Incremental Development Models: The Iterative Approach and the Risk-Driven Spiral Model Compared

UPI Study › Courses › System Analysis and Design › Free lessons › Lesson 2.7 Free with a free account

Incremental Development Models: The Iterative Approach and the Risk-Driven Spiral Model Compared

System Analysis and Design · Chapter 2: Business Analysis Tools and Development MethodsSection 2.7 · lesson 13 of 699 min · free with a free account

← Getting Started with Analysis Tools: Definitions and FlowchartsNext lesson: Verification at Every Stage: The V-Shaped Model in the SDLC →

Earn 3 college credits for System Analysis and DesignRecommended for credit by NCCRS · Computer Science 390 · self-paced · one payment, lifetime access
Enroll in System Analysis and Design · $250 →Download the syllabus (PDF)

In this lesson

Delivering in stages, refining in passes, and letting risk drive the loop.

  • Explain how the incremental model delivers functional additions
  • Explain how the iterative approach differs from the incremental model
  • Describe the Spiral model's 4 quadrants, and how risk analysis steers it
  • Compare the models and identify when each fits best

Transcript

Everything said in the video, section by section (1,196 words). Read along, or search it.

Business Analysis Tools and Development Methods

Incremental, Iterative, and Spiral Models

Waterfall delivers a system once, at the end. This section looks at three models that deliver in repeated cycles instead. The incremental model, the iterative approach, and the risk-driven Spiral model. They sound similar, and they are often confused. By the end, you should be able to say exactly what changes in each cycle, and when each model fits.

The Incremental Model

Delivering Working Slices

Start with the incremental model. It builds a system as a series of functional pieces, called increments. Each increment adds new, working capability to what has already been delivered. So instead of waiting for the entire system, the organization starts using a smaller, working slice early. Take a university course-registration system. The first increment might deliver only the ability to browse available courses. The second adds the ability to put courses in a cart. The third adds payment, and final enrollment. Students can browse courses while payment is still being built. That works because each increment is a complete, working piece, not an unfinished fragment of the whole. Users get value early, and the team gets real feedback on the parts already delivered.

The Iterative Approach

Refining the Same Functionality

The iterative approach works differently. It repeatedly revisits and refines the same functionality, rather than adding new functionality each pass. An early iteration produces a rough working version. Each later iteration improves it, based on feedback and testing. This continues until the functionality is judged good enough to finalize. Back to the registration system. An iterative pass on course browsing would not add payment. It would refine browsing itself. Faster search, clearer filtering, and fewer bugs. And those improvements come from how early users actually used it. The feature stays the same feature. It just gets better with each pass.

Easily Confused

Incremental vs. Iterative

Both models deliver in repeated cycles, which is why they are easy to confuse. The distinction is what changes from one cycle to the next. An increment adds functionality that was not there before. An iteration improves functionality that was already there. The incremental model is driven by a plan to deliver the full feature set in stages. The iterative approach is driven by feedback and testing on what already exists. Rework risk differs too. Incremental rework risk is lower, because each piece is meant to be built once. Iterative rework is higher by design. Revisiting the same work is the point. In practice, the two are often combined. A system is delivered incrementally, and each increment is refined through iterations before it is finalized.

Figure 2.4

The Spiral Model: 4 Quadrants per Loop

The Spiral model combines the repeated cycles of the iterative approach with a deliberate, structured focus on risk. Instead of a straight line of phases, the project moves through repeated loops. Each loop is larger in scope and cost than the last. And every loop passes through the same four quadrants. Quadrant one, determine objectives, alternatives, and constraints. Quadrant two, evaluate alternatives, and identify and resolve risks. Quadrant three, develop and verify the next-level product. And quadrant four, plan the next phase. Then the next, larger loop begins, back in quadrant one. Picture it as a spiral, winding outward. Each lap covers more ground than the last.

Inside Each Loop

What Happens in Each Quadrant

Let's look inside each quadrant. In quadrant one, the team defines what this loop is meant to achieve. It also identifies the different ways that goal could be achieved. In quadrant two, it weighs those alternatives against the risks each one carries. And it addresses the most serious risks, before committing further. In quadrant three, it builds and tests the working piece of the system this loop produces. In quadrant four, it reviews what was built and learned. Then it decides the objectives for the next, larger loop. Notice that building only happens in quadrant three. Half of every loop is about deciding, not doing.

