Leads and lags LEARNOVITA

Developing Project Schedule : Role of Float, Leads, and Lags [ OverView ]

Last updated on 03rd Nov 2022, Artciles, Blog

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Ravichandran (Data Engineer - Financial Performance and Analytics )

Ravichandran has a wealth of experience in cloud computing, BI, Perl, Salesforce, Microstrategy, and Cobit. Moreover, he has over 9 years of experience as a data engineer, financial performance, and analytics.

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    • In this article you will learn:
    • 1.What is Lead?
    • 2.Using a Lead Time in Scheduling Activities.
    • 3.What is the Lag Time?
    • 4.What Are the Leads and Lags Used For?
    • 5.Durations and Dependencies.
    • 6.Finish-to-Start Sequence.
    • 7.Sample Schedule with a Lead and Lag Times.
    • 8.Conclusion.

What is Lead?

The definition of a ‘lead’ is that a successor activity is started before a predecessor activity has be finished. For a project managers to use a lead, the logical relationship between activities must have been a discretionary finish-to-start or start-to-start dependency .In the other words, there is no physical limitation or constraint that a predecessor needs to be finished before a successor activity can start.

Using a Lead Time in Scheduling Activities:

Identifying a lead times can enable a project managers to (re)schedule activities to utilize that time. Thus earlier start of a such successor activities compresses the overall time it takes to finish a sequence of activities .

What is the Lag Time?

  • A lag is explained as a required delay in a start of the successor activity. Lag time is a number of hours, days, weeks (or any other unit of time) that has to pass while there are no resources associated with a successor activity during that time.
  • There are the numerous reasons why lags are to be a scheduled, e.g. due to physical or resource constraints, to wait for the another activity to fully or partially finish or for certain dates and events to be happen.
  • In the network diagram, the lag would be indicated by the positive number because it is time that needs to be added.

What Are the Leads and Lags Used For?

  • According to the PMBOK, ‘leads and lags’ is the technique that is used in a processes ‘sequence activities’, ‘develop schedule’ and ‘control schedule’ (PMBOK®, 6th ed., ch. 6.2, 6.5, 6.6). Leads and lags indicate a flexibility, or the lack thereof, when it comes to the scheduling activities while taking a logical relationships into account. Logical relationships are identified and prioritized using a precedence diagramming method, for instance.
  • Examples of application of leads and lags are schedule network analysis technique and a critical path method (source), as well as an optimization strategies. These may be include, for instance, schedule a compression techniques such as fast-tracking.
Lag Time

Example of a Leads and Lags in an IT Project:

  • A typical finish-to-start relationship can be assumed for the activities related to design, development, implementation, testing, and go-live of IT solutions. It looks like the ‘natural order of things’ that, as first step, a solution is designed, then developed, then implemented, tested and an eventually deployed.
  • In a practice, however, projects tend to be constrained by a tight timelines. If things go wrong part-way through a project, even compressing or crashing a schedule may be required. Being aware of leads and lags of activities enables a project managers to “save” project execution time.

Durations and Dependencies:

In the project, the durations are estimated as below :

  • Technical design of a module A: 10 days.
  • Technical design of a module B: 5 days.
  • Development of a module A: 15 days.
  • Development of a module B: 20 days.

The dependencies are:

  • A technical designs of module A and B, respectively, must be a completed before a respective module can be developed.
  • The technical design of a module B makes reference to that of a module A.
  • Thus, the module A design needs to be a 50% completed before module B design can be started and The development of a module A depends on module B, which needs to be 25% completed before a team can start the development of module A.
What is Lead

Finish-to-Start Sequence:

Due to these interdependencies, the project manager could assume that there is inherent finish-to-start relationship among these activities. If it were sequence based on a finish-to-start relationships it would look as below:

1. Design module A

2. Design module B

3. Develop module B

4. Develop module A

However a descriptions give a hint that there are also be a leads and lags. These can be considered when a planning schedule:

    ActivityDependencies and durationsLag timeLead time
    Design module A Takes a 10 days A first activity A first activity
    Design module B can start 5 days after a design of module A started and takes 5 days 5 days (after start of A) 5 days (before finishing A)
    Develop module B can start when a design of module B is completed and takes 20 days no lag no lead
    Develop module A can start when a module B is 25% completed and takes 15 days 5 days (after start of B) 15 days (before completion of B)


A lead of the successor activity is represented as a negative number on the network diagram. Utilizing a lead times when scheduling activities will accelerate a completion of a series of activities.A lag is the necessary delay in the start of successor activity that needs to be a scheduled accordingly. It is shown with the positive number in a network diagram.They are therefore essential techniques for a proper and efficient scheduling of activities, an optimization of a schedule and as an input to find a critical path.

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