Calculating early start, early finish, and late start, late finish times on a precedence network is a tedious process on construction projects with many activities. In this video, Jim describes the process, and explains that this step is typically handled by scheduling software.
- We're now at what is probably the most tedious step in the process, but in a network model calculating early and late start and finish times is really what enables you to calculate just how long a project is expected to take along with the dates that each activity can and must start in order to finish on time. The steps we've covered so far require your input whether you're using a software scheduling program or you're doing it by hand, at this point though if you were using a software program and you had accurately entered your work breakdown structure, turned it in to that ordered list of activities, created the relationships between each of the activities by defining those immediately preceding activities and you'd assigned durations to each of them, you'd be in the position to let the software take it from here.
However for this course I would like you to understand what's being calculated and how and why so you understand what the output means or at the very least I want you to understand the terms that are used for the times that are being calculated. So let's start with the basic concept that I think tends to the trip up all of my students unless they actually listen closely to my next statement. Early start, early finish and late start and late finish times have nothing to do with variations in the duration of the activity.
In other words, I think people tend to equate these terms was some sort of variability in durations. Like if this activity could take between 7 and 10 days to complete its early finish is day 7 and it's late finish is day 10. This is not correct. These terms when we use them here in the scheduling process have nothing to do with variations in durations. Durations stay fixed. Instead these are calculations related to finding the critical path in the schedule and identifying those activities that are not on the critical path.
Let me break that down a little before I run through an example. The early start time of an activity is the earliest time that an activity can begin based on the relationships that you identified in that network diagram we did a few steps ago. The early finish time is just a day that the activity will finish based on the adding its duration to its early start day. So in our previous example if we use a beginning of the day system of numbers and we assign day one to the site fence, it's early start day is the beginning of day one.
It has a duration of one day so it's early finish day is the beginning of day two. The site fence was the predecessor to rough grading so based on that relationship the early start day of the rough grading is the start day of day two. The late start time is the latest that an activity can start without hurting the overall project schedule and the late finish time is just the latest date that an activity can finish without impacting the schedule.
When an item's in the critical path, it's early start and late start are going to be the same and it's early finish and late finish are going to be the same. Again back to our simple example, if the site fence starts or finishes late, it will push back the rough grading and it will ultimately push back the end date of the whole project. In an absolutely sequential schedule, one where there is only ever one activity occurring at a time and one where each activity has only has one predecessor, there is no need to calculate early start versus late start.
They'll always be the same but that's just not typical of most construction projects. If we try to make them simple and sequential we're going to draw out and lengthen a schedule. The purpose of scheduling using this network diagram is to find those items that can be done simultaneously so that we can compress the schedule in a logical and workable manner. Remember the history lesson from the beginning of the course with one of the first computers ever used in business.
Being able to create a schedule with a 25% savings. It did this by taking all of the hundreds of activities and finding the ones that can be done simultaneously, pulling those down into the body of the schedule and shortening the overall project duration. But we're looking for when we run these calculations are those items that can move a little bit without affecting the overall project completion date. These are the items that are not on the critical path and we say that these items have some float time.
So without running any calculations yet that's try to visualize what this is on our tennis court project. In my overall schedule I define some relationships they say that the concrete had to be done before I can start the surfacing and the surfacing has to be done before I can install the fencing and the fencing has to be done before I can start the final landscaping. These all represent finish to start relationships like I described earlier. I have to finish one activity before the next one starts and if one of these activities finishes late, it will push the entire schedule.
The early start and late start of each of these activities are the same. They have no float. This means that they must start on time or the project will be delayed but remember I also have that tennis court hardware that we discussed, the posts and the nets that have to be installed. If you look at the table this activity has a predecessor activity. I can't start it before I finish the coatings on the courts but again if you look at that original activity list we created you will see that none of the other activities on the list have this hardware as they are required predecessor.
So no other activities depend on finishing this activity before they start. I can start the hardware really anytime after the coating is done as long as I started in time so that it finishes before the end of the project. This means that the hardware activity has some float or some people will also refer to this as slack. I can start late without impacting the end date of the project or to a point. Now that last day that I can start the installation of the hardware and still finish on time, that's the late start date.
It's early start date and it's late start date are different. The difference is the number of float days that we have available on this task. That's what these terms mean. So how do we use the network to create and calculate these figures? It's actually not that hard but it is tedious. It involves making a forward pass through the network to calculate early start and early finish days just like I described earlier. Then you have to work back backwards through the network to find items with float days.
Finally you make another forward pass through the network. If an activity has float you calculate the late start by adding the total float to the early start and you calculate the late finish by adding the number of float days to the early finish. Like I said this can get a little tedious with hundreds of activities and in order to make it work you do have to run completely through the entire network making all three passes and that takes time. You could also start to see how difficult it would be to add an activity that you left off or to move something.
You would have to run the calculations for the entire network all over again with every change. That's a big limiting factor when it comes to using network models for scheduling but the use of scheduling software means that these calculations can be run and rerun almost instantly anytime that you want to make changes. We'll look at the computer model of this network later in the course so you can see how a scheduling program runs these calculations but if you do want more information on how to run these calculations by hand, an exercise files been provided for you to read through.
When using scheduling software all you have to do to create a successful and usable network model is make sure that you understand the input. Get those activities broken down correctly and assign those IPAs accurately to each activity. Then assign accurate durations. As you've seen in this segment the activities and the numbers from that table are all that's needed to build the network and to calculate the critical path.
This course identifies the steps needed to develop a proper plan, and demonstrates how that plan is transformed into a construction schedule. Throughout the course, instructor Jim Rogers shares examples of his own successes and failures in the areas of construction planning and scheduling, so as to lend real-world context to the concepts he covers.
- Types of schedules
- Planning versus scheduling
- Work breakdown structure
- Developing a schedule
- Creating a network model
- Assigning durations, costs, and resources
- Identifying the critical path
- Letting the software do the calculations
- Checking and updating the schedule
- Scheduling's impact on productivity