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Tuesday, May 17, 2011

Chapter 35: Sequencing Activities

The next step after defining activities is to sequence them in the proper order. You cannot test a system without building it first. So if your schedule testing before the development phase, your project is almost doomed. In this chapter, we are going to learn about the activity sequencing step.

So, lets get started!!!

Sequencing Activities

The activity sequencing process is used to arrange the schedule activities in the appropriate order, which takes into account the dependencies among the activities. For example, if Activity B depends upon the product of Activity A, then Activity A must be performed before Activity B.

Let us take a look at the diagram below that explains the steps in sequencing the activities involved in our project.


As you can see, the activities list, scope statement and other organizational process assets are all the input to this phase. You then apply some tools & techniques to come up the sequence.

Tools and Techniques for Sequencing Activities

Dependency determination is the prerequisite to determine sequencing. Therefore, most of the tools and techniques used for sequencing are focused on determining and displaying the dependencies.

Determining dependencies

To properly sequence the schedule activities, you need to determine the dependencies among them. Look at the image below:



Activity A is the Predecessor and Activity B is the successor. In other words, activity B cannot happen unless Activity A is over and hence the terms. This essentially means that A must start before B.

By definition, the successor activity must start after the predecessor activity has already started. But exactly when can the successor activity start after the predecessor activity has already been started?

Well, both the predecessor and the successor have a start and a finish, and there are at maximum four possible combinations between the start and finish points of the predecessor and the successor activities. Accordingly, there are four kinds of dependencies, also called precedence relationships or logical relationships, listed here:

Finish to start - The initiation of the successor activity depends upon the completion of the predecessor activity i.e., the successor activity cannot be started until the predecessor activity has already been completed.
Finish to finish - The completion of the successor activity depends upon the completion of the predecessor activity i.e., the successor activity cannot be completed until the predecessor activity has already been completed.
Start to start - The initiation of the successor activity depends upon the initiation of the predecessor activity i.e., the successor activity cannot be initiated until the predecessor activity has already been initiated.
Start to finish - The completion of the successor activity depends upon the initiation of the predecessor activity i.e., the successor activity cannot be completed until the predecessor activity has already been initiated.

These types of dependencies describe the logical relationships between activities.

Dependencies can be grouped into three categories:

Mandatory dependencies - These are the dependencies inherent to the schedule activities. For example, a software program must be developed before it can be tested. Mandatory dependencies are also referred to as hard logic.
Discretionary dependencies - These are the dependencies at the discretion of the project team. For example, the development for two parts of the system can happen in parallel and then integrated at a later point in time. It is not mandatory for the second module to wait until module one is over. Some of the guidelines for establishing discretionary dependencies can come from the knowledge of best practices within the given application area and from the previous experience of performing a similar project. Discretionary dependencies are also referred to as soft logic, preferential logic, or preferred logic.
External dependencies - An external dependency involves a relationship between a project activity and a non-project activity i.e., an activity outside the project. For example, lets say your application is migrating to a newer version of application server and it is supposed to be delivered & set up next week. You cant practically deploy and test your application until the server is delivered and set up. This is an example of an external dependency.

The dependency between two schedule activities is an example of the logical relationships defined earlier in this chapter. Logical relationships can be displayed in schematic diagrams, called project schedule network diagrams, or just network diagrams. A common method to develop network diagrams is called the precedence diagramming method (PDM).

Precedence diagramming method (PDM)

The precedence diagramming method is the method to construct a project schedule network diagram in which a box is used to represent an activity and an arrow is used to represent dependency between two activities. The boxes representing activities are called nodes.

Let’s look at a sample diagram to understand them better.


Above is a sample network diagram constructed by using PDM, in which Activity A is a predecessor of Activity B, Activity C is a predecessor of Activities D and E, and so on.
In general, PDM supports all four kinds of precedence relationships discussed earlier, but the most commonly used dependency relationship in PDM is finish to start. The start-to-finish relationship is rarely used.


Applying leads and lags

In the real world, some activities may need or lend themselves to what are called leads and lags to accurately or effectively define the logical relationships. For example, the finish-to-start dependency means that the successor activity starts where the predecessor activity finishes. Applying a lead means you allow the successor activity to start before the predecessor activity finishes, and applying a lag means you start the successor activity a few days after the predecessor activity finishes. Sometimes you might need to make such adjustments in the schedule for effectiveness and efficiency.

Schedule network templates

You can use standardized network diagram templates to expedite the process of activity sequencing. You can also use network diagrams from previous projects and modify them for the project at hand.

Output of Sequencing Activities

The goal of the activity sequencing process is to determine the dependencies among the schedule activities and sequence the activities accordingly. The sequencing is presented in network diagrams.

The output items from the activity sequencing process are:
Project schedule network diagrams - These diagrams, discussed in the previous section, can be created manually or by using an appropriate project management software application. Depending upon a project’s size, you might have multiple network diagrams for it.
Updates to project documents - During the process of sequencing activities, you may identify new necessary activities, split an activity into two, modify activity attributes, add new attributes, or identify a risk related to an activity. Accordingly, you may need to modify some project documents, such as the activity list, activity attributes, and the risk register.

Previous: Defining Activities

Next: Estimating Activity Resource Requirements

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