In its simplest form the calculation of the schedule is a matter of establishing a start date for the project and adding the durations of the activities in the schedule, being careful not to include the activities that are scheduled to be done in parallel with other activities. The schedule consists of determining the start and finish dates for each of the activities and can be calculated with or without considering the influences of resources or the lack of them. The scheduling of each activity must consider the logic of the schedule. That is, the scheduling of each activity depends on the activities that it depends on for its schedule. These logical relationships are best seen in the "precedence diagram".
Tell me more …
The initial schedule is calculated based on the logic of the schedule and does not consider the constraints that result from the resources available to work on the project. Probably the easiest way to learn to calculate a schedule is to do one. We will walk you through the calculation of the project schedule the same way computer software does it and explain things as we go along.
Referring to NETWORK DIAGRAM, we will calculate the early start and early finish dates for the schedule and then go on to calculate the late start and late finish dates as well. These are normally the dates that are calculated. What they show is the earliest dates that a particular activity can be started and finished and the latest that a particular activity can be finished. The pair of early schedule dates is referred to as the early schedule and the pair of late dates is referred to as the late schedule.
These two schedules are useful because they tell us something about the flexibility of our schedules. Activities that have no difference between the early schedule and the late schedule do not have any flexibility. If an activity has the same dates for its early schedule as it has for its late schedule, it means that the activity cannot be delayed even one day without causing the project completion date to be delayed. If there is a difference between the early schedule and the late schedule, there is some flexibility, which means that this particular activity could be delayed as much as the late schedule without affecting the finish date of the project.
The first thing that must be done in any project schedule is to declare a project start date (or finish date if that is mandated). If you do not do this when you are using project management software, the software will assign today’s date as the project start date. This is usually not the date for the project to start. In the example we have determined that the project is to start on January 1, 2003.
Next we look for the activities that have no predecessor. These are the activities that do not have any arrowheads going to them. There can be more than one activity with this condition. Some people prefer to create an activity with zero duration called "start" and another one called "finish". When this is done, there is only one activity that is first and one activity that is last. Software for project management will not do this automatically. It is not necessary for proper schedule calculations since the computer will come up with the same schedule dates whether you use start and finish milestones or not. In our example this is activity 1.
The start of activity 1 is January 1, shown as 1/1. The duration of this activity is ten days, so the completion of the activity is scheduled for 1/10. This is the early start and early finish date for activity 1. You may wonder why you don’t simply add ten days to January 1 and get a completion date of January 11. This is not the case when doing date calculations since it is assumed that the work will start in the morning of January 1 and finish on the afternoon of the day it finishes. The work is done on the first and last day of the activity schedule. The next activity, since it starts in the morning, will start on the next day. This is a generally accepted convention used by all schedulers, and it is the way your project management software will calculate dates.
Schedules can also be done by tracking time on the same basis as the hands of a clock. If an activity started at 1:00 P.M. and took two hours, it would finish at 3:00 P.M., and the next activity would be able to start immediately afterward at 3:00 P.M. plus a few microseconds. When schedules are calculated on calendar days, the schedule is generally using the former method. When schedules are done in hours, they are usually done with the latter method.
Activities 2 and 3 have a finish-start dependency with activity 1. This means that neither of them is permitted to start until activity 1 is completed. By our convention, activities 2 and 3 have early start dates of January 11, the morning after January 10. Adding the durations of activities 2 and 3 to their early start dates gives us early finish dates of January 25 and January 15 respectively.
Similarly, activities 4 and 5 have early start dates of January 26 and January 16. Adding the durations to the early start dates gives us the early finish dates of February 19 and February 15.
Activities 6 and 7 both depend on activity 4, so both of them have early start dates of February 20. Adding the duration of the activities gives us the early finish for each of March 1 and February 27. This is not a leap year so February has 28 days.
Activity 8 is a little different in that it is dependent on two activities, 4 and 5. Since activity 8 must not start until both 4 and 5 are finished, the earliest it can start is the morning after activity 4 is completed, and its early start date is February 20 and its early finish date is March 3.
Activity 9 can start immediately after activity 5. Its early start and finish dates are February 16 and March 4.
