Functional vs. Non-Functional Requirements

What are functional vs. non-functional requirements of software engineering? Why is it essential to have functional and non-functional requirements for your project to succeed? Which is more important? What is the difference between functional vs. non-functional requirements? This article will answer all of these questions.

Ultimately, the goal of any project is to deliver a quality product that meets the customer’s expectations. Customers communicate their requirements to project teams through functional requirements, which are essential to ensure the development team is progressing in the right direction.

Unclear requirements can lead to a poorly defined scope, which can cause many difficulties from the project’s beginning. And a poorly defined scope can lead to delays and rapidly increasing costs. As a result, your customer might not have the funds or time to invest in the desired high-quality products.

Most customers have both wants and needs. For example, the customer may request to reduce the scope once they have received the cost estimate. Most often, removing specific non-functional requirements will reduce the complexity adequately. Having many NFRs can increase costs. Conversely, insufficient NFRs may result in a poor user experience.

It will benefit both client and the supplier to understand the differences between functional vs. non-functional requirements. In addition, understanding the difference allows better scope definition, cost optimization, and, ultimately, happy customers. 

A comprehensive and sensible collection of functional and non-functional requirements is the most important thing a project should have to avoid failure.

Every project requires clearly understood, balanced, and well-thought-out requirements by everyone involved. But perhaps, the most important thing is to avoid de-scoping requirements halfway through the project.

But what does it mean to distinguish between “functional” and “non-functional” requirements? It will become easy to define once you understand the distinction.

You can use functional requirements to typically specify a behavior or function. Officially, a functional requirement specifies what the system must do.

A perfect example of a functional requirement is: “Display the name of the flash drive, its total size, space, and format.” An additional example would be to “add a customer to a system when they order a product.”

Perhaps an even more relatable functional requirements example for a fuel canister includes the “ability to contain fluid without leaking.”

What are Functional Requirements?

A functional requirement can be defined as “Any requirement specifying what the application ought to do.“

Successful products will have strict requirements, which holds true for all projects. Therefore, it is no surprise that 68% percent of projects meeting exact requirements achieve the required quality standard.

Another way to put it is that a functional requirement describes a system’s behavior when certain conditions have been met. For example, “Send email to a new customer who signs up” or “Open a new bank account.”

A functional requirement for an object many people use daily, such as a thermos, would be the “ability to contain coffee or tea without leaking.”

What are the advantages of Functional Requirements?

These are the advantages of creating a functional requirement document:

• It allows you to determine if the application has all the functionalities specified in the functional requirements.

• You can use a functional requirement document to describe the functionality of a system and its subsystems.
• Both functional requirements and requirement analysis can help you identify missing requirements. This helps define the expected system behavior and service.
• The most cost-effective time to correct errors is during the functional requirement-gathering stage.
• Facilitate project management by supporting user goals, tasks, and activities.
• You can express functional requirements in Use Case forms or user stories, provided they show externally visible functional behavior.

What are some examples of Functional Requirements?

Some examples would include the following:

• When a user attempts to log in, it authenticates them.
• In the event of a cyberattack, the system shuts down.
• When users register for a program for the first time, they receive a verification email.
• The software validates customers automatically against the Salesforce system.
• The Sales system must enable users to track customers’ sales.
• Only employees at the Managerial level can see revenue data.
• The system should be compatible with the bank’s API.

What are the 11 most common types of Functional Requirements?

• Administrative functions
• Authentication
• Authorization levels
• Audit Tracking
• Business Rules
• Certification Requirements
• External Interfaces
• Historical Data
• Legal and Regulatory Requirements
• Reporting requirements
• Transaction cancellations, corrections, and adjustments

How do non-functional differ from functional requirements?

The difference between them is non-functional requirements describe the system’s operation, while functional requirements describe what the system should be doing.

NFR is defined as “how the system should behave” and becomes a limitation on the system’s behavior. NFRs could be captured as a quality attribute of a system.

