attribute that defines high system loads where the system still performs.
- Security – How the system stores and shares sensitive information, an IT department’s adherence to specific standards, and the security best practices to protect sensitive data.
- Usability – The product’s ease of use and a definition of the intended user experience (UX).
Functional and Non-Functional Examples
Non-functional requirements focus on different acceptance criteria compared to functional requirements because the two describe separate scenarios of how the system works.
Let’s help you to the same page by sharing examples of functional or non-functional requirements.
Functional Requirements Examples
Some software features and functions examples of functional requirements include:
- System emails are sent each time an order is placed on the website or app.
- The ability to subscribe to a newsletter with an email.
- Site users use their mobile numbers to verify accounts.
- The feature to enter a username and password to authenticate a login.
- A button to report a system problem.
- CRUD users can change, read, update, or delete account information.
- Adjust cart items while shopping online, or check them out.
- A filter to reduce results in a search engine.
- A function to download or print a page from the system.
- The function that allows users to authenticate accounts with external platforms.
- An app sends updates, promotional content, or reminders to users.
Non-Functional Requirements Examples
Some function and feature examples of non-functional requirements include:
- How fast the system responds to input commands and increasing system loads.
- Security measures, including password generators, security questions, and account locking.
- The system’s portability across multiple devices and platforms.
- Whether the system is compatible with other applications running on the same device.
- The amount of capacity a system has to store new settings and preferences.
- A system’s reliability related to the probability of failure and the number of and time between critical failures.
- How the system responds to external interfaces and environmental changes.
- How the system localises languages, geographic locations, time zones, and currency.
- Criteria that manage features like navigation, performance quality, and tool purposes.
Functionality and Non-Functionality Advantages
Both requirements have advantages for your system or project.
Functional Requirements Advantages
Defining functional requirements has a few advantages, including:
- Clearly defined functional requirements prevent misunderstandings between stakeholders, developers, designers, and testers.
- Everyone on the development team can focus on the same objectives to finalise the functional features and achieve the project goals.
- Good communication is key to any successful technical project, and clear functional requirements ensure well-defined communication and understanding among a development team.
- Estimations won’t lead to failure or risky development strategies if the development team understands the user’s needs and requirements.
- Project teams can resolve assumptions early when a functional requirement defines precise instructions, which saves time and money.
- Documented functional requirements provide a written record to reduce the need for stakeholder meetings.
- A functional requirements document (FRD) makes the development process, timeline, and costs more predictable.
- Thoroughly captured functional requirements during the discovery phase can eliminate or reduce the likelihood of risks or challenges.
Non-Functional Requirements Advantages
The advantages of non-functional requirements include the following:
- To ensure the system follows legal or regulatory requirements.
- To guarantee the system’s performance, reliability, and availability.
- To ensure the system has easy operations and a good user experience.
- To help formulate software system security policies.
- To help fit the system’s purpose and user needs.
- To use quality attributes to direct the intended behaviour.
- To make the system easy to maintain.
Requirements and Non-Functional Requirements SRS Documents
A software requirements specification document is the core of what software engineering teams use to develop product features, allocate system attributes, and conduct API testing and security testing.
The requirements specification document is an overall description of what’s necessary for project success.
Functional and Nonfunctional Requirements Documents
Functional and nonfunctional requirements documents are written in Agile models but may contain visual representations.
You can learn how to write a software requirements document (SRD) or see what you can expect.
Here are effective requirements documentation you can expect:
Software Requirements Specification Document (SRS)
The software requirements specification (SRS) documents outline nonfunctional and functional requirements.
The SRS includes descriptions of all capabilities and functions the software must provide.
It also describes the non-functional constraints and assumptions to present a system overview.
What must the SRS documentation describe?
- Purpose – System overview, background, definitions, and the particular solution, which may change in the Agile development stage.
- Overall Description – Company rules, assumptions, constraints, project scope, project vision, and the acceptance criterion.
- Specific System Requirements – Quality attributes, database requirements, functional requirements, administrative functions, and system safety.
The SRS document must also include the WBS documentation, user stories, and use cases to help stakeholders understand every aspect.
Also, you may not see a comprehensive SRS document until the system performs long enough to gain user feedback about user expectations and end-user perspective.
Fast visual representations could delay improvement opportunities if the SRS document finalises before reaching a particular solution to further enhance the system’s functionality.
Software developers may also release software versions with some administrative functions until gathering enough information from user feedback for a final solution.
In that case, the SRS document may be delayed, which is a solid strategy in Agile methodology.
