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Understanding and strategically applying lead and lag time is a fundamental project management skill that directly impacts scheduling efficiency and on-time delivery. By mastering these concepts, project managers can create more realistic timelines, optimize resource allocation, and improve overall project outcomes. This guide breaks down these essential scheduling techniques.
Lead time is the amount of time a successor task can be advanced relative to its predecessor. In simpler terms, it's when you can start a later task before the preceding one is 100% complete. This technique creates overlap in the schedule, which can significantly reduce the project's total duration.
A common example is in software development. The coding phase (successor task) does not necessarily need to wait for the entire requirements-gathering phase (predecessor) to finish. Once a core set of requirements is defined, coding can begin on those modules, running in parallel with finalizing requirements for others. This overlapping schedule, managed effectively, accelerates the project timeline. According to our assessment experience, identifying lead opportunities is crucial for compressing schedules without compromising quality.
Conversely, lag time is a mandatory delay or waiting period between the completion of a predecessor task and the start of its successor. Unlike unplanned delays, lag is a planned part of the schedule, often due to dependencies beyond the team's immediate control.
Lag time is typically required for three main reasons:
| Reason for Lag | Description | Example |
|---|---|---|
| Process Constraints | A necessary waiting period for a process to complete. | After pouring concrete for a foundation, a construction team must wait for it to cure (lag time) before framing the walls can begin. |
| Resource Constraints | A delay while waiting for essential resources to arrive. | A manufacturing line is halted (lag time) awaiting a shipment of critical components from a supplier. |
| External Dependencies | A delay based on a fixed date or an external event. | A marketing campaign launch (successor task) must wait for a specific product announcement date (lag time) to pass. |
Understanding lag is vital for setting realistic stakeholder expectations and accurately modeling the critical path—the longest sequence of tasks that determines the project's minimum duration.
These concepts are best visualized using a Precedence Diagramming Method (PDM), which illustrates four types of logical relationships between tasks:
What is a Finish-to-Start (FS) relationship with lead and lag? This is the most common relationship. The successor task can only start after the predecessor finishes. Adding lead time would be illogical here. However, lag time is frequently applied. For example, "Training documentation can begin (successor) 5 days after (lag) software development is complete (predecessor)" to allow for a testing phase.
What is a Start-to-Start (SS) relationship with lead and lag? Here, the successor task can only start after the predecessor has started. Lead time can be applied to allow the successor to start earlier relative to the predecessor's start. For instance, "Quality assurance testing (successor) can start 2 days after (lag) coding begins (predecessor)" to allow a baseline of code to be written.
How are Finish-to-Finish (FF) and Start-to-Finish (SF) relationships used? These are less common. In a FF relationship, the successor can only finish after the predecessor finishes. A SS relationship is rare, where the successor's finish is dependent on the predecessor's start. Lead and lag can be applied to these, but they are more complex to manage.
Effectively leveraging lead and lag time offers several key advantages for project managers and their organizations:
To optimize your next project schedule, first map all task dependencies to identify logical relationships. Then, actively look for opportunities to introduce lead time and honestly account for all necessary lag time. This disciplined approach is a hallmark of advanced project scheduling and a reliable method for enhancing project predictability.
