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Process Models

A process model describes the shape of the work: which phases exist, in what order they run, and where feedback enters. It is the skeleton that a methodology puts flesh on. Waterfall says requirements come before design. Scrum says who is in the room and for how long.

Every model below is an answer to one question: when do we find out we were wrong?

The models

Model Shape Feedback arrives Best fit
Waterfall Sequential phases, each finished before the next At acceptance, near the end Stable, well-understood, contract-fixed requirements
V-Model Waterfall folded, each phase paired with its test level Per verification level, still late for requirements Safety-critical and regulated work needing verification evidence
Incremental Requirements split into increments, each delivered in full After each increment Known scope that can usefully be delivered in pieces
Iterative The same product refined repeatedly After each iteration Requirements that must be discovered by building
Unified Process Four phases, iterations inside, risk-driven Every iteration Larger projects wanting iteration with formal artifacts
Other models Spiral, prototyping, RAD and more Varies Specialized risk or exploration profiles

The models side by side

graph TD
    subgraph W[Waterfall]
        W1[Requirements] --> W2[Design] --> W3[Build] --> W4[Test] --> W5[Deploy]
    end
    subgraph INC[Incremental]
        I1[Increment 1 built and delivered] --> I2[Increment 2] --> I3[Increment 3]
    end
    subgraph IT[Iterative]
        R1[Rough whole product] --> R2[Refined] --> R3[Refined again]
    end

Waterfall learns once, at the end. Incremental learns after each slice. Iterative learns after each pass over the same ground.

Incremental is not iterative

The two words are used interchangeably and mean different things.

  • Incremental builds the final product a slice at a time. Each slice is finished, and the picture grows.
  • Iterative builds a rough version of the whole, then refines it. Each pass covers the same ground at higher quality.

Most modern delivery, including Scrum, is both. A Sprint delivers a new slice and improves what already exists.

Plan-driven and empirical

Plan-driven Empirical
Assumption Requirements can be known before building Requirements are discovered by building
Control Conformance to a plan Frequent inspection of a working increment
Cost of a late change High, since finished artifacts must be reworked Lower, since increments are small and tested
Needs Stable domain, clear contract Available customer, automated tests, deployable increments

Neither is a default. A pacemaker's firmware and a consumer web app carry different risks, and the model should follow the dominant risk. See Choosing a Methodology.

Check Your Understanding

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What is the difference between incremental and iterative delivery?

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Why does the V-Model suit safety-critical work?