Non-functional Requirements – Why do we need them?

Written by Ananya Pani on October 17, 2018. Posted in Articles.

Non-functional requirements, what comes to our mind when we talk about NFR (Non-functional requirements)?

What do you put down in non-functional requirements when you are documenting requirements in your project?

When we say non-functional we typically mean those requirements that are not related to functionality of the system, then what exactly are these and why do we need them.

Now imagine a system where it publishes results for 10 graders school certification examination in a country like India, and the kind of load it will have when the results are declared.

So what all requirements do we have to consider for this system apart from functionality, it surely has to have high performance requirement to handle the load as well as good security/authentication system in place.

This is the kind of requirements which is generally referred as Non-functional requirements, the requirements which are important for the user community or for smooth functioning of the system like usability, reliability etc.

The non-functional requirements should always be described in clear terms like the system should be able to handle 0.1 million users simultaneously and the response time has to be less than 2 seconds for each user.

Here is a good list of common non-functional requirements:

NFR Category	Non-functional Requirements	Short explanation	Applicable to situation	Can be tested by
Constraint	Price	Target price for the solution	Most	Checklist
Constraint	Resource constraints	Constraints imposed on development such as constraints imposed due to small screen size of mobile devices.	Most	Checklist
Compliance	Compliance	Regulatory compliance.	Health care (HIPAA, FDA)	Integration testing across systems
Compliance	Documentation	Documentation requirements.	Health care, Aviation, Automotive	Test cases for color blindness
Compliance	Legal and licensing issues or patent-infringement-avoidability	Adhering to compliance requirements.	Most	Test cases verification of record data update
Maintainability	Analyzability	Ability to investigate a failure	Most	Backup recovery test
Maintainability	Changeability	Ability to change one component without affecting others, and without causing unexpected failures	Most	Review the application code
Maintainability	Deployment	Ease with which an application can be deployed and upgraded.	Most	Review the application code
Maintainability	Escrow	Source code of an application is kept securely and available to buyer under certain conditions	Purchased an out-sourced product	Checklist
Maintainability	Extensibility / Modifiability	Ability to extend the product easily	Most	Consumer feedback
Maintainability	Supportability	Ability to support applications for specific period, locations etc.	Most	Checklist
Maintainability	Testability	Ease of test automation	Most	Checklist
Performance	Performance / response time (performance engineering)	Time taken to respond to a user request.	Most	Checklist
Performance	Resource utilization	% of available capacity used	Most	Checklist
Performance	Scalability	Ability to support specified number of users / transactions	Systems supporting large number of users	Review the application code
Portability	Interoperability	Ability to work with existing systems.	Most	Review the application code
Portability	Platform compatibility	Ability to work with stated platforms.	Most	Review the application code
Portability	Replaceability		Most	Review the application code
Reliability	Availability	% of time the system is available.	Critical systems	Latency testing
Reliability	Backup	Frequency at which data must be backed-up.	Universal	Test on desired platforms
Reliability	Disaster recovery	Time taken to restore the application after a disaster.	Most	Multiple browser test
Reliability	Failure management (Fault tolerance)	Ability to manage failures.	Most	Checklist
Reliability	Quality (e.g. faults discovered, faults delivered, fault removal efficacy)	Target defect density	Mission critical applications	Code and design Review
Reliability	Recovery / recoverability (e.g. mean time to recovery – MTTR)	Ability to recover quickly	Mission critical applications	Review the application code
Reliability	Reliability (e.g. mean time between failures – MTBF)	Ability to provide service when needed	Mission critical applications	Review the application code
Reliability	Replaceability	Ability to replace a faulty part on the fly	Mission critical applications	Review the application code
Reliability	Resilience	Ability to withstand attacks	Mission critical applications	Checklist
Reliability	Robustness	Ability to operate continuously even under adverse conditions.	Most	Simulation of internet attack If hardware is present, heat cycle testing
Reliability	Stability		Most	Checklist
Security	Audit and control aka Accountability	To track changes made to data.	Finance, Health care	Review the application code
Security	Authenticity		Most	Review the application code
Security	Confidentiality	Protect data from being exposed to unauthorized users	Most	Review the application code
Security	Integrity	Maintaining correctness of data	Most	Penetration test
Security	Privacy	Ability to keep personal data secure.	Health care	Penetration test
Usability	Accessibility	Application being usable by persons with special needs such as color blindness.	Government	Review the application code
Usability	Ease of use	Limit number of clicks to maximum 3 clicks to complete any transaction	Most	Review the application code
Usability	Emotional factors (like fun or absorbing)	Making application likable by a certain audience.	Education	Checklist
Usability	Internationalization	Ability to operate the application in different countries such as handling multiple time zone, currency, languages etc.	Most	Review the application code
Usability	Learnability	Different user groups should be able to use the product with or without training	Most	Review the application code
Usability	Safety	Ensure safe usage of the product and prevent damages caused by the application. For example, safety features for a navigation system.	Where there are dangers to human life. Aviation, Automotive, Health care etc..	Key board control test
	Certification	Certification on a particular technology such as certified on Azure.	Most	Untrained user test
	Localization	Ability to satisfy needs of a particular country or domain (say petroleum industry)	Most	Review the application code
	Re-usability	Ability to re-use existing components and create new re-usable components	Most	Review the application code
Portability	Installability	Ability to install or un-install easily	Most	Review the application code

Whereas some of the common functional requirements include:

Business Rules, logic, Validation
Calculations/formulae
Error message/handling
Transaction corrections, adjustments and cancellations
Admin functions/access, super user access
Authentication
Authorization levels
Audit log of transactions
External Interfaces
Reporting Requirements
Legal/Regulatory/Compliance Requirements

Advantages

Makes the system user friendly/easy to use and acceptable
Absence of them makes it lot more difficult to use for users and sometimes system may get abandoned due to the absence of these features

Disadvantages

Gets missed out often in requirements gathering exercise
Difficult to articulate or define quantitatively

Ananya Pani

I am an entrepreneur, building my brand to be a global name in the Business Analysis domain. My mission is to help business analysts to build a successful professional career. I have helped 2000+ BAs in choosing a better career and help them achieve professional success. I co-founded Adaptive US, a Business analysis skill development organization. I manage sales and operations at Adaptive US. Under my leadership, Adaptive is now established as a world-leader.