A lag time is the amount of time whereby a successor activity is required to be delayed with respect to a predecessor activity.
Following are Important points about this definition which many of us tend to miss. These points are the main source of confusions.
It defines which activity is dependent on the other one and eventually decides the effect of lag. E.g., in the below diagrams activity B is dependent on A; vice versa is not true
Lag always describes delay. Which means the addition of time and denoted by “+“ sign in the network diagram
It can be a predefined delay. Which we need to put between the activities to fine-tune the project and to meet the quality. A classic example of this is we need Lag between plaster of wall and paint so that cement can set up first
Project rarely puts Lag without any reason. As no one wants to delay the project without proper justification. So we put Lag to do some predefined need or some strategic goal (specific time to launch the product)
We have four types of relationships, and we can apply to any. Refer below diagram. The dark/shaded area is the effect of using Lag to Activity B with respect to Activity A
All these aspects of Lag, we can put for Lead too. Let’s see how
lead time is the amount of time whereby a successor activity can be advanced with respect to a predecessor activity.
Following are the key points from this definition:
It defines which activity is dependent on the other one. And, eventually decides the effect of Lead. E.g., in the below diagrams activity B is dependent on A; vice versa is not true
Lead always describes advancement. Which means a reduction in time taken and denoted by “-“ sign in the network diagram
Lead can be predefined advancement, which we need to put between the activities to fine-tune the project and to meet the quality. A classic example of this is starting of document editing immediately once one part of the document is ready. No need to wait till the creation of the full document. As documents having say 5000 pages if we wait for that long and then again take time to edit may lose its purpose
We can also use Lead while using a schedule compression technique like fast-tracking. Though when put as part of schedule compression. It has risk associated (Rework) with it. E.g., starting development when one part of the design is ready (earlier planned to start development at the end of full design)
We have four types of relationships, and we can apply Lead to any. Refer below diagram. The dark/shaded area is the effect of using Lead to Activity B with respect to Activity A
After handling so many questions on Lead and Lag, I realized the main point to fail is:
Visualize the situation to apply one type of dependency along with relationships.
Frequently Asked Questions:
Q: What does relationships means? i.e. FS, FF, SS, SF
A: We need to remember –
For relationship first letter defines predecessor behavior and second successor. E.g., for Finish to Start, Finish of predecessor and start of successor dependency. We need to start reading it that way.
Q: What “+” & “-“denotes in network diagrams
A: “+” is used for lag and “–“ is used for lead. E.g. if the relationship is SS+2, naturally we should read it as Start of Activity B with Lag of 2 with respect to start of Activity A.
Q: Why we need to put Lead and Lag?
A: It varies. It’s applied due to the predefined constraint (in case of Lag majorly). And sometimes we use it to achieve project deadlines (In case of Lead)
Q: Is lead same as fast tracking?
A: Lead is a type of dependency. And, we use Lead while fast-tracking schedule compression technique. Many times, we need to reschedule activities which are happening in serial. Using fast-tracking, we make them happen in parallel. So, we can meet the deadlines. Lead in network diagram helps in achieving that.
For more details on Lead and Lag with practical examples, simple watch following video: