What is Lead & Lag?

In so many recent discussions I realised that even though Lead & Lag are easy concepts confusion around it exists with many of PMP® aspirants. Especially when we use these two dependencies in network diagrams and even more when these are associated with different types of relationships. Let’s try to understand today these two techniques inside out and resolve all so called confusions revolving around this

What is Lag? As per PMBOK® Guide Fifth Edition

“The amount of time whereby a successor activity is required to be delayed with respect to a predecessor activity”

Important points about this definition which many of us tend to miss are as below. These points are the main source of confusions

• With respect to is an important aspect. It defines which activity is dependent on other and eventually decides the effect of lag. E.g. in below diagrams activity B is dependent on A, vice versa is not true
• Lag always describes delay, which means addition of time and denoted by “+“ sign in the network diagram
• This can be a predefined delay which is required to put in between the activities to fine tune the project and to meet the quality. Classic example of this is we need lag between plaster of wall and paint so that cement can set up first
• Obliviously it’s rare that project puts lag without any reason as no one wants to delay the project without proper justification. So Lag is definitely put to accomplish some pre-defined requirement or some strategic goal (specific time to launch the product)
• We have 4 types of relationships and lag can be applied with any. Refer below diagram. Dark/shaded area is effect of applying lag to the Activity B with respect to Activity A

If we understand all the aspects of Lag, it’s very easy to put it for Lead too. Let’s see how

What is Lead?As per PMBOK® version 5

“The amount of time whereby a successor activity can be advancedwith respect to a predecessor activity”

Important points about this definition which many of us tend to miss are as below. These points are the main source of confusions

• With respect to is an important aspect. It defines which activity is dependent on other and eventually decides the effect of lead. E.g. in below diagrams activity B is dependent on A, vice versa is not true
• Lead always describes advancement, which means reductionin time taken and denoted by “-“ sign in the network diagram
• Lead can be predefined advancement which is required to put in between the activities to fine tune the project and to meet the quality. Classic example of this is starting of document editing immediately once one portion of document is ready. No need to wait till creation of 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
• Lead is also used while using schedule compression technique like fast tracking. Though when put as part of schedule compression, it has risk associated (Rework) with it but we can’t avoid project challenges e.g. starting development when one portion of design is ready (earlier planned to start development at the end of full design)
• We have 4 types of relationships and lead can be applied with any. Refer below diagram. Dark/shaded area is effect of applying lead to the Activity B with respect toActivity A

I realised after handling so many questions on Lead and Lag that the main point we fail is, to visualize the situation when one type of dependency is applied along with the relationships.

Let’s see some key questions which create confusion.

Q: What does relationships means? i.e. FS, FF, SS, SF
A: We need to just remember, for relationship first letter defines predecessor behaviour 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 & Lag?
A:It varies, as explained above, sometimes it’s applied due to predefined constraint (in case of Lag majorly) and sometimes its applied to achieve project deadlines (In case of Lead)

Q: Is lead same as fast tracking?
A:Lead is a type of dependency which is used while schedule compression technique fast tracking because in fast tracking we need do reschedule the activities originally happening in serial, to happen in parallel so that deadline can be met/compressed. Lead in network diagram helps in achieving that

This is precisely the compilation of Lead and Lag on the whole. I hope this blog will help you in clarifying your fundamentals about Lead and Lag and help you in solving network diagram question in a better way. Please feel free to put your follow up questions on our Forum