For example, one non-functional requirement of a bicycle helmet could be “must not break when dropped from 2 meters.”

What are Non-Functional Requirements?

A non-functional requirement can be defined as: “Any requirement specifying how the system must perform a specific function.” Non-functional specifications specify how a system should behave and determine the limitations of its functionality.

In addition, NFRs encompasses all remaining provisions not covered under the functional requirements. These criteria evaluate the system’s operation and not specific behaviors. For example, “users should be able to retrieve data from the system within three seconds.”

For the gas canister previously mentioned, NFR would be to: “Contain hot liquid but not heated up to more than 100 degrees Fahrenheit.” Despite not being able to meet non-functional requirements, basic functionality remains unaffected.

Why are non-functional requirements necessary if the product’s functionality isn’t dependent on them? It is about usability, as non-functional requirements would affect the user’s experience. They define the system’s behavior and general characteristics and inform the features.

When non-functional requirements are clear-cut and properly implemented, they will make the application easier to use and ensure it performs well.

The user’s expectations are the main focus of non-functional requirements. Therefore, the non-functional specifications become the product’s properties.

Let’s look at a simple example of a functional requirement. When a user clicks on a button, the system loads a page. The corresponding NFR specifies the acceptable response time for the webpage to load. A delay in loading can lead to a poor user experience even if all functional requirements are met.

What are the advantages of Non-Functional Requirements?

There are many benefits to these requirements:

• NFRs are necessary to ensure compliance and legal or regulatory requirements.
• They guarantee the application’s reliability, availability, performance, and security.
• NFRs ensure a seamless experience and easy operation of the software for users.
• They assist in the formulation of security policies for the software system.

What are some examples of Non-functional Requirements?

What are the 25 most common types of Non-Functional Requirements?

• Accessibility – Measures how easily a solution is available to people with Motor, Cognitive and Visual impairments.
• Reliability, availability, maintainability – How often does the system fail? What are the downtimes, and how often is it possible to prevent them?
• Capacity, current, and forecast.
• Compliance, Regulatory – Describes product conformity to laws.
• Data Integrity – Describes how data accuracy, completeness, consistency, and overall accuracy are handled.
• Documentation – Describe what the system accomplishes and how to utilize it.
• Disaster recovery – A collection of policies, tools, and procedures permitting the recovery or continued operation of vital technology infrastructures and systems following an accident or natural disaster.
• Efficiency – How effectively the system uses scarce computational resources like CPU cycles, memory, and disk space.
• Extensibility – Ability to extend a system and the effort needed to implement the extension.
• Fault tolerance – Ability to meet requirements despite system or hardware failures.
• Interoperability – The ability of software to communicate with other components or systems.
• Manageability – Ability to monitor and manage software to ensure the system runs smoothly and securely.
• Performance – Measure how well a system responds to data based on utilization and throughput.
• Privacy – What applications will be integrated with, how will data be stored, and who has access to it?
• Portability – Which operating systems, browsers, and hardware do you use to run the software? Is it compatible with other programs and processes in these environments as well?
• Recoverability – The capability of software to restore damaged operational data or achieve the desired level of performance.
• Resilience – The ability of a solution to absorb the effect of a problem within a system and still provide acceptable service levels to the business.
• Response time – Measures how long a server takes to reply to every single transaction or query.
• Robustness – The ability of a computer system to deal with errors during execution or with incorrect input.
• Scalability – Assesses the system’s ability to handle high workloads without compromising performance.
• Security – How do you protect your system and data from hackers?
• Serviceability, Supportability – The ability of a service/technical expert to install the software system in a real-time environment, monitor the system while running, identify any technical issues in the design and provide a solution to resolve the issue.
• Stability – Determine if a software application can execute or accomplish tasks within specified conditions or for a set duration.
• Testability – The extent that a system supports testing in the development process.
• Usability – How close does the intended use of the system match the software’s implementation?