Quadrant 2

Risk Analysis Before Commitment

Quadrant two is what sets the Spiral model apart from a plain iterative approach. Before committing resources to building anything in a loop, the team deliberately asks what could go wrong. Technical risk, cost risk, and schedule risk. Then it works out how to reduce, or test, that risk. That might mean building a small technical prototype, to test whether an approach will actually work. It might mean consulting an expert. Or it might mean researching how similar risks were handled elsewhere. Either way, the risk is examined before the money is spent, not after. A prototype that fails in quadrant two is a success. It saved the project from a much larger failure later.

Why It Pays Off

Fail Small and Early, If You Must Fail

Here is why that matters so much. Each loop is larger and more expensive than the one before it. So catching a serious risk early, in a small, cheap loop, costs far less than finding it after heavy investment. Suppose risk analysis in an early loop shows that a proposed approach is not workable. The project can change direction, or be canceled altogether, while the loop is still small. That is the core advantage of the Spiral model. It protects projects where one major unknown could otherwise sink everything, after significant investment. Notice the echo of the cost of change curve from earlier in this course. Early discovery is cheap discovery.

Comparing the Models

Incremental / Iterative vs. Spiral

Let's compare the models directly. Incremental and iterative models are driven by delivering usable functionality in stages. The Spiral model is driven by explicit risk analysis before each stage. Incremental and iterative models suit projects where requirements are reasonably well understood, and can be split into independent pieces. Spiral suits large, high-uncertainty projects, where technical or business risk could derail everything. Planning overhead is lower for incremental work. It is mostly about sequencing the increments. It is higher for Spiral, because every loop includes a formal risk evaluation. With incremental work, a major risk is usually discovered during development. With Spiral, it is deliberately surfaced before heavy investment. And in size, incremental projects range from small to large, while Spiral projects are usually large, complex, or unprecedented.

Choosing a Model

Match the Model to the Uncertainty

So when do you choose each one? Incremental and iterative models fit when overall requirements are reasonably well understood. The system can be broken into independent pieces, and early feedback on working features is valuable. A retail website adding a loyalty-points feature to an already stable order system is a good fit. The risk is modest, and the feature can be delivered and refined in stages. The Spiral model fits when uncertainty is high enough that one unexamined risk could threaten the whole project. Think of unfamiliar technology, or a novel integration between systems that have never been connected before. Or a large-scale system, where a wrong early decision would be very costly to reverse. In those cases, the extra overhead of formal risk analysis is worth it. Because the alternative, discovering the risk late, would cost far more.

Key Takeaways

What to Carry Forward

To pull this together. The incremental model adds new functionality with each delivery. The iterative approach refines functionality that already exists. The Spiral model moves through repeated loops, each passing through four quadrants. Determine objectives, evaluate risk, develop and verify, and plan the next phase. Risk analysis in quadrant two lets a project change direction, or stop, while a loop is still small and cheap. And incremental and iterative approaches suit well-understood projects that split into independent pieces. The Spiral model suits large, high-uncertainty projects.

About System Analysis and Design at UPI Study

System Analysis and Design (Computer Science 390) is a self-paced online course of 69 video lessons in 9 chapters, from the analyst's role and requirements to UML modeling, interface and database design, testing, maintenance and project management. It carries a NCCRS credit recommendation for 3 semester credits.

Every lesson transcript is free on pages like this one, and the videos are free with a free UPI Study account. Enrolling adds a practice quiz after each lesson, the graded work and final exam that earn the credit, lifetime access, and an official transcript on completion. UPI Study has helped over 130,000 students with college credits. ACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities.

Enroll now → Course details Download the syllabus All Computer Science courses Every course with PRO →

Compare: Software Engineering vs Systems Analysis

Why take it here

Why students take System Analysis and Design with UPI Study

From business problem to working systemSystem Analysis and Design teaches how analysts study a business need and turn it into a system design: requirements, UML and data-flow models, interface and database design, implementation and project management. It is a core course in information systems and IT degrees.
Credit at a fraction of the priceThe same 3 credits can cost $750 or more at a US college. Here it is one payment, self-paced, with lifetime access.
Finish your degree soonerTransfer credits reduce the courses you still owe, so you can graduate earlier or lighten a heavy semester.
Strengthen an applicationA completed college-level course on an official transcript shows admissions and scholarship committees you can do the work.
Study on your own scheduleNo start dates, no deadlines. Watch the lessons, practice, and take the final when you are ready.
130,000+students helped with college creditsACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities.

