Project Planning or Network scheduling techniques
Project scheduling is concerned with the techniques that can be employed to manage the activities that need to be undertaken during the development of a project.
Scheduling is carried out in advance of the project commencing and involves:
- Identifying the tasks that need to be carried out;
- Estimating how long they will take;
- Allocating resources (mainly personnel);
- Scheduling when the tasks will occur.
Once the project is underway control needs to be exerted to ensure that the plan continues to represent the best prediction of what will occur in the future:
- Based on what occurs during the development;
- Often necessitates revision of the plan.
- Effective project planning will help to ensure that the systems are delivered:
- Within cost;
- Within the time constraint;
- To a specific standard of quality
The three basic project planning techniques are Gantt chart, CPM and PERT - All monitor progress and costs against resource budgets.
Gantt chart
Gantt charts are also called Bar charts. The use of Gantt charts started during the industrial revolution of the late 1800's. An early industrial engineer named Henry Gantt developed these charts to improve factory efficiency. Gantt chart is now commonly used for scheduling the tasks and tracking the progress of energy management projects. Gantt charts are developed using bars to represent each task. The length of the bar shows how long the task is expected to take to complete. Duration is easily shown on Gantt charts. Sequence is not well shown on Gantt Charts. Gantt charts produced in this form are: graphical; easy to read; and easy to update. Gantt chart is the simplest and quickest method for formal planning. Gantt charts can be very useful in planning projects with a limited number of tasks and with few inter-relationships. This chart typically depicts activities as horizontal lines whose length depends on the time needed to complete the activities. These lines can be progressively overprinted to show how much of activity has been completed.
A Gantt chart is a horizontal bar or line chart which will commonly include the following features:
- Activities identified on the left hand side;
- Time scale is drawn on the top (or bottom) of the chart;
- A horizontal open oblong or a line is drawn against each activity indicating estimated duration;
- Dependencies between activities are shown;
- At a review point the oblongs are shaded to represent the actual time spent;
- A vertical cursor placed at the review point makes it possible to establish activities which are behind or ahead of schedule.
Critical Path Method (CPM)
DuPont developed a CPM designed to address the challenge of shutting down chemical plants for maintenance and then restarting the plants once the Maintenance had been completed. Complex project requires a series of activities, some of which must be performed sequentially and others that can be performed in parallel with other activities. This collection of series and parallel tasks can be modelled as a network. CPM models the activities and events of a project as a network.
Activities are shown as nodes on the network and events that signify the beginning or ending of activities are shown as arcs or lines between the nodes.
Steps in CPM Project Planning
- Specify the individual activities - All the activities in the project are listed. This list can be used as the basis for adding sequence and duration information in later steps.
- Determine the sequence of the activities - Some activities are dependent on the completion of other activities. A list of the immediate predecessors of each activity is useful for constructing the CPM network diagram.
- Draw the Network Diagram - Once the activities and their sequences have been defined, the CPM diagram can be drawn. CPM originally was developed as an activity on node network.
- Estimate activity completion time - The time required to complete each activity can be estimated using past experience. CPM does not take into account variation in the completion time.
- Identify the Critical Path - The critical path is the longest-duration path through the network. The significance of the critical path is that the activities that lie on it cannot be delayed without delaying the project. Because of its impact on the entire project, critical path analysis is an important aspect of project planning.
- Update CPM diagram - As the project progresses, the actual task completion times will be known and the network diagram can be updated to include this information. A new critical path may emerge, and structural changes may be made in the network if project requirements change.
CPM Benefits
- Provides a graphical view of the project.
- Predicts the time required to complete the project.
- Shows which activities are critical to maintaining the schedule and which are not.
CPM Limitations
While CPM is easy to understand and use, it does not consider the time variations that can have a great impact on the completion time of a complex project. CPM was developed for complex but fairly routine projects with minimum uncertainty in the project completion times. For less routine projects there is more uncertainty in the completion times, and this uncertainty limits its usefulness.
PERT
The Program Evaluation and Review Technique (PERT) is a network model that allows for randomness in activity completion times. PERT was developed in the late 1950's for the U.S. Navy's Polaris project having thousands of contractors. It has the potential to reduce both the time and cost required to complete a project. In a project, an activity is a task that must be performed and an event is a milestone marking the completion of one or more activities. Before an activity can begin, all of its predecessor activities must be completed. Project network models represent activities and milestones by arcs and nodes. PERT is typically represented as an activity on arc network, in which the activities are represented on the lines and milestones on the nodes. The milestones generally are numbered so that the ending node of an activity has a higher number than the beginning node.
