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

• 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.

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.

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:

Activity	Dependencies and durations	Lag time	Lead 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)

Conclusion:

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.