Estimating task durations to find the critical path

If your project’s running order is nailed down (or stuck down at least), it's time to move on to estimating your tasks' durations and costs.

Do you have your Post-its up on the wall?

This post is the third in our series on planning and managing projects. If you've not read the preceding two posts and haven't created a Work Breakdown Structure, or a project running order, be sure to do those first.

1. How to create a Work Breakdown Structure
2. How to build a project running order
3. Estimating tasks durations and finding the critical path (you are here)

Are your task durations going to be in days or weeks?

A crucial decision you'll need to make early in the project planning process is whether you'd like tasks to have an associated duration of days or weeks.

This goes back to the granularity of your tasks; remember, your project plan isn't meant to be a minute-by-minute or even a day-by-day checklist of events. It's a high-level overview of what has to happen, and when you think of it, nobody can get that much done in a day! While there is no hard and fast rule, weeks are probably the better option for most.

There will be exceptions to this, and it's at your discretion, but we'd recommend opting for weeks when in doubt.

The two types of time

Before we jump into how to estimate durations and costs, we need to point out that there are two types of time to consider.

1. Time elapsed = duration (usually weeks)

The first type of time is elapsed time, i.e. the amount of time it takes in weeks (occasionally days) for something to be completed. For example, say you're an architect applying for planning permission to build an extension on a client's home. From start to finish, you expect the process to take 18 weeks. No one will be working flat out for that period, but the overall duration of getting planning permission will take that long. That's the elapsed time, aka duration.

2. Time worked = effort (usually hours)

The second type of time is time worked, i.e. the number of hours of effort someone needs to spend on a given task. Going back to your planning permission application, while the duration will be 18 weeks, the actual effort spent actively working will likely be far less.

You may spend several hours a day for the first week getting drawings ready and speaking with your client. But as soon as you submit the application, you'll stop working and need to wait for your friends at the council to do their thing.

Of course, the application could be rejected, and it might come back to you, and you need to spend another 8 hours or so. But even then, the overall time worked will only be around 20 hours.

You wouldn't bill your client for 18 weeks of work, even though that's how long it took overall - you'd bill them for the 20 hours of time worked, i.e. effort.

Try to avoid confusing the two

These two types of time can be easily confused, especially for companies where teams typically work on a single project solidly and the duration and the effort are basically the same. However, even when that is the case, try your best to separate them, as they aren't the same!

Estimating task duration

For simplicity, we'll assume that you've done as we've suggested and your task durations are in weeks.

By this point, you've got your running order up on a whiteboard or wall, with all the tasks connected with arrows into and out of them. Now is the time to estimate their duration. You can do this with everyone responsible for delivering the project in the room at the same time or grab people when you get to their section of the project.

Write your task durations on the Post-it notes

What you want to do is write down how many weeks you and your team think tasks will take on each Post-it (task) you have on the board.

Estimating durations shouldn't be guesswork

While we've said "estimate", we don't mean guess. Your estimates should be based on your experience delivering similar tasks and projects. If you don't have experience providing the type of thing you're planning, speak to someone who does, so you can get the best estimates possible.

Break tasks down if estimations are proving difficult

Another option if you're struggling to estimate the duration of tasks due to their complexity is to break them down into more tasks and, if needed, create a sub-plan. This is the brilliant thing about the Post-it notes on a whiteboard - they're easy to change and adapt when necessary.

Estimating task effort

While you're working through your project with your experts, it makes sense to ask them at the same time how much effort will be required to deliver each task - this will avoid having to bring them back again later.

Remember, you're interested in how many hours (or perhaps days for longer projects) they need to work to deliver the tasks, and from this, the cost of the work can be calculated using each person’s hourly or daily rate.

Of course, time spent working on tasks isn't the only type of cost you'll likely incur. Now is a perfect time to discuss external costs like materials, freelancers, software, and everything else you'll need. You may not have the figures to know precisely what your external costs will be, but getting a list of them enables you to get that information from suppliers or others in the team.

Finding the critical path

The critical path of your project isn't the most important tasks, nor those that'll cost the most - it's simply the sequence of tasks that will take the longest duration to complete.

Working from left to right, follow the branches of your work running order that has the longest overall duration. Depending on your plan's complexity, you might need to do a few passes, but you should quickly find the chain of tasks that'll take the longest time - the critical path.

Example of finding a critical path

This is an example project relating to the creation of a new business.

Finally, once you know the critical path, put red lines between each task on it so everyone can see it clearly.

Next up - calculating contingency

That's it for this post. You should now have a logical project running order, duration estimates, effort and external cost estimates and your critical path. The next step is calculating contingency and adding it to your project, which we'll get into soon.