Characteristics of PERT
- It forms the basis for all planning and predicting and provides management with the ability to plan.
- It enables management for best possible use of resources to achieve a given goal within time and cost limitations.
- It provides visibility and enables management to control ''one-of-a-kind" programs as opposed to repetitive situations.
Steps in the PERT Planning Process
- Identify activities and milestones - The activities are the tasks required to complete the project. The milestones are the events marking the beginning and end of one or more activities.
- Determine activity sequence - This step may be combined with the activity identification step since the activity sequence is known for some tasks. Other tasks may require more analysis to determine the exact order in which they must be performed.
- Construct the Network Diagram - Using the activity sequence information, a network diagram can be drawn showing the sequence of the serial and parallel activities.
- Estimate activity times - A distinguishing feature of PERT is its ability to deal with uncertainty in activity completion times. For each activity, the model usually includes three-time estimates:
- Optimistic time (OT)
- Most likely time (MT)
- Pessimistic time (PT)
- Determine the Critical Path - The critical path is determined by adding the times for the activities in each sequence and determining the longest path in the project. The critical path determines the total time required for the project.
- Update as project progresses - As the project unfolds, the estimated times can be replaced with actual times. In cases where there are delays, additional resources may be needed to stay on schedule and the PERT chart may be modified to reflect the new situation.
Benefits of PERT
- Expected project completion time.
- Probability of completion before a specified date.
- The critical path activities that directly impact the completion time.
- The activities that have slack time and that can lend resources to critical path activities.
- Activities start and end dates.
Limitations of PERT
- Time and labour-intensive effort are required.
- Upper-level management decision-making ability is reduced.
- There exists a lack of functional ownership in estimates.
- There exists a lack of historical data for time–cost estimates.
- The assumption of unlimited resources may be inappropriate.
- There may exist the need for too much detail.
The differences between PERT and CPM
- PERT uses three-time estimates (optimistic, most likely, and pessimistic). From these estimates, an expected time can be derived. CPM uses one time estimate that represents the normal time (that is, better estimate accuracy with CPM).
- PERT is probabilistic in nature, based on a beta distribution for each activity time and a normal distribution for expected time duration. This allows us to calculate the "risk" in completing a project. CPM is based on a single time estimate and is deterministic in nature.
- Both PERT and CPM permit the use of dummy activities in order to develop the logic.
- PERT is used for Research and Development projects where the risks in calculating time durations have a high variability. CPM is used for construction projects that are resource dependent and based on accurate time estimates.
- PERT is used on those projects, such as Research and Development, where percent complete is almost impossible to determine except at completed milestones. CPM is used for those projects, such as construction, where percent complete can be determined with reasonable accuracy and customer billing can be accomplished based on percent complete.
Network Re-planning Techniques
There are two network re-planning techniques based almost entirely upon resources: resource levelling and resource allocation.
- Resource levelling is an attempt to eliminate the manpower peaks and valleys by smoothing out the period-to-period resource requirements. The ideal situation is to do this without changing the end date. However, in reality, the end date moves out and additional costs are incurred.
- Resource allocation is an attempt to find the shortest possible critical path based upon the available or fixed resources. The problem with this approach is that the employees may not be qualified technically to perform on more than one activity in a network.
Not all PERT/CPM networks permit such easy rescheduling of resources. Project managers should make every attempt to reallocate resources so as to reduce the critical path, provided that the slack was not intentionally planned as a safety valve. It is important to note here that transferring resources from slack paths to more critical paths is only one method for reducing expected project time. Several other methods are available.
These are as follows:
- Elimination of some parts of the project
- Addition of more resources
- Parallelization of activities
- Shortening critical path activities
- Shortening early activities
- Shortening longest activities
- Shortening easiest activities
Increasing the number of work hours per day in this regard, under the ideal situation, the project starts and end dates are fixed, and performance within this time scale must be completed within the guidelines described by the statement of work. Should the scope of effort have to be reduced in order to meet other requirements, the contractor incurs a serious risk in that the project may be cancelled, or performance expectations may no longer be possible.
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