Project Scheduling

Project scheduling is the time phased processing of operations subject to precedence interactions, time constraints, and resource constraints in order to achieve particular goals (Badiru, 2011).

The fundamental network assessment of the project is typically carried out in three stages: stage of network planning, stage of network scheduling, and stage of network control (Badiru, 2011).

Network planning is also often pointed as activity planning. This involves identifying the specific project activities. The activities needed and relationships of precedence are defined in this stage. Precedence requirements can be defined by technological, procedural or imposed limitations. Finally, network diagrams are utilized to visually represent the activities.

Network scheduling is carried out using computational methods for forward passes and backward passes to calculate the earliest and latest starting and finishing times of each activity.

To describe a project, three kinds of data are required (Hillier, & Lieberman, 1995):

• Activity information: Divide the project (at the required point of detail) into its individual operations.
• Precedence relationships: Define the predecessor(s) for each activity.
• Time information: Forecast each activity’s duration.

Some principles to adopt, when creating an AOA network diagram, are as follows (Elbeltagi, 2009):

• Each activity must have a distinctive number such as i and j, where j (number at the arrow’s head) is greater than i (number at the arrow’s tail ).
• A gap between numbers (i.e. 5, 10, 15, etc.) is suggested. This will enable missed operations to be accommodated.
• Keep away from back arrows

Activity on node network (AON): This technique is also called the method of the preceding diagram.

• The dummy activities in AON representation are not necessary.
• AON can be drawn and read more readily.
• In AOA, activity can only begin once all of its predecessors have been completed.
• AON enables the depiction of overla /lag.
• AON enables four kinds of relationships to be represented while AOA enables only finish to start relationship.

For these purposes, AON project network diagrams have become extremely popular with professionals.

Projects consisting mainly of deterministic activities (e.g. maintenance projects) use CPM, which ignores statistical considerations and depends on a single estimate of the mean time needed to carry out the activity in hand. Details of the CPM calculations are provided in the previous chapters.

The PERT system relies on three estimates for each activity: an optimistic (minimum) time, a highly probable (modal) time, and a pessimistic time (Moder, & Phillips, 1964).

The network analysis depends on the well- known “central limit theorem”. The theorem says: “Where a series of sequential independent activities lie on the critical path of a network, the sum of the individual activity durations will be distributed in approximately normal fashion, regardless of the distribution of the individual activities themselves. The mean of the distribution of the sum of the activity durations will be the sum of the means of the individual activities and its variance will be the sum of the activities’ variances” (Elbeltagi, 2009).

The major steps in PERT analysis can be summarized as follows (Badiru, 2011):

1. Determine three estimates; optimistic “a” modal “m”, and pessimistic “b” durations for each activity.
2. Calculate the expected value for each activity by using a, m, and b.
3. Compute the variance of each activity by using the formula for .
4. Calculate the expected project duration, Tx. Similar to CPM, the duration of the project is calculated by summing up the expected modal time of critical activities.

Therefore, if there are multiple critical paths in the project network, the critical path with the largest variance must be chosen to determine the project duration and the variance of the project duration.

It is possible to use several approaches to determine the duration estimates necessary for PERT. The following are some of the strategies (Badiru, 2011):

• Predictions provided by an experienced person
• Estimates derived from historical data
• Estimates obtained from statistical methods such as regression
• Estimates provided by the results of simulation scenarios
• Approximations obtained from heuristic assumptions
• Customer requirements

PERT derives the following advantages (Chand, 2014):

• Allows managers to analytically plan their projects and evaluate all the factors that affect the progress of the project. The network analysis process involves the project planning to be carried out from start to finish in considerable detail.
• Provides management with a tool to predict the effect of schedule changes and be prepared to resolve such circumstances. The likely trouble areas are placed earlier which gives enough time to apply some preventive or corrective steps.
• Task relationships are represented visually for easier assessment, and individuals can logically deduce their role in the overall task requirements at various steps.
• The PERT time is based on a there estimates, and therefore, it is the most objective prediction under the uncertainties and results in a higher degree of forecast accuracy.
• PERT provides improved communication results. The network diagram provides a mutual interest for different stakeholders such as project managers, customers, contractors, etc., and they all understand the role and contributions of each other.
• The network will illustrate situations that require higher priority attention whereas intention can be applied to key activities without overlooking the lower priority tasks. This provides management a chance to shift focus to any critical activity in order to finish the entire project on time.

There are some constraints or issues that arise in PERT (Chand, 2014):

1. The estimation of duration and resources is uncertain. Keep in mind that these values are just assumptions and outcomes can only be as good as the assumptions
2. Final cost may exceed traditional planning and control techniques. Due to the current nature of networking and network analysis, it requires a high level of planning skills and more details that would raise the cost of time and resources for labors,
3. It is not appropriate to use PERT for fairly simple and repetitive projects such as fixed-sequence works.

                        PERT                                      CRM

                                                PERT                                      CRM