Iterative vs. incremental development: What's the difference?

What is iterative development?

Iterative development is a software development strategy that focuses on releasing a product before it's perfect, then improving it over time through a series of small changes.

For example, imagine a team tasked with implementing a new web app. Under an iterative development approach, the team would begin by prototyping and building all the functionality that it thinks users will value, including both critical and nonessential features. Then, it would release the complete app.

However, the team doesn't expect the initial application release to be perfect from users' perspective -- far from it. As a result, it plans to make improvements over time based on feedback that users provide once they begin using the app. Each round of feedback would drive an iterative round of changes, resulting in an ongoing feedback loop.

What is incremental development?

Incremental development is a software development strategy that focuses on implementing application components or capabilities one by one, with the goal of perfecting each existing component before moving onto the next one.

For example, a team that uses an incremental development strategy to create a website would start by identifying which features are absolutely essential. It would then build those core capabilities and carefully test and optimize them before releasing the app to users.

At that point, the app could be considered a minimum viable product, or MVP -- meaning it meets basic user requirements but might still be missing important features. To enhance the application, the team would add new components or features over time. It would do so incrementally by building and optimizing each additional capability before implementing the next one.

Iterative vs. incremental development: What's the difference?

Iterative and incremental development are both means to the same ends: Developing a software project and progressing through the development cycle. Both are also compatible with popular Agile methodologies, such as Kanban.

However, iterative and incremental development work in distinct ways. Key differences include the following:

• Units of work. Iterative development usually breaks projects into sprints, with each sprint targeting changes to a broad set of application features or components. In contrast, with incremental development, the units of work are the application components that developers implement one by one.
• Time to market. Incremental development can help bring a minimum viable product to market faster, because developers release only the most essential features first. That said, it might end up taking longer to build out a complete, feature-rich final product using an incremental approach. With iterative development, the initial release often takes longer but includes a broader set of capabilities -- although those capabilities might not be as carefully tested or optimized.
• Incorporation of user feedback. Feedback from users is important for both iterative and incremental development, but developers incorporate it in different ways. With iterative development, they collect user feedback as early as possible and use it to inform the next iteration of application updates. Under an incremental approach, they might release an MVP prior to soliciting user feedback.
• Testing and optimization. Incremental development typically involves rigorous testing of each application component prior to release, along with efforts to optimize it. Developers usually perform basic testing during iterative development too, but they prioritize releasing features to users quickly, then improving them through subsequent releases, rather than attempting to perfect features prior to their initial release.
• Codebase organization. While there are no strict rules about how to organize and manage code under an iterative or incremental approach, the incremental development process tends to encourage the separation of the code that powers each feature. When developers implement each one by one, they're more likely to add new modules for each feature. Iterative development often results in code that is more integrated because developers work on multiple features at the same time.

Below is a summary comparison table:

Pros and cons of iterative and incremental development

The iterative and incremental model each come with various pros and cons.

The main advantages of iterative development include the following:

• Speed. Because developers don't commit to optimizing application capabilities prior to release, they can bring products to market faster.
• Flexibility. Iterative development makes it easier to modify application components -- or even abandon them entirely -- in response to user feedback or changing market conditions. For this reason, teams that prioritize product management and a product mindset tend to benefit more from iterative development -- which simplifies the process of making changes to a product over time, even if the changes weren't initially planned.

The main drawback of iterative development is that it increases the risk of releasing low-quality code that results in a poor user experience. When developers focus on getting features into production as quickly as possible and assume they can fix issues later, the code they write might not end up meet the minimum acceptable quality for users.

Iterative development can also be less efficient, especially in cases where application requirements and user expectations are clear from the start. If developers know what they need to build, it's usually easier and less time-consuming to build it all at once, rather than iterating through multiple rounds of changes. That said, application requirements often are not clear from the beginning of a project, and the feedback loops that are part of iterative development can be a valuable way of establishing direction.

As for incremental development, it provides these important benefits:

• Clear focus. An incremental approach results in clear deliverables, because developers know that each stage of the development project will focus on implementing a specific component or feature.
• Stability. By implementing features one by one and ensuring that each feature meets quality requirements before moving onto the next, incremental development reduces the risk of pushing low-quality code into production.

A key challenge posed by incremental development is that it's a less flexible approach that makes it hard to respond quickly to changing user priorities. For instance, if an application component that developers initially view as critical turns out not to be so important, they will have wasted time building and optimizing it. By contrast, iterative development would have helped them learn -- based on early user feedback -- that the feature is not as valuable as the development team expected.

In a similar vein, incremental development does not work well when developers lack a clear sense of what to implement. It might result in projects where coders build features that meet the requirements established during the planning phase but don't actually provide value to users or the business -- a classic example of what might happen when developers become overly invested in a project mindset and project management.

How to use iterative and incremental development together

It's not unusual for a team to break an application into increments, then develop each one using an iterative approach.

Although iterative and incremental development work in different ways, they're not mutually exclusive. It's possible to adopt a combination of both approaches within the same software development project.

There are two basic ways to go about this:

• Increment, then iterate. First, a team can begin with an incremental approach in which it decides which application components to build. Then, it can work on each component iteratively by releasing it to users before perfecting it and soliciting feedback to improve the component over time. One by one, the team would iteratively release and perfect each application part. This strategy offers the predictability that comes with breaking projects into increments, while also allowing the team to benefit from the flexibility afforded by iterative development.
• Iterate, then increment. Working from the other direction, it's also possible to develop a broad set of application features using iterative techniques. Then, once that functionality is complete and improved upon through feedback loops, the team can add other components in increments. This approach helps to incorporate user feedback earlier in the project, while also providing the stability and clear deliverables that come with an incremental approach.

Of these two options, the first is more common. It's not unusual for a team to break an application into increments, then develop each one using an iterative approach. Iterating first and then adding new components as increments usually only happens if there is no clear project direction initially -- in which case, early delivery and feedback gathering is valuable as a way of setting clearer goals -- but developers expect to establish a concrete vision over time.

Chris Tozzi is a freelance writer, research adviser, and professor of IT and society. He has previously worked as a journalist and Linux systems administrator.