The Project Schedule Part 6: The Critical Path


With tasks, resource assignments, durations/effort and dependencies defined, your project scheduling software will create a schedule. The path of dependent tasks that in aggregate take the longest amount of time is called “The Critical Path”. This is because if any one of these tasks is completed later than originally scheduled, your project end date will move.

When you are in the “Execution and Monitoring” Phase of your project, regular examination of the state of your critical path tasks is a top priority. It is important to routinely check in with the assigned resources to determine if the “days to completion” is still valid. If you wait until the task is late, it will be too late to do anything about it. Your only choice would be to examine the other tasks on the critical path to see if any task times can be reigned in to make up for the lost time. We will examine techniques you can use for this in the next post.

A technique you can use to make your critical path less volatile is to estimate your individual tasks more aggressively and aggregate the extra time you would have assigned to individual tasks into one “critical path buffer”. If critical path tasks come in early, you can add the time saved to the buffer. If critical path tasks come in late, you subtract time from the buffer. Using this technique, your schedule will not move with any one late task and it will encourage team members to work faster and ignore distractions. Also, the health of the buffer would be the key metric instead of the health of each individual task.

You can measure the health of the critical path buffer with two metrics:

  1. % or original buffer remaining
  2. Divide the “% of elapsed project time” into “% of buffer used”. If this is a number less than one you are tracking well. If it is greater than one it is an early warning sign to take action

For example: Your project has used 30% of it’s original buffer but you are only 10 weeks into a 50 week project (only 20% of the project schedule has elapsed), You divide 30%/20% and this equals 1.5 (greater than one) meaning you need to take remedial action.

If your project only used 15% of your original buffer, 15%/20% = 0.75, which is less than one indicating a healthy schedule.

When using this technique, it is very important to regularly update the “days to complete” for each critical path task so you can have confidence in the status of your buffer.

Whatever technique you use, constant monitoring of the health of the critical path is one of the most important tasks for the Project Manager.


The Project Schedule Part 5: Dependencies


Now that you have your lowest level scheduled tasks defined and have assigned resources, it is time to define the dependencies between tasks. This is where MS Project really comes in handy as it will create the schedule for you based on task dependencies and resource availability.

There are four types of dependencies:

  • Finish to Start – This is the most common and the default in MS Project. The predecessor must finish before the successor can start. For example, “Applying Primer” must finish before “Painting” can start.
  • Start to Start – Predecessor must start before the successor can start. For example “Mortgage Application” must start before “Credit Check”.
  • Finish to Finish – Predecessor must finish before the successor can finish. This can be true of tasks that run in parallel but both are needed for the subsequent task.
  • Start to Finish – Predecessor must start before the Successor can finish. This one is rarely used and frankly should be avoided.

When you initially define the dependencies, take care to only define “true” dependencies. If you have one person assigned to all the tasks you may be tempted to make all of the tasks dependent since the resource must complete one before starting another. Don’t do this. Let MS Project do this via resource leveling. The reason for not doing this is you may get additional resources later so some tasks can run in parallel. If you made them all dependent, the schedule will not show this possibility.

MS Project can now take your tasks, resource assignments, estimates and dependencies and create an initial schedule. I say initial because you often will find with your first cut that the finish date does not occur within the Sponsor’s expected time frame. In the next topic I will discuss the concept of the “Critical Path” and what the Project Manager can do to rein in the target date.

The Project Schedule Part 4: Resource Assignments and Estimates


Once you have defined your Work Breakdown Structure (WBS) the next step in creating a schedule is to assign resources and estimates to each task (the lowest level items in your WBS). A starting point in assigning resources is to consult the “Roles & Responsibilities” matrix you created as part of the Project Charter. You are looking for the stakeholders that are in the category of “helping you execute your project”.

You may find that some WBS tasks need resources that have not yet been specifically identified as assigned to the project. In this case you will meet with the managers of the areas that have the resource expertise and agree on the assignment. In other cases you may need to contract for professional services. This should have been included in your Project Budget and the Procurement Plan. If not, you will have to use the formal Project Change Management process to alter the budget to include these services.

Microsoft Project allows you to assign resources to a task. You can also assign the percentage of the resource’s time that will be dedicated to the task. In addition, you can assign more than one resource to a given task.

Once you have resources assigned to a task you can attach estimates. You can estimate using “Duration” (the length of time the task will take independent of the resources assigned) or “Effort” (the amount of hours or days a task will take given undivided attention of the resources). Which one you use will depend on the type of project, type of task and organization culture. As a general rule I like to use “Effort” and let MS Project calculate the duration given the resource percent allocation and other tasks assigned to that resource.

When defining estimates, take into account the expertise of the resource(s) assigned to that task. An expert resource may complete a task much faster than a novice. Sometimes estimates are placed on a task prior to the resource assignment to get an idea of how the schedule will look. If you do this, remember to re-estimate once the specific resource is assigned.

There are many techniques to creating estimates. I will not address them all here. The most common are:

  • We’ve done this task before so we know how long it takes
  • The assigned resource supplies the estimate. Be careful with this one. People tend to be overly optimistic on how much time a task will take.
  • A small group of people with expertise in that task are asked to independently estimate the task, then the group discusses the discrepancies and comes to a consensus
  • Management may dictate the duration of the task in which case the PM may have to assign resources with more expertise or add resources (if feasible) to meet the deadline.

You can also use a 3-point estimate (optimistic, pessimistic and most likely) and calculate your estimate using PERT (I recommend you Google this term for the details). For a more advanced scheduling technique, you can investigate “Critical Chain and Buffer Management”. This is an advanced technique that will need organizational buy-in from the top down.