Activity 10 is interesting. It depends on activities 6, 7, and 8. As before, we must investigate all three of these activities before determining the early start date of activity 10. Activities 6 and 7 have normal finish-start relationships with activity 10. With respect to activity 6, activity 10 could start on March 2. With respect to activity 7, activity 10 could start on February 28.
With respect to activity 8, we have a different kind of relationship. This relationship is a finish-start relationship with a lead of three days. The lead subtracts three days from what zero lead or lag would have. In this case the zero lead or lag relationship would be a normal finish-start relationship and the early start date for activity 10 would be March 4. Introducing the three-day lead means that activity 10 is allowed to start three days earlier than it would be allowed to start without the lead time. So, with respect to activity 8, activity 10 has an early start date of March 1.
Since activity 10 depends on all three relationships being completed, activity 6 determines the early start date of activity 10, and its early finish date is March 11.
Activity 11 depends on activity 10 and has an early start and finish date of March 12 and March 16.
Activity 12 depends on activity 9. The relationship between these two activities is a start-start with a lag of twelve days. In a start-start relationship the dependent activity is allowed to start as soon (on the same day) as the independent activity starts. This would be February 16, the same date as the start of activity 9. There is a lag of twelve days in the relationship so the early start date of activity 12 is February 28, and its early finish date is March 6.
The calculation of the early schedule is often called the forward pass since it moves forward in time to the end of the project. The project completion date is the latest early finish date in the schedule. In this case it is March 16, and activity 11 is the latest finishing activity in the early schedule. It should be noted that the latest early finish date in the project schedule determines the project completion date, not the early finish of the activities that have no successors. Leads and lags in schedules can cause activities that have successors to be scheduled to be later than activities that have no successors.
Since we have a forward pass, it seems only natural to expect there will be a backward pass as well. The backward pass is just what it sounds like. We will start at the end of the schedule and work back to the beginning, calculating the latest possible dates that activities can be scheduled without delaying the project completion date of March 16.
Activities 11 and 12 have no successors. Since they have no successors, both of them could be finished at the latest possible date. This would be the project completion date, March 16. The late finish for both of these activities is set to March 16. Durations of the activities are subtracted from the late finish dates to get the late start dates much as we added durations to the early start dates to get the early finish dates. Activities 11 and 12 have late start dates of March 12 and March 10.
The latest that activity 10 can finish in order to support a late start date of March 12 for activity 11 is March 11. Subtracting the duration gives a late start for activity 10 of March 2.
Activity 9 has a start-start relationship with a lag of twelve days with activity 12. The latest that activity 9 can start in order to support the late start of March 2 is twelve days earlier so activity 9 must have a late start date of February 26 and its late finish date is March 14. Notice that here we are not concerned with the finish date of activity 9 because the start of activity 12 depends on the start, not the finish, of activity 9.
Activity 8 has a finish-start relationship with activity 10 but there is a minus-three-day lead associated with the relationship as well. Since the latest that activity 10 can start is March 2, the late finish of activity 8 must be three days later than it normally would without the lead. This means activity 8 finishes on March 4. Subtracting the twelve-day duration, we get a late start of February 21.
Activities 6 and 7 must support a late start of March 2 in activity 10 so they must both have late finish dates not later than March 1. The late start dates for activity 6 and 7 are February 20 and February 22 respectively.
Activity 5 must support the late start dates for both activity 8 and activity 9. Since activity 8 has a late start of February 21 and activity 9 has a late start of February 26, activity 5 must have a late finish of February 20. Subtracting its duration gives it a late start of January 21.
Similarly activity 4 must support the late start dates of activities 6, 7, and 8, which are February 20, 22, and 21. Therefore activity 4 must have a late finish date of February 19, and its late start date must be January 26.
Activity 3 supports activity 5 and has late start and finish dates of January 16 and 20.
Activity 2 supports activity 4 and has late start and finish dates of January 11 and 25.
Activity 1 must finish to allow activity 2 and 3 to start on their late start dates of January 11 and 17, so it must have a late finish of January 10 and a late start of January 1. This is how the schedule is calculated in the computer software for project management. It makes sense to make this calculation before resources are introduced to the project schedule because there is a lot of work involved in entering the resources, and the schedule with resources will never get shorter than the schedule with no resources. If the logical schedule is too long to satisfy the stakeholders, the problem can be addressed in the logical schedule before resources are introduced. This will save considerable time and effort.