← Years 7–8 Creating Digital Solutions

Non-functional requirements

A rule about how well a solution must work, such as how fast, easy to use or reliable it is. It is not about what the solution does, which is a functional requirement.

Part of: 7-8 Digital Technologies: Defining and decomposing problems — Module 9, QCE Digital Solutions Unit 1 — User Experiences and Interfaces — Module 27

Understanding Non-functional requirements in Years 7–8 Creating Digital Solutions

What it does: Not crashing and working almost all of the time describe reliability, which is one kind of non-functional requirement. Saving a score is an action the app does (functional). Saving in under 1 second is how well it does it (non-functional). Performance means speed. Only the first choice gives a time. The other one just says what the quiz does. Big, easy-to-tap buttons describe usability. Having a start button is a feature, so it is functional. Speed is a performance requirement, so the wrong sub-type was used. Usability is about how easy the solution is to use.

Common mistakes: Paying online is something the app does, so it is a functional requirement. A non-functional rule describes how well it works, such as 'Pages load in 2 seconds'. Staying online is about reliability, not usability. A usability rule is about ease of use, such as 'Buttons are easy to see'.

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Learning
Duo
Which one is a non-functional requirement for a school canteen ordering app?
The app must let students add food to an order.The app must load the menu in under 3 seconds.
Matches
Non-functional requirement: The app must let users send messages.
↓
Sending messages is something the app does, so it is a functional requirement. A non-functional requirement should say how well it works, such as 'Messages must send in under 3 seconds'.
Applying
Meanings
Term
non-functional requirement
Meaning
A rule about how well a solution must work, such as how fast, easy to use or reliable it is. It is not about what the solution does, which is a functional requirement.
Practice
Usability: A Year 7 student can order lunch in 3 taps.
Testing
Gaps
A requirement describes
Check Answer
Spots
2Type: non-functional
3Rule: Students can pay online

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Quick Questions

1. Which one is a non-functional requirement for a school canteen ordering app?
A. The app must load the menu in under 3 seconds. ✓
B. The app must let students add food to an order.
Loading in under 3 seconds says how well the app works (performance). Adding food says what the app does, so it is a functional requirement.
2. True or false: A non-functional requirement describes how well a solution must work.
A. False
B. True ✓
Non-functional requirements cover qualities like speed, ease of use and reliability. Functional requirements describe what the solution does.
3. Which requirement is about usability?
A. A Year 7 student should be able to use the app without any help. ✓
B. The app must save each student's score to a list.
Usability is about how easy a solution is to use. Saving scores is something the app does, so it is functional.
4. Which requirement is about performance?
A. The quiz must show the result within 2 seconds of the last answer. ✓
B. The quiz must show the result after the last answer.
Performance means speed. Only the first choice gives a time. The other one just says what the quiz does.
5. A game must not crash during a 30-minute play session. What kind of requirement is this?
A. Functional (a feature)
B. Non-functional (reliability) ✓
Not crashing is about how dependable the game is. That is reliability, which is a non-functional requirement.

Match the Pairs

The app must load in under 2 seconds. → A performance non-functional requirement: it says how fast the solution must work.
A new user must be able to use the app without training. → A usability non-functional requirement: it says how easy the solution must be to use.
A reliability requirement says the solution must keep working without failing. → The app must work 99% of the time without crashing.
A functional requirement says what the solution does, while a non-functional requirement says how well it does it. → Functional: The app lets users save a score. Non-functional: Saving takes under 1 second.
Non-functional requirement: The app must let users send messages. → Sending messages is something the app does, so it is a functional requirement. A non-functional requirement should say how well it works, such as 'Messages must send in under 3 seconds'.
Usability requirement: The website must load in 1 second. → Loading speed is performance, not usability. A usability requirement should be about ease of use, such as 'Buttons must be big and clearly labelled'.

Key Term

non-functional requirement
A rule about how well a solution must work, such as how fast, easy to use or reliable it is. It is not about what the solution does, which is a functional requirement.

Fill in the Blanks

1. A non-functional requirement describes how well a solution must work.
Hint: Type the term for a requirement about how well a solution works, not what it does.
2. Requirement: Pages must load in 2 seconds. Quality: performance
Hint: Type the quality that is about speed and how quickly the system responds.
3. Requirement: Buttons must be easy to read. Quality: usability
Hint: Type the quality that is about how easy a system is to use.
4. Requirement: The app must not crash. Quality: reliability
Hint: Type the quality about a system working correctly every time without failing.
5. Lets users log in = functional Loads in 2 seconds = non-functional
Hint: Type the kind of requirement that says what the solution does.

Spot the Error

Question 1
A App: Canteen order app
B Type: non-functional
C Rule: Students can pay online
Line C — Paying online is something the app does, so it is a functional requirement. A non-functional rule describes how well it works, such as 'Pages load in 2 seconds'.
Question 2
A Type: non-functional
B Category: usability
C Rule: Site never goes offline
Line C — Staying online is about reliability, not usability. A usability rule is about ease of use, such as 'Buttons are easy to see'.
Question 3
A Category: performance
B Rule: Search is quite quick
C Type: non-functional
Line B — A performance requirement should be measurable. 'Quite quick' is vague. It should give a clear limit, such as 'Search takes under 2s'.
Question 4
A Category: reliability
B Rule: App must not crash
C Test: Count how fast it loads
Line C — Counting load speed tests performance, not reliability. A reliability test checks that the app keeps working, such as running for 1 hour with no crash.
Question 5
A Game: Quiz app
B Functional: Loads in 3 seconds
C Non-functional: Easy to read
Line B — How fast something loads is a non-functional requirement. A functional requirement says what the app does, such as 'Show the score'.

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