Functional Decomposition and Work Breakdown Structure (WBS)
A functional decomposition is a process used to break down complex problems, systems, or structures into understandable parts on a visual representation of the product features.
Decomposition creates a system functionality visual representation in software engineering.
The visual is called a work breakdown structure (WBS).
The WBS document illustrates how complex features and functions break into easily digestible features and functions for stakeholders without technical knowledge.
Requirements Formats: User Stories and Use Cases
Functional and nonfunctional requirements are more easily understood by all stakeholders when you capture them in an easy-to-use format, including use cases and user stories.
Use Cases
A use case lets stakeholders better understand how the development team creates the product.
A use case describes how the system outputs behave when users enter inputs to define functional requirements.
For example, a use case has three primary elements:
- Actors – External users interacting with the system.
- System – The system is described by functional requirements that describe the system behavior.
- Goals – Goals outline the purpose of the user and system interaction.
Business analysts may represent use cases with a use case diagram or use case specification.
A use case specification represents event sequences and information related to the basic flow use case, such as:
- The description
- Pre and post-interaction conditions
- Basic interaction pathways
- Alternative pathways
- Exception pathways
On the other hand, use case diagrams contain fewer details.
It reveals a high-level overview of actors, use cases, and system relationships.
The diagram has the following elements:
- Use cases drawn with ovals to represent various interaction scenarios between users and the system.
- System boundaries with a box outline to group various system use cases.
- Actors depict external users or simultaneous users interacting with the system.
- Association lines to show different relationship types between users and use cases.
User Stories
A user story helps stakeholders visualise the features from a user perspective.
A user story defines an external perspective scenario where users interact with the software system.
For example, the user story describes how users want the system to respond to them.
A user story flows like this: As a <user>, I want <goal> so that <reason>.
Here’s an acceptance criteria checklist example for a search feature user story:
- A search field is accessible on the top tab.
- The search begins when users click on submit.
- A default placeholder is gray and reads, “Start typing here.”
- The placeholder makes algorithm suggestions or disappears when users begin typing.
- English is the search language.
- Users can type up to 200 characters/symbols.
- No support exists for special symbols.
- The system displays “Search input can’t include special symbols” if users input special characters.
Also, every story must meet the INVEST criteria:
- Independent – Can you implement and schedule stories separately?
- Negotiable – Can all stakeholders agree to prioritise negotiation over specifications?
- Valuable – Does the story add value to the customer, user, or stakeholder?
- Estimable – Can you estimate the story?
- Small – Is the story small enough for short production releases?
- Testable – Will the story test clear and acceptable?
Summing Up Non-Functional vs Functional Requirements
Collecting the best related non-functional requirement and functional requirement list could define the ideal project outcome.
To capture objective helps any project or software development reach a greater chance of success. Our Requiment tool helps you gather functional and non-functional requirements.
Sign Up today to start gathering requirements for your next software project.
Also, read What Are Functional and Non-Functional Requirements for a summary of this article.
Functional VS Non-Functional Requirements FAQs
What Is the Difference Between Non-Functional and Functional Requirements?
Functional requirements define the system’s usable and interactive elements, while nonfunctional requirements describe the directives, constraints, and assumptions guiding functional features.
What Is a Non-Functional Requirement?
A non-functional requirement defines the specifications that describe a system’s constraints or operational abilities.
It also describes criteria used to judge a system’s operations and doesn’t refer to the behaviours or functions within that system.
What’s the Difference Between System and Functional Requirements?
You capture functional requirements during an analysis phase, whereas system requirements include the functional and non-functional requirements from the analysis to use during development.
What Are Nonfunctional Requirements Metrics?
Nonfunctional requirements metrics include:
- Availability
- Capacity
- Compatibility
- Maintainability
- Manageability
- Performance
- Recoverability
- Reliability
- Scalability
- Security
- Serviceability
- Usability
What Is the Difference Between Functional and Non-Functional Testing?
Non-functional testing includes performance, reliability, usability, and stress tests to check the system’s properties.
Functional testing includes API, system, applications, features, end-to-end, and integration tests to check the system’s processes.
Who Gathers Non-Functional and Functional Requirements?
Business analysts typically gather nonfunctional and functional requirements before a project commences.
Knowing how to gather as a business analyst is a critical skill the analyst must possess.
However, software developers, product owners, and project managers may collect data with an automated tool like Requiment.
Sign Up today to start gathering requirements.
Where Can I Learn to Gather Functional and Non-Functional Requirements?
Michigan State University includes requirements-gathering for mandatory and non-mandatory capturing type functional and nonfunctional requirements on the academic programs catalog.