This is the last installment of the series on scheduling without using software. At the end of this post if you read parts 1 through 3, you pretty much know the logic that goes into the scheduling process.
In the last few posts I’ve described the creation of a schedule from scratch just using logic, the PDM, and relationships. I’ve assumed all the constraints were removed such that each task started as soon as possible, and the resources and budgets were in place to get going. I showed how to do a forward pass, and a backward pass, and laid down the foundation for creating a network. Now I’ll talk about the critical path (CP) and float (total and free float). Let’s return to the example I’ve been using.
What is the critical path? By definition the critical path is:
• The longest path on a project
• The path defining the minimum amount of time a project requires for completion
• The path with zero or little float
Determination of the critical path is vital for proper project control. The PM will realize that knowing the critical path (CP) along with Total Float; will serve to communicate successful planning to the clients. CPs can change depending on the completion of certain tasks ahead of time, or late completion of others, so the CP is dynamic.
Using my previous three posts to guide you, here’s what the network looks like when you do a forward pass using the home construction WBS previously provided with the duration in days.
You’ll notice that the total time it takes to make this house is 113 days. The forward pass solution is shown above. Now let’s do a backward pass to determine the critical path, and the float in the network.
The backward pass reveals some interesting items worth noting. For example when going “backwards” from the two tasks HVAC and Landscaping the next task being Interior, we chose 97 days instead of 105 days. Can you see why? The answer is below. Look back at “Project Schedule Creation – Part 3” to review if you need to.
Keep in mind that we’ll need to account for the convergent activities and also ensure that we’ve done the math properly. When we finish the backward pass we’ll be able to determine the early and late dates for each task, identify the critical path, the Total Float for each task, and the Free Float for the network.
The Types of Float
What is float? Sometimes float is also referred to as slack. To be thorough we need to define Total Float (TF) and Free Float (FF).
Total Float is simply the total time a task can be delayed without affected the schedule end date. Total Float will change as tasks are updated in the status schedule. When tasks start or end early, or start or end late, the Total Float will change. From a practical perspective if Total Float is greater than zero you can permit that task to slip in time. If the Total Float is zero, you cannot permit any slips in time. In this scenario the task is most likely critical. If the Total Float is less than zero you are behind schedule and will need to re plan the effort.
Total Float is calculated by the following formulas:
TF=LF-EF or TF=LS-ES
Since the Total Float is calculated with the network in mind, a task having a Total Float of zero or less will affect the end date of the project. Total Float is shared by all of the activities along a specific path in the schedule. Additionally we also need to consider how a constraint can affect the Total Float. If a constraint like “finish no later than” is placed on a task and one of the tasks violates the constraint, you could finish beyond the scheduled end date. This will result in producing a Total Float less than zero for that network path.
Free Float is the amount of time a task can be delayed without affecting or delaying the early start of the successor activity. Free Float is defined by the following formula:
Let’s look at examples for both Free and Total Float in the house construction network diagram. If I wanted to calculate the Total Float for the “Interior” task I would simply use TF=LS-ES or TF=80-80. The answer is zero, (0) so the Total float for the task is zero. In a similar manner if I were to calculate the Total float for the “HVAC” task I have zero float since TF=LS-ES or TF=98-98.
The total Float for the “Landscaping” task on the other hand is TF=106-98, or 8 days. I can delay this task as much as 8 days before I affect the network. Calculating the Free Float between the “Interior” task, and “HVAC”, and applying the formula FF=ES-EF-1 I have at once FF=98-97-1, or zero. Between those two tasks I have zero Free Float. When I calculate the Free Float between “Interior” and “Landscaping” I have the same answer. FF=98-97-1, or zero. On the other hand there is Free Float between “Landscaping” and the final task which is the “Open House for Sales” task. In this case FF=113-104-1, or 8 days of Free Float. Now that we’ve identified the Free Float, what is the critical path for the construction project? It’s in the diagram below.
The critical path is shown as the path with red dashed lines above. Note it is the path with tasks having zero Total Float, and it is also the path on the project with the minimum amount of time. If the project were to start the day after New Year’s Day the project time line for all the activities in table format would look like the table below.
That’s all there is to it! All you need to do is practice to convince yourself that it works. Then again you can take all the information and put it into MS Project and verify your calculations. Why would I want to do this manually at all? Like everything else that’s become highly automated, sometimes it’s necessary to check your work manually. I find that sometimes the software does foolish things with the inputs provided, because there are numerous algorithms working behind the scenes that confound the information provided. I like to also know that it makes sense given the constraints placed on the project schedule, so occasionally; I’ll do a paper calculation to determine if it makes sense. Is this all the information concerning schedules needed to create a successful schedule? No, wait there’s more.
Successful Scheduling Steps
If the following steps are taken, every schedule created by the team is bound to be successfully constructed for execution. This doesn’t mean an automatic guarantee that your project will be successful, only that you have identified the scope of work and the time it will take to execute your project. You will achieve your goal if you:
1) First define the project activities
2) Using the tasks identified, sequence them in a logical order
3) Involve Subject Matter Experts (SMEs), and stakeholders to estimate the resources, budgets, and assets necessary for each activity
4) Identify the duration for each activity once you completed step 3. You can only estimate the duration of a task if you know what it takes to accomplish it
5) Develop the schedule, finalize the network
6) Baseline the schedule, and execute the project plan
The schedule is created after verifying that the WBS reflects the scope of the work. If the WBS correctly reflects the work scope, then the next step is to logically establish the relationships. Next identify the durations, and link the tasks in their logical order using their relationships, accounting for constraints. All of this sound involved but if you are careful and get others to help , its not too hard to do. Poorly planned and executed schedules show a lack of detail to clients, and reflects on you and the team. If you want to get into greater depth on this subject or want to try practice problems, complete with solutions, check out my book on Amazon, “The Project Scheduling Primer”.
I won’t get into the various ways of utilizing the schedule correctly here (tips for execution). That will be a post for another time. Please drop me reply if you found this beneficial, I’m always looking to provide quality PM techniques to my readers.