Non-functional requirements, as mentioned above, specify the system’s “quality characteristics” or “quality attributes.” 

Many stakeholders are interested in ensuring NFRs are met, especially in large systems where the buyer is not necessarily the user.

NFRs are, therefore, essential and should not be underestimated. Using non-functional requirement groupings is one way to ensure no NFRs are left out.

How should a Scrum Team Deal with Non-Functional Requirements? 

Scrum development team deals with Non-functional requirements in a few key ways:

Prioritize the requirements 

Prioritizing NFRs is important for scrum teams. Not all of them may be equally valuable or priorities; therefore, it is essential to rank the NFRs according to their importance. With this approach, development teams can focus on the most important requirements and ensure that each gets the right attention.

Break down the requirements

Breaking NFRs into smaller chunks helps development teams understand how to implement them. It also allows them to divide the work into manageable chunks and keep track of their progress. This can be done by dividing the requirements according to priority, complexity, or other criteria.

Keep stakeholders involved

Since NFRs are often related to how users will interact with the products or how they will look, it is important to involve all relevant stakeholders (product owners, customers, and other stakeholders with a vested interest in the project) when making decisions about these requirements. This will ensure that everyone’s needs are considered and that the end products meet their expectations.

Acceptance Criteria 

The NFRs can also be part of a software product’s acceptance criteria, which are the conditions for acceptance by users, customers, or other stakeholders. This can help define how the requirement will be implemented, how it will be tested, and how success or failure will be measured. A constraint that is part of the acceptance criteria may make it hard for stakeholders to see the effort spent solving NFRs.

Set a definition of Done 

Having a “Definition of Done.” This is a set of criteria for determining the completion of tasks. DoD is key to ensuring that you meet all NFRs. This definition should include how and when the scrum team will test each requirement, how they will measure success or failure, and how they will communicate any changes to the stakeholders.

Use prototypes to elicit and validate non-functional requirements 

Prototyping can be used to elicit and validate non-functional requirements. By creating prototypes that simulate how the product will work, stakeholders can better understand how their needs will be addressed by the end product. Prototypes also allow them to provide feedback on how they would like the product to look or how it should function. Therefore, this feedback is helpful to ensure that you meet all NFRs.

There are benefits to using prototypes to gather non-functional requirements:

• getting accurate feedback and validation from stakeholders
• uncovering unknown user needs
• reducing the risk of project failure due to overlooked requirements
• helping scrum teams and stakeholders stay on the same page

Be flexible and adaptable to new information 

It is also important for the development team to remain flexible and adaptable as they handle NFRs. Since these are often changing or evolving as the product progresses, it is essential for the scrum team to be able to adjust and refocus on what matters most at any given time. This can help ensure that all non-functional requirements are met promptly and efficiently.

Ensure every Increment meets the requirements

Ensure that each project increment meets all the non-functional requirements and provides the expected value. Even if it takes extra time and effort, you should ensure that no NFR is left behind. This can save time and money in the long run by avoiding potential problems resulting from missing or incomplete requirements.

Make work visible in your product backlog 

The product backlog should list all the non-functional requirements that need to be included in the next version of the product. This visibility helps everyone stay focused on what needs to be done and how much effort is required.

Independent User Story

It is important to test how the products behave against each requirement independently, as this allows you to identify any discrepancies and make changes before they become costly issues. This can be done by creating user stories for each non-functional requirement and writing tests around them.

Regression Testing 

Finally, it is important to ensure that any changes made to the product do not cause issues with existing non-functional requirements. This can be done through regression testing, which Identifies any unexpected consequences of code changes and modifications through newer user stories (e.g., new or reopened bugs). To ensure that the application is ready for use as part of a combined unit, this exercise is also carried out as part of the ‘hardened sprint.’ It ensures that the product works as intended and meets all its NFRs.

