Lead, Lag, Lollygag: Task Dependencies in Project Management

Having trouble locking onto the different dependencies when it comes to tasks and how it relates to resource allocation and scheduling in project management? I hear ya! This is definitely one of those times when trying to sound uber-professional can make things that you do literally all the time in your every day life suddenly feel like you’re trying to memorize a new language. Here’s a very brief breakdown of how you can better understand Lead, Lag, and Dependencies between tasks.

Lead
A task’s lead refers to how far into whatever comes before it, the task can start. If Task A takes 7 days and Task B has a lead time of 2 days, then it can start 5 days into Task A. It only feels weird for our brains because no one talks like this. Lead time is something we talk about in general life, but we don’t really think about it when we do and I’d even argue we don’t use the term in the same way that it’s used in this context. When we talk about lead time what we’re really asking is ‘how far into doing that thing, can I start this thing?’. We actually do this in real like all the time. If I’m waiting to pick up a food order that they told me it’s going to take them 20 minutes to make it, and I live 5 minutes away - what is my lead time for driving to the restaurant? It’s 15 minutes!

Lag
Lag is how long you need to wait after one task is done in order to start the next one. I’m getting ready for a wedding and I need to paint my nails and then put on my dress, but I can’t put my dress on with wet nails. The 10 minutes it takes for my nails to dry before I can get dressed is the lag.

Mandatory Dependency
This can sometimes feel similar to lag, but lag refers to time and MD refers to the actual tasks themselves. A Mandatory Dependency is exactly what it sounds like: Something you need to do before you can do the next step. The relationship between the two tasks is inherent, and a hard requirement in order to move forward with the project. If I’m painting a room in my house, I have to clear all of the walls first. Painting the walls cannot commence until I take all of the frames, bookshelves, etc, off of the walls. Clearing the walls is an inherent and required step in painting a room.

Discretionary Dependency
These tasks can be done at the same time (parallel), or one after the other (sequentially) and it’s up to the Project Team to decide which is best for the specific project at hand. Maybe resources are a consideration, maybe you want to see how task A turns out before moving onto task B even though you don’t need to. If we’re cleaning the house we can vacuum the living room and mop the floors in the kitchen at the same time, or we can do them one after the other. If we have two people available and are pressed for time, we will probably decide to run the two tasks parallel. If only one person is available, then we’re going to have to run them sequentially. Neither is dependant on the other - it’s up to us based on our resources to decide which is the best course of action.

Breaking this stuff down into every day applications is incredibly helpful for understanding how it all works. We do so much of this stuff all the time and we plan our own lives, allocating resources like time, money, and manpower, but we don’t even realize that we’re doing it. Putting it into real-world concepts can take you from memorizing, to understanding and actually applying the terms correctly when you sit down to take a PMP exam.