CHAPTER 4: SEQUENCE PROJECT ACTIVITIES AND CREATE A SCHEDULE
1hr. 30min.
A4.KCI-4. Sequence the activities and plan the duration
We now take the following steps:
Draw up a network with dependencies.
Determine the critical path.
Process the availability.
DRAW UP A NETWORK WITH DEPENDENCIES
In every project, there are a number of dependencies. Certain activities can only start when others have been completed. To ensure that the team members are on the same wavelength as one another, it is recommended that the dependencies are identified and listed.
To make the structure of the time schedule in a phase clear, the project manager can show the dependencies in a precedence diagram.
If you study the network, you can see two paths you can follow to get to the end point. The activities B and C below, and the activity A above. Each path has a duration, and the longest determines the duration of the project. If, in this example, A takes three weeks, B takes three weeks and C takes two weeks, then the duration of the project is five weeks (duration A (3) < duration B (3) + duration C (2)).
This also means that for the total duration of the project, it does not matter if activity A overruns its time. There is a margin of two weeks before this has an impact on the end of the project. This margin is lost if activity A starts more than two weeks late. If activity B takes four weeks and C three weeks, then the total duration will be seven weeks instead of five, an overrun of two weeks. It is the path B – C, therefore, that determines the duration of the project.
The activities on which the duration of the project depends, are called critical activities and taken together, they form the critical path. Every time when you are working with the schedule, you look at the critical path; these are the activities which you need to pay the most attention to in order to control the time.
It is possible that the critical path will change during the project, as a result of activities, which initially were not critical, overrunning their time.
The start of certain activities often depends on milestones or decision points, and these are also included in the network diagram. As a decision point, or milestone, has no time associated with it, you include it as an activity with a duration of zero. However, for the actual decision making, time is usually planned either with a lag dependency, or with a separate activity.
DETERMINE THE CRITICAL PATH
The calculation of the critical path (the minimum duration of the project) is carried out in a number of steps:
Forward pass.
Backward pass.
Determine slack.
The calculations are done in the rectangles of the network.
ES= Earliest Start
EF= Earliest Finish
LS= Latest Start
LF= Latest Finish
9.1. Calculate the earliest possible start and end time (forward pass)
The earliest possible start for A is 0 and the earliest time this activity can be finished is the end of the third week. The earliest possible start for B is 0 and, at the end of the third week, B is finished and C can start. At the earliest, C can be finished at the end of week five. You calculate, therefore, the earliest start by looking at the highest early finish of all preceding activities. When you add the duration to this, you obtain the earliest possible finish time for that activity.
Therefore, this project can at the earliest be finished at the end of week five. For this forward pass, we always use the top row in the rectangles.
9.2. Calculate the latest possible start and end time (backward pass)
For the backward pass, we use the bottom row of the rectangles. We now calculate back and look at what the latest possible finish time is for each activity without endangering the minimum duration of the project. You do that by using the earliest of all latest possible start times of the following activities. Subtract the duration from this, and you have the latest possible start time.
9.3. Calculate the slack (float)
Next, we calculate how much slack time, or float, the different activities have. The latest possible end date of activity C (and the project) is the end of week 5. This activity therefore has no slack; every delay means that the project will overrun. The whole of the path from B to C has no slack. That is different from the path going via A, which has a slack of 2. So A only has to be finished at the end of week 5 before there is a danger that the project finish will not be on time.
There are two types of slack:
Total Float (slack): the time an activity can overrun without the project being delayed.
Free Float (slack): indicates when a further overrun will delay the start of other activities.
The path that is build up by the activities with zero float (slack) is the critical path. This determines the minimal duration of the project.
10. PROCESS THE AVAILABILITY
When making a critical path schedule, no account is taken of the availability of resources. In the event of limited resources being available, it is possible that activities which could be carried out in parallel, have to be carried out by the same person and, therefore, have to be executed one after the other. This will change the critical path, and the process of balancing is called “resource levelling”.
In the diagram, I have translated the schedule into a capacity diagram.
For each period, you are now able to compare the availability with the capacity you require. If, for this project, you only have two people available, you have to move up certain activities. First, of course, you look at activities that do not lie on the critical path so as to use any free slack time, and then you use the total slack and by doing this you lengthen the project duration. An alternative approach is to look at the uncertainty in the estimates, and then delay the less risky activities along with those that have a short duration. You can also better delay activities that are further down the track, than activities that are earlier in the schedule.
Modern scheduling software supports you in calculating the effect of all the different options. It is true to say, however, that this can never replace the skill of a planner, and ultimately you have to make an educated decision which activities to delay in order to match the available resources.
Application
You can convert the above into actions on the project/programme/portfolio for which you are currently responsible, by carrying out the following steps:
Order the activities into sequence.
Determine the dependencies.
Calculate the critical path.
Process the availability.
