Two Tips for Reducing Project Schedule Risk

Project schedules are always a big focus in project execution. There’s so much time, money, and relationships tied to them, especially in big warehouse projects which often include construction, equipment, and software.

Nobody likes delays, but there’s a lot ‘under the hood’ of a schedule that can make it go sideways. People, environment, other projects, vendor availability, lead times, and just plain bad luck are just a handful of the things that make project leads tear their hair out.

Simple! Easy! Right?

But we have good news! Here are a couple tips to avoid surprises and improve the resiliency of your project schedules.

Analyze sub-critical activity chains and buffer them

Project managers and stakeholders often ask “What’s the critical path” to get an idea of what to focus on in the project. The critical path is the longest chain of events required to accomplish the project, which drives overall schedule, so it makes sense that those events are the center of attention.

If we also think of the critical path as the bottleneck or constraint on the project*, then we have to keep it working all the time. This means protecting resources on the critical path and making sure everything is ready to go when the critical activities need them.

This means that sub-critical activity chains can be the dark-horse schedule drivers. Let’s explore why.

The sub-critical activities become critical when they are late in delivering something that the critical path needs. Here’s an example from a warehouse startup:

Imagine a warehouse is being started with conveyors being installed and forklifts being purchased. These are both long-lead items, and–great news!–they’re both delivered on time. But wait a minute! The short-lead-time activity of recruiting and hiring supervisors and forklift drivers is late. Now the critical path runs through recruiting, hiring, and training. Everyone was focused on the material-handling commissioning, but now PIT training is holding up the go-live. Surprise!

This could have been avoided if the sub-critical path of hiring and training had been early.

To ensure your critical activities are protected, review your schedules for sub-critical event chains. “Sub-critical” means the events that produce an output required by the critical activities. When you find those sub-critical events, give them extra schedule or resource buffer to ensure they are ready.

Task chain in a project Gantt Chart
Sample project Gantt chart kicking off 10/28, with prudent buffer

If you neglect the sub-critical chain, chances are good that you end up with surprise delays on the critical path waiting for the formerly “non-critical” activities to be completed.

Sub critical task without buffer
Sample project Gantt chart kicking off 10/28, likely to disappoint!

The takeaway here is that adding buffer to the non-critical activity chains makes your schedule more resilient to change without impacting the overall delivery.

Conduct statistical schedule analysis

Schedules often come from vendors or project managers in a deterministic form. In other words, you get activities on the schedule with a duration for each activity, and that’s it. That’s the schedule. What’s missing is a statistical schedule analysis.

Statistical schedule analysis will tell you how likely that schedule is to occur, or how confident the schedule owner is in it. You want to be able to articulate that level of confidence so you know how to plan schedule and cost contingencies.

Schedule confidence can be expressed as a probability. For example, you want to be able to look at a schedule and know whether it is a 50th-percentile probability (P[50]) or a 95th-percentile P[95] probability schedule. The 50th percentile probability schedule accounts for fewer outcomes, and therefore conveys a lot less confidence, than a P[95] schedule.

The way to do this is by applying a probability distribution to your key activities minimum, maximum, and most likely durations, and then simulating many outcomes. This is called a “Monte Carlo” simulation or model. There are software packages to help with this or you can research and do it in MS Excel (or ask experts, ahem, for help).

The resulting probability distribution of schedule outcomes can tell you the probability of an outcome of a schedule. This will inform your decision on what “the schedule” is, and how much contingency in dollars and time you should put on top of it to plan for risky events.

Schedule probability distribution

A word of caution though – Garbage In, Garbage out: if your basic schedule assumptions don’t reflect what could happen with the activity, your resulting confidence calculations won’t be as helpful.


Doing these two things will remove a huge amount of uncertainty in your plan. They will enable you to identify the level of confidence of a schedule and to de-risk it by buffering the activities supporting your critical path. This in turn helps avoid surprises and be more clear in communicating level of risk of the activities you’ve planned to deliver.


*Credit to Eli Goldratt and his Critical Chain methodology for the concept. And an excellent resource for project risk management is this book [link] by Jeremie Averous.


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