Key Differences Between Functional vs Non Functional Requirements

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Key Differences of Functional vs Non-Functional Requirements

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Last updated on April 2nd, 2024

Do you know?

  • 47% of the projects fail due to re-prioritization, and 39% are delayed due to unclear requirement estimates during the planning phase. (PwC)

  • Unclear requirements increase the project timeline and budget significantly. The average cost of project overruns is 27%. (HBR)

  • Shifting requirements coupled with technical complexity remains the second most pertinent issue for IT project failures and cost overruns. (McKinsey)

Clearly, the project specifications and requirements can prove to be drivers or roadblocks to the realization of a successful project. They establish a formal agreement between the service provider and client to reach a common goal. The detailed project requirements also help mitigate the project delays and financial risks.  

But the real challenge begins when you encounter a standoff between functional and non-functional requirements. Both functional and non-functional requirements are crafted by the business analysts based on client communication, needs, and wants. 

But what exactly is the difference between these two in the context of software development, and why are both types critical for a software project’s success? Let’s find out.

What Are Functional Requirements?

The functional requirement specifies “what the system is supposed to do.” It describes the expected behavior of the function of the system when specific actions are executed. Consider this; when a user registers on a website and receives an email notification for registration confirmation, it is specified as a functional requirement. 

In a nutshell, the software will not work if the functional requirements are not met. Some of the functional requirements include:

  • Business and reporting requirements

  • Certification requirements

  • Levels of authorization 

  • Audit tracking

  • Legal and regulatory requirements

Examples of Functional Requirements

  • Sales software should allow users to record customer and sales information.

  • Automatic validation of customers against the software.

  • Only senior managers will be able to access the revenue data.

  • The software system should be integrated with payment gateways through APIs.

  • The software system should be compliant with Section 508.

What Are Non-Functional Requirements?

The non-functional requirement specifies “how a system will perform the specific function.” It describes how the system is supposed to behave and the limitations of the functionality. While the functional requirements define the fundamental behavior of the system, non-functional requirements set out the ways to carry out the software functions. 

For example, an email is sent when a user registers on the website, but non-functional requirements will clarify when (1 minute after sign-up) the email will be sent. The non-functional requirements cannot be considered the backbone of the system but are highly user-oriented. 

Some of the non-functional requirements include:

  • Reliability and performance

  • Usability

  • Maintenance

  • Serviceability

  • Recoverability

  • Scalability

  • Integrity and interoperability of data

  • Security

Examples of Non-Functional Requirements

  • The assigned login password should be changed after the initial successful login.

  • The website should be capable of handling 20 million users without hampering the system performance.

  • Unsuccessful login attempts by the user should be recorded in the audit trail.

Key Differences of Functional vs Non-Functional Requirements

Functional requirementsNon-functional requirements
Represent the features and elements of the software.Represent the quality characteristics of the system.
Requirements are established by users.Requirements are established by the technical team.
Easy to define.Difficult to define.
Mandatory.Not mandatory.
Measure the functionality of the system.Evaluate the system performance.
Involve functional testing such as API and system integration.Involve non-functional testing such as security, stress, and performance.
Clarify the activities that should be performed by the system.Set specific rules and regulations for the software system.

How Are Functional and Non-Functional Requirements Written?

Both functional and non-functional requirements must be written by the business development teams, analysts, and technical personnel. They can be written in various forms, including user stories, specification documents, and use cases. 

Software Requirements Specification (SRS) Document

It is the standard form for writing software requirements. It indicates the functions that need to be included in the product and how the system should perform them. This document is a detailed description of all the features of the product. 

SRS document translates the needs and wants of the client into an understandable language. It covers every detail of the product and helps estimate the software development’s final time and cost. 

Use Cases

Uses cases are part of the agile development process and reflect every possible way of user interaction with the system. The use cases detail the flow that leads to the purpose of the software. 

For example, if an eCommerce platform like Amazon needs to be built, it is crucial to consider various actors like buyers, sellers, suppliers, auditors, distributors, etc. The actions of all these actors will be forecasted and specified in the document.

Final Words

A critical rule of thumb is that comprehensive documentation is the key to project success. The combination of functional and non-functional documentation clarifies the steps that need to be taken by the development team and how the product should work. 

Proper documentation also increases customer satisfaction for end-users, businesses, and stakeholders.

Want to convert an idea into a real-time software product? Get a free consultation today.

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