6.4c Software Development Lifecycle — Edexcel 1CP2

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What is the Software Development Lifecycle?

The Software Development Lifecycle (SDLC) is a structured process for planning, creating, testing, and delivering software. It ensures software is built systematically rather than ad hoc. Edexcel covers three key models: Waterfall, Iterative (Agile-style), and Spiral.

The Waterfall Model

The Waterfall model is a linear, sequential approach where each phase must be fully completed before the next begins. You cannot go back.

Analysis

Identify what the system must do — gather requirements from the client. Produce a requirements specification.

Design

Plan the solution — system architecture, data structures, algorithms, user interface.

Implementation

Write the actual code based on the design documents.

Testing

Systematically test the complete system against requirements.

Evaluation / Maintenance

Review against original requirements; fix bugs; update for new needs.

✅ Advantages	❌ Disadvantages
Simple, easy to understand and manage	Very inflexible — changes are very costly
Clear milestones and deliverables	Client doesn't see product until the end
Well-documented at every stage	Problems found late are expensive to fix
Good for projects with fixed requirements	Assumes requirements are known upfront — rarely true

The Iterative (Agile) Model

The iterative model builds software in small cycles (iterations/sprints). Each cycle produces a working version; feedback is collected and fed into the next cycle.

Requirements can change between iterations — more flexible
Client sees and tests working software early
Bugs found early within each short cycle are cheaper to fix
Risk is reduced: if one iteration fails, little is lost

✅ Advantages	❌ Disadvantages
Flexible to changing requirements	Scope creep — project can keep growing
Client feedback at every stage	Total cost and timeline hard to predict
Working software delivered quickly	Requires close client involvement throughout
Problems found early and fixed cheaply	Documentation may be less thorough

The Spiral Model

The Spiral model combines iterative development with risk analysis. Each loop of the spiral has four phases: Planning → Risk Analysis → Engineering → Evaluation.

Explicitly identifies and addresses risks at each iteration
Good for large, complex, or high-risk projects
More expensive and complex to manage than Waterfall or iterative

Comparing the Models

Feature	Waterfall	Iterative	Spiral
Flexibility	Low	High	Medium–High
Client involvement	Start and end	Throughout	Throughout
Risk management	Poor	Moderate	Explicit
Best for	Fixed requirements	Evolving requirements	Large/risky projects
Cost of changes	Very high	Low	Medium

Maintainability

Good software should be easy to maintain after delivery. Techniques that improve maintainability:

Comments — explain what code does and why
Meaningful variable names — total_score not ts
Modular design — functions/procedures that each do one thing
Consistent indentation — Python enforces this but good style reinforces it
Version control — tracking changes so previous versions can be restored

Exam tip: A common Edexcel question is "State one advantage and one disadvantage of the Waterfall model compared to the iterative model." The key contrast: Waterfall is rigid but well-planned; iterative is flexible but harder to manage costs/scope. Always give specific reasons, not just "it's better".

⚠️ Common Mistakes

Saying Waterfall has "feedback loops" — it does NOT; it is strictly linear
Confusing iterative with just "doing it twice" — it's a cycle of build-test-feedback repeated many times
Forgetting to mention risk analysis as the key feature of the Spiral model
Stating advantages/disadvantages without explaining WHY — "inflexible" alone is not enough; say "requirements cannot change once development begins"

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Worksheet — 6.4c Software Development Lifecycle

8 Edexcel-style questions · instantly marked

Q1List the FIVE stages of the Waterfall model in the correct order.[5]

✅ Mark scheme

1. Analysis [1]; 2. Design [1]; 3. Implementation [1]; 4. Testing [1]; 5. Evaluation/Maintenance [1]. Must be in correct order for full marks — order is the key feature of Waterfall.

Q2State ONE advantage and ONE disadvantage of the Waterfall model.[2]

✅ Mark scheme

Advantage: any of — simple/clear structure, clear milestones, well-documented, good when requirements are fixed [1]; Disadvantage: any of — inflexible/requirements cannot change, client doesn't see product until the end, bugs found late are expensive to fix [1].

Q3What is the key difference between the Waterfall and iterative models of software development?[2]

✅ Mark scheme

Waterfall is linear and sequential — each stage must be complete before the next begins and changes are not made once started [1]; Iterative builds software in repeated small cycles, with feedback gathered after each working version and requirements can change between cycles [1].

Q4A hospital is building safety-critical patient monitoring software. Suggest which SDLC model would be LEAST suitable and explain why.[2]

✅ Mark scheme

Iterative model [1]; because requirements and features change between iterations, which could be dangerous in safety-critical software where all requirements must be fixed, verified and fully tested before deployment [1]. Accept: Waterfall would be least suitable because it offers no mechanism for fixing major design flaws found during testing.

Q5What distinguishes the Spiral model from both Waterfall and iterative models?[2]

✅ Mark scheme

The Spiral model explicitly includes a risk analysis phase in each iteration [1]; unlike Waterfall (no iteration) or iterative (no formal risk analysis), Spiral evaluates and mitigates risks at every cycle before proceeding [1].

Q6Give THREE techniques a developer can use to make code easier to maintain.[3]

✅ Mark scheme

Any three of: meaningful variable/function names (e.g. total_score not ts) [1]; comments explaining what code does [1]; modular design — functions that each do one task [1]; consistent indentation [1]; version control [1]. Accept other valid maintainability techniques.

Q7A software company is developing a social media app where user requirements change frequently. Explain why the iterative model is more suitable than Waterfall.[3]

✅ Mark scheme

Iterative is more suitable because: requirements can change between cycles without large cost [1]; users can test working versions early and provide feedback to shape the next iteration [1]; bugs and design problems found in one cycle can be fixed cheaply before the next cycle begins [1].

Q8What happens in the Analysis stage of the Waterfall model and why is it so important?[3]

✅ Mark scheme

Requirements are gathered from the client to establish exactly what the system must do [1]; a requirements specification document is produced [1]; it is crucial because in Waterfall all subsequent stages depend on these requirements — if the analysis is wrong, the entire project will produce the wrong product and changes later are very costly [1].

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Term	Definition

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What is the Software Development Lifecycle?

The Waterfall Model

The Iterative (Agile) Model

The Spiral Model

Comparing the Models

Maintainability

Worksheet — 6.4c Software Development Lifecycle

Mini Test — Software Development Lifecycle