Non-functional requirements
Non-functional requirements describe how well a system should work rather than what it should do.
They set the expectations for quality, performance, reliability, and overall user experience.
Think of them as the rules that make a product feel fast, safe, easy to use, and dependable, not the features themselves, but the qualities that make those features work smoothly.
If functional requirements are the 'what', non-functional requirements are the 'how well'.
Resources
To make sure the target solution (service) is defined appropriately to meet its business need, it is encouraged that a Business Impact Assessment is conducted as early as possible.
In doing so, this will help establish the appropriate Service Tier based on its business need.
All solutions must conduct a business impact assessment to determine the appropriate Service Tier for a solution.
In turn the service tier defines the technical requirements the target solution must adhere to.
The accompanying resources provide guidance to achieve this.
The use of NFRs across delivery
Across delivery, you should:
- understand business criticality at the earliest opportunity to inform the appropriate Service Tier
- use the NFR catalogue to establish system requirements and drive service standardisation
The Business Analysis process for completing non-functional requirements is as follows:
- BAs work with business owners to understand the Business Criticality and Service Tier Assessment
- hold a BA-led discussion with the delivery team to determine solution-specific non-functional and functional requirements
- build a composite set of requirements
- procurement or build
- testing and assurance
- service transition or go live
- performance matrices (Continuous Service Improvement)
Service tiering
The service tiers listed below provide an outline definition of the non-functional characteristics for each service tier.
Once a Service Tier for the solution has been defined, all non-functional requirements relating to that service tier must be adhered to as a minimum or an exception waiver sought.
The non-tier-specific category identifies NFRs that relate to all solutions regardless of service tier.
Access the full list of service tiers here: DDTS Service Tiers
NFR categories
All of the non-functional requirements are grouped by a set of high-level categories that define particular characteristics of the service.
View the full list of NFR categories.
View the full list of non-functional requirements.
Availability guidance
A service is defined 'Available' when users are able to access and use all the functions of the System.
Service Availability is measured as a percentage of the total time in a Service Period, in accordance with the following formula:
Service Availability % = (MP - SD) × 100 / MP
where:
- MP = total number of minutes, excluding Permitted Maintenance, within the relevant Service Period
- SD = total number of minutes of service downtime, excluding Permitted Maintenance, in the relevant Service Period
When calculating Availability:
- Downtime arising due to Permitted Maintenance is subtracted from the total number of hours in the relevant Service Period.
- Downtime arising due to a catastrophic national or international outage of both the primary site and the disaster recovery site is subtracted from the total number of hours in the relevant Service Period.
- Availability is only measured for the Live Environment and on a 24x7 basis.
Get support
To request additions or amendments to the NFR list, or for help applying non-functional requirements, contact the Business Analysis Community.