The Mystery Behind Little’s Law and WIP Limits

Erez Morabia
6 min readNov 17, 2023

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The Work In Progress (WIP) limits practice has its roots in Lean manufacturing principles, particularly the Kanban system, which originated in Japan at Toyota in the 1940s and 1950s. The concept was introduced by Taiichi Ohno, a Toyota engineer, as part of the Toyota Production System (TPS), which aimed to optimize efficiency, minimize waste, and improve overall productivity in manufacturing.

The WIP limits practice, as applied in Kanban and later adopted in various Agile methodologies, addresses the issue of overburdening teams with too much work in progress. When teams take on more tasks than they can efficiently handle, it often leads to a decrease in overall productivity, longer cycle times, and increased stress on team members.

WIP limits work by setting a maximum threshold for the number of tasks that can be in progress simultaneously. Once this limit is reached, the team must finish some existing tasks before starting new ones. This constraint helps maintain a balanced flow of work and ensures that the team focuses on completing and delivering tasks before taking on additional work.

WIP Limits Advantages

Using Work-in-Progress (WIP) limits in software development offers several advantages, helping teams to streamline their processes, enhance efficiency, and deliver higher-quality software. Here are some key advantages:

  1. Faster Time to Delivery:
    WIP limits prevent teams from taking on too many tasks simultaneously. By focusing on completing work before starting new tasks, teams can reduce cycle time and deliver software more quickly.
  2. Improved Flow and Throughput:
    WIP limits encourage a steady and balanced flow of work through the development pipeline. This minimizes bottlenecks and ensures that work progresses smoothly from one stage to the next, ultimately increasing throughput.
  3. Increased Focus and Reduced Multitasking:
    With WIP limits in place, team members are less likely to juggle multiple tasks at once. This reduction in multitasking allows individuals to concentrate on completing one task at a time, leading to increased focus and efficiency.
  4. Early Identification of Issues:
    WIP limits act as an early warning system. When a stage exceeds its WIP limit, it indicates a potential problem or bottleneck. This early identification allows teams to address issues promptly, preventing delays in the overall development process.
  5. Enhanced Collaboration:
    WIP limits encourage collaboration within the team. Team members are more likely to work together to complete tasks within the established limits, fostering a sense of shared responsibility for the project’s success.
  6. Better Work Prioritization:
    WIP limits force teams to prioritize their work. By limiting the number of tasks in progress, teams are compelled to focus on the most important and valuable work, resulting in a more efficient allocation of resources.
  7. Reduced Wasted Effort:
    By completing tasks before starting new ones, teams reduce the risk of having a significant amount of partially completed work. This minimizes wasted effort and resources on tasks that may be deprioritized or changed before completion.
  8. Predictable Delivery:
    WIP limits contribute to a more predictable development process. Teams can better estimate the time it takes for work items to move through the workflow, leading to more accurate project planning and delivery predictions.
  9. Continuous Improvement:
    WIP limits are a fundamental aspect of the Kanban methodology, which emphasizes continuous improvement. Teams can experiment with and adjust WIP limits over time, learning from their experiences and continually optimizing their development processes.
  10. Increased Quality:
    With a more focused and controlled workflow, teams can dedicate more attention to each task, resulting in higher-quality deliverables. Reduced multitasking and improved collaboration also contribute to a higher level of quality in the software development process.

In summary, the advantages of using WIP limits in software development include faster time to delivery, improved flow and throughput, increased focus, early issue identification, enhanced collaboration, better work prioritization, reduced wasted effort, predictable delivery, and a continuous commitment to improving processes.

Little’s Law

The Work In Progress (WIP) limits in Agile are backed up by the mathematical theory of Little’s Law. Little’s Law is a theorem in queueing theory, a branch of operations research and applied mathematics. It was formulated by John D. C. Little in 1961 and has widespread applications in various fields, including manufacturing, telecommunications, and, notably, software development.

The law provides a relationship between three key metrics in a queuing system: the average number of items in a queue (L), the average rate at which items arrive (λ), and the average time an item spends in the system (W).

The formula for Little’s Law is: L=λW

In the context of Agile software development, Little’s Law can be applied to understand and manage cycle time, which is the time it takes for a user story or task to be completed from the moment it enters the development process until it’s delivered. Little’s Law is particularly useful in Agile environments where there is a continuous flow of work, and teams strive for quick and predictable delivery of features.

The key components of Little’s Law in the Agile context are:

  1. Cycle Time (W): In Agile, this is the time taken to complete a user story or task. It includes the time a story spends in the development queue, being actively worked on, and undergoing testing before it is considered “done.”
  2. Arrival Rate (λ): This represents the rate at which new user stories or tasks are entering/leaving the development process (throughput or completion rate). It’s the pace at which the team is committing to and starting new work.
  3. Work in Progress (WIP) Limit: Little’s Law assumes a stable system, and in Agile, implementing Work in Progress (WIP) limits is a common practice to maintain this stability. WIP limits help prevent the team from taking on too much work simultaneously, ensuring a steady flow through the development process.

Little’s Law in Agile can be expressed as:
Cycle Time (W)= WIP (L) / Throughput (λ)

Teams should use Little’s Law to optimize their cycle time by adjusting and limiting the WIP. For example, if the team wants to reduce cycle time, they can simply limit the WIP of specific phases in the development workflow.

Leveraging WIP Limit for Scrum

Here’s how you can use WIP limits with Scrum:

  1. Map Columns to Workflow Stages:
    Identify the columns on your Scrum board that represent different workflow stages. These stages typically include To Do, In Progress, Testing, and Done. Each column represents a step in the development process.
  2. Set WIP Limits for Columns:
    Assign WIP limits to relevant columns on your Scrum board. It’s common to set WIP limits for columns like In Progress and Testing to control the number of tasks being actively worked on and tested. Determine the appropriate WIP limit based on the team’s capacity and the flow of work. In each of the workflow stages, I recommend setting an initial WIP limit number that is twice the size of the people involved in that stage. For example, if we have 4 developers and 3 testers on the team, the In Progress column will be set with a WIP limit of 8 and the Testing column will be set WIP limit of 6.
  3. Daily Stand-up:
    During the daily stand-up meetings, discuss the status of tasks on the board, paying attention to items that are in progress or testing. Ensure that the team is aware of and respects the WIP limits to maintain a balanced and controlled flow.
  4. Manage Exceptions:
    While WIP limits provide guidelines for the number of tasks in progress, there may be situations where exceptions are necessary. For instance, a critical bug might need immediate attention. In such cases, it’s important to have a process for handling exceptions and communicating the reasons for exceeding WIP limits.
  5. Continuous Improvement:
    Regularly review and adjust (usually reduce) WIP limits based on the team’s experience and performance. If the team consistently doesn’t hit the WIP limits, consider reducing WIP limits to optimize flow.
  6. Reflect in Retrospectives:
    Include discussions about WIP limits in sprint retrospectives. Gather feedback from the team regarding the effectiveness of WIP limits and explore ways to further improve the flow of work.

Using WIP limits on Scrum boards helps teams manage their work more effectively, reduce context switching, and improve the overall flow of tasks through the development process.

Conclusion

In conclusion, the strategic implementation of WIP limits, grounded in the robust theory of Little’s Law, stands as a cornerstone in the Agile toolkit. It is a pragmatic approach that not only aligns with mathematical principles but, more importantly, empowers teams to navigate the complexities of software development with precision, agility, and a focus on continuous delivery with maximum speed.

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