What is the difference between the definition of “Done” and the acceptance criteria? 

The Definition of “Done” or DoD is a widely adopted practice in software engineering. It identifies the completion of each task and how the QA process should assess the output, ensuring it meets all the requirements.

Acceptance criteria are specific conditions that must be satisfied for a requirement to be accepted. This includes functional and non-functional requirements, like how the system should respond in a certain situation or how quickly it must process data.

Defining how the scrum team deal with non-functional requirements is essential. This includes implementing and testing each requirement and evaluating the final product’s performance and quality standards.

What is the difference between Functional and Non-Functional Requirements?

The table below aims to depict what the differences are by comparing them head-to-head.

Parameters	Functional Requirements	Non-Functional Requirements
What it is	Verb	Attributes
Requirement	It is mandatory	It is optional
Defined by	The subject matter experts	Architects, Technical Leaders, or Software Developers
Defined at	Component level	System level
Capturing type	It is documented in use cases	It is documented as a quality attribute.
End-result	Product feature	Product properties
Capturing	Easily captured	Difficult to capture
Objective	It helps you verify the functionality of the software	It helps you to verify the performance of the software
Area of focus	User requirements	Focus on user expectation
Documentation	Describe what the product does	Describes how the product works
Type of Testing	Functional Testing like System, Integration, End to End, and API testing	Non-Functional Testing like Performance, Stress, Usability, and Security testing
Test Execution	Test Execution is done before non-functional testing	After the functional testing
Product Info	Focuses on Product Features	Focuses on Product Properties

What is the relationship between Functional and Non-Functional Requirements?

Often, a functional requirement is accompanied by a non-functional one. Keep in mind that the difference is essentially what and how. They are both essential. Here are some examples:

Functional Requirement	Related Non-Functional Requirements
The server shall send a welcome email when a new account is created.	Sending welcome emails must be done within 5 minutes of registration.
If the order status changes to fulfillment, the company printer will print the details.	Packing slips must be printed on both sides of white paper sheets measuring 5″ x 8.5″
The system should allow a customer to complete and submit a service request form.	The form must load in 3 seconds after it is requested from the server.

How to go about gathering requirements?

A guided brainstorming session is a great way to gather requirements and bring all stakeholders together.

Four basic functional requirements can be divided into:

Business requirements: Define the ultimate goal, such as an inventory management system, an e-commerce platform, or a physical product. You can also find things such as approval workflows or authorization levels.

Administrative functions: These are the everyday things the system will do, such as reporting requirements.

System requirements: Include software engineering or hardware specifications, system reactions, or system action.

Once you have clearly defined the functional requirements, it’s time to start gathering nonfunctional requirements questions.

Usability: This section focuses on the interface’s appearance and user interaction. What colors are the screens? What should the size of the buttons be?

Reliability/Availability: What would an acceptable downtime be? What are the uptime requirements?

Scalability: Can the system handle a growing number of simultaneous users? For physical installations, spare hardware is included with the possibility of future expansion.

Performance: What are the acceptable response times?

Supportability: Can you access external resources remotely or receive support in-house?

Security: What security requirements exist for both physical installation and cyber-security installation?

What is the distinction between Business Requirements vs. Functional Requirements?

The difference between business requirements and functional requirements is the question they answer. Business requirements define what a business desires (the objective). In contrast, functional requirements deal with how the business can achieve the desire.

How do you write non-functional and functional requirements?

There are many ways to write functional or nonfunctional requirements.

A requirements specification document is the most popular way to describe functional vs. nonfunctional requirements. In general, it describes the functionality required. It contains the project goal and a brief overview of the project. The requirements specification document also provides applicable constraints and assumptions. To aid non-technical stakeholders’ scope understanding, the requirements specification document should contain visual representations.

The work breakdown structure (WBS) is closely related to a requirements specification document. This allows you to break down the entire process by “decomposing” requirements into their components until they can’t be broken down anymore.