Where the credit goes

Universities that work with NCCRS

System Analysis and Design carries a NCCRS credit recommendation. These universities work with NCCRS; some are UPI Study's signed partners. ACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities.

Thomas Edison State UniversityThomas Edison State UniversitySigned partnerUniversity of the PeopleUniversity of the PeopleSigned partnerCharter Oak State CollegeCharter Oak State CollegeSigned partnerStrayer UniversityStrayer UniversitySigned partnerColumbia Southern UniversityColumbia Southern UniversitySigned partnerIndiana TechIndiana TechSigned partnerPenn StatePenn StateWorks with NCCRSSouthern New Hampshire UniversitySouthern New Hampshire UniversityWorks with NCCRSSUNYSUNYWorks with NCCRS +many more cooperating universitiesFind my college →

Find my college → Not listed? Ask Athena in the corner of this page, or book a 15-minute call.

FAQ

Questions students ask us

About Incremental Development Models: The Iterative Approach and the Risk-Driven Spiral Model Compared

What is the incremental model in software development?
The incremental model builds a system as a series of working functional pieces, called increments, each adding new capability to what has already been delivered. For a course-registration system, the first increment might allow browsing courses, the second adds a cart, and the third adds payment and enrollment. Users gain value early from complete, working slices, and the team gets real feedback on the parts already delivered.
What is the difference between incremental and iterative development?
Incremental development adds functionality that was not there before, while iterative development repeatedly refines functionality that already exists. Incremental work follows a plan to deliver the full feature set in stages, and each piece is meant to be built once. Iterative work is driven by feedback and testing, so revisiting the same feature, such as improving search and filtering, is intentional. In practice the two are often combined.
What are the four quadrants of the Spiral model?
Each loop of the Spiral model passes through four quadrants: determine objectives, alternatives, and constraints; evaluate alternatives and identify and resolve risks; develop and verify the next-level product; and plan the next phase. Each loop is larger in scope and cost than the one before. Building happens in the third quadrant, so much of every loop is about deciding rather than doing.
When should you use the Spiral model instead of an incremental approach?
The Spiral model fits large, high-uncertainty projects where one unexamined risk could threaten the whole effort, such as unfamiliar technology or a novel integration between systems. Its formal risk analysis in every loop adds planning overhead, but it surfaces serious risks while loops are still small and cheap, letting the project change direction or stop. Incremental approaches suit well-understood requirements that split into independent pieces.

About the System Analysis and Design course

How many college credits is System Analysis and Design?
System Analysis and Design (Computer Science 390) carries a NCCRS credit recommendation for 3 semester credits. ACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities.
How much does System Analysis and Design cost and what is included?
$250, paid once, with lifetime access: all 69 video lessons, a practice quiz after each, the graded work and final exam, and an official transcript when you finish. Students who want several courses choose UPI Study PRO instead.
Which universities accept UPI Study credits?
UPI Study has signed partner agreements with universities such as Thomas Edison State University, University of the People and Charter Oak State College, and works with universities such as Penn State, SNHU and SUNY. ACE and NCCRS have over 2,100 cooperating colleges that will consider accepting credits; the final decision is with the universities. Use "Find my college" on this page to check yours.
How long does System Analysis and Design take?
It is self-paced with no deadlines. The 69 lessons total about 9 hours of video; most students add reading and practice time and finish in a few weeks to a few months, on their own schedule.
How are the exams done?
Quizzes and assignments are online in the course. The final exam is taken online with proctoring; a person on the UPI Study team checks assignments, usually within two days.
How do my credits reach my college?
When the course shows 100% complete, request the official transcript from your dashboard. It is sent to the college you name, and you can send it to more than one. Check your college first with "Find my college" on this page, or ask Athena.

Ready for the full course? Earn 3 credits in System Analysis and Design.

69 video lessons, a quiz after each one, and the graded work that earns the credit. Recommended for credit by NCCRS.

Enroll in System Analysis and Design →Download the syllabusEvery course with PRO →
System Analysis and Design
3 credits · NCCRSEnroll →