Functional vs Non-functional Requirements: Key Differences

Every project is always created with one thing in mind: yield a supreme result that relies on the end user.

Functional vs Non-functional Requirements: Key Differences

Every project is always created with one thing in mind: yield a supreme result that relies on the end user.

And the secret sauce is always the same:

To create a high-quality solution, all parties involved should develop a panoramic and clear view of both functional and non-functional requirements. Otherwise, the repercussions of your failure may result in financial losses, damaged reputation, and infuriated users.

But what exactly is the difference between functional vs non-functional requirements? And most importantly, how to define them?

Bear with us because we’re going to break down those beasts further in the article.

What Role Do Requirements Play In The Software Development Process?

When you’re just taking off, you are likely to have only a very high-level idea of your needs. To define the boundaries, you need to perform meticulous competitor research to finesse those needs to a tee and list your requirements.

Those requirements are the main driving factor for your success. Think of it as a building construction process. If you have foundation problems, they will pull apart or compact walls and support beams. These shifts will result in sagging floors and ceilings or, in our case, in outright project failure.

Besides, clearly defined requirements glue together every member of your team and serve as a beacon for everyone.

But there’s more:

  • According to Statista, almost half of all software projects fail due to poorly defined requirements;
  • On top of it, vague requirements stretch the project timeframe and resources up to 60%, as Academia Edu suggests
  • According to the same source, vague requirements guzzle up more than 40% of the tech development budget.

Sounds discouraging, right? But let’s see what sweeteners you get when you pay due diligence to your requirements.

  • Assigned roles and clear terms. Requirements leave no room for innuendo between team members and stakeholders.
  • Reduced communication time. It’s more time-effective to chart out your development path with a business analyst beforehand. This practice eliminates unnecessary communication during the development stage and ensures a more efficient allocation of resources.
  • Accurate estimates. In-depth requirements make sure your costs and time have a more outlined silhouette.
  • No rework. Errors are less perilous if you detect them at the beginning of your development process.

Overall, stated requirements keep developers and stakeholders on the same page, save you thousands of dollars and hundreds of hours as well as make sure your product is fine-tuned to your and customers’ needs.

The Many Faces Of Requirements

To make the most of your documentation, you need to grapple with different kinds of requirements. In general, the main types include:

  • Business requirements - those are ones that aren’t linked with the specifications. They have to do with the high-level statements of goals and needs that should help the company to boost its profit or meet the standards.
  • Stakeholder requirements or user needs - describe how users interact with the product.
  • Solution requirements:

Functional requirements

Non-functional requirements

  • Transition requirements - describe all temporary capabilities that are required to transition from an old system to a new solution.

The Big Comparison

Now that we’ve covered the basics, let’s go over the main differences between the two.

  1. Functional requirements (FRs) outline what the software system must do, while non-functional requirements (NFRs) put limits on how the system will do it or specify its activities.

Thus, a prominent example of FRs would be:

The system must send a notification each time a specific condition is met (e.g., the money has arrived at your bank account.).

Related NFRs for the system might be:

Notification must be sent with a delay of no more than 1 hour after the condition is met.

  1. A functional requirement describes the behavior of the system as it relates to the functionality of the system. An NFR is related to a performance characteristic of the system.

Typically, non-functional requirements belong to areas such as:

  • Availability
  • Compliance
  • Documentation
  • Extensibility
  • Fault tolerance
  • Compatibility
  • Durability
  • Scalability, etc.
  1. Non-functional requirements are sometimes expressed in a form of metrics to make them more tangible.They may also relate to non-performance aspects of a system, but rather to its improvement over time (e.g., maintainability, extensibility, scalability, etc.)
  2. Non-functional requirements focus on user expectations since they rely on product properties.
  3. Functional requirements are faster to specify as they are based on a specific business idea. They go with all the product features and the ways users interact with that functionality. On the contrary, NFRs rely on experience.

Advantages of Functional Requirement

Again, both types of requirements are crucial to be expressed clearly. Let’s see what benefits you can reap when writing the FRs:

  • Make sure that the product includes all features that were stated in the documentation.
  • Help find the missing requirements.
  • Detect errors at an early stage, thus saving time and money.
  • Simplify project management, since they reinforce user goals and tasks.

Advantages of Non-Functional Requirement

NFRs can be helpful in the following ways:

  • Guarantee the product's compliance with set standards.
  • Help communicate the scalability characteristics of the app.
  • Lay the ground for good user experience and ease of use.
  • Improve the efficiency of the system.
  • Increase the security of the application.

How To Identify Non-functional Requirements

Since functional requirements are a lot easier to develop, let’s focus on non-functional ones.

To define most of NFRs, you should:

  1. Classify all NFRs into three types, operation, revision, and transition. In this case, all parties involved will have a common ground for requirement discussion.
  2. Review a predefined catalog of common non-functional requirements categories, divide them into categories, and then outline questions for each type.
  3. Include visuals such as process flows or system context diagrams to develop a better idea of the environment.
  4. Elicitation questions also facilitate productive discussion of non-functional requirements as part of a structured requirements workshop.
  5. Repurpose the requirements from previous projects, since NFRs are quite similar for all software systems.
  6. Leverage automated testing tools to monitor the app performance and dwell on more non-functional requirements.

As the last step, you need to actually define non-functional requirements and further validate them.

The Bottom Line

Crystal clear vision of all requirements is a mandate if you want to reach a positive outcome at the arrival. Functional requirements are much easier to identify since they define basic system behavior. On the other hand, non-functional requirements are subjective and more challenging to outline. The latter also makes the product affordable, easy to use, and accessible.

In any case, requirements should be comprehensible for all interested parties and be clearly stated in the documentation.

However, if you lack the expertise to document all requirements, you can always hire a professional to elicit and develop them for you.

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