User stories are another approach to writing requirements. A user story is written from the user’s perspective and describes the desired behavior of the system.

An effective User Story states, “As a _______, I want to _______.” The beauty of user stories is they do not require technical knowledge to be written. User stories can be used with requirements specifications to help end users define their needs.

Use cases can be compared to User Stories because they don’t require technical knowledge. Instead, these use cases are an overall description of what a user does while they do a task. A good example is “purchase product,” which would describe each step during the purchase process.

What are the best practices for functional and non-functional requirements?

Regardless of their form, the key to effective functional requirements is clear, concise, and easily understood. Every audience is different. However, in every case, the more the requirements are in the natural language, the better. Active voice is preferable to passive voice when it comes down to language. By using active voice, you can make requirements more straightforward and concise by ensuring an actor in every requirement statement.

Write precise and complete requirements. Avoid vagueness when writing them. Avoid confusing people by not including unnecessary information. When writing user stories or a requirements document, use “must” rather than “should.” Use the same terminology and units throughout the document, format, and language.

You must communicate the requirements. What does it mean if stakeholders insist the system should be “fast”? What does it mean when stakeholders state the website must handle “many” users? Quantifying and reducing these requirements is necessary.

You can test good requirements. But how do you know when it meets all the conditions? It is crucial to write requirements in a clear way so you won’t combine them. This will make turning your needs into reality easier and help with later testing.

You should ensure your requirements cover every scenario. That means you must include requirements detailing what the system must not do. However, be careful not to be too specific. But be specific about the features users need. A more extensive product is not always better. Larger projects often result in increased costs, diminished impact, and a product with too many difficult features to use and confuses users. It should be possible to track back each requirement to one project objective. It is helpful to compare the industry with your competitors to understand their work better. Sometimes you have to aim for the same quality as your competition. Other times you reach a point of reference from which you can improve.

When speaking with stakeholders and users, it is crucial to understand better their needs and how they relate to project objectives. Sometimes, they present a solution when they should be offering a problem. Offering the problem allows for a development team to brainstorm the best solution. You should also be aware of their authority to submit requirement requests. A project manager usually has the final say. It is important to discuss this before adding more requirements. Document and create a requirements traceability matrix. Requirement traceability matrixes help you contact the person right person when questions arise. It is often an iterative process to develop specifications. Ensure the requirements written by you are both feasible and do not conflict.

It can be difficult to define requirements clearly. However, you can make them easily understandable by non-technical stakeholders. Visuals are encouraged to reinforce information.

Last but not least, don’t consider nonfunctional requirements irrelevant, regardless of their name. For example, although an application that takes 2 minutes to load may meet its functional requirements, it would not be usable.

How to make Non-Functional Requirements Visible in Scrum?

One of the best ways to make scrum non-functional requirements visible is by adding your development team’s NFRs to a checklist. Also, you can add this checklist to each user story and verify it before moving a user story to Done.

In addition to a checklist, AccelaTest can help answer the question of “how to make non-functional requirements visible in scrum” through Shared Steps. Shared Steps are the backbone of a successful development team which you can use in multiple test cases.

How can AccelaTest help with functional vs. non-functional requirements?

One of the most critical parts of testing is having traceability between requirements and test cases. AccelaTest is a Test Case Management Solution with integrations with project management systems like Jira to facilitate the traceability between requirements and test cases. In addition to having traceability, AccelaTest offers reusable test cases that you can use to document and test your Non-Functional Requirements. Start documenting your requirements and adding traceability to AccelaTest today.

Conclusion:

When it comes to functional vs. non-functional requirements, both types are critical to a project’s success. Non-functional requirements should not be cut or scrimped on to make a project fit a budget because a project without NFRs will lead to poor quality. You don’t have to complicate the process of writing solid requirements. Remember that every requirement must be clear, concise, and easy to understand.