Project Management (ENG) — Ünite 3 Soru-Cevap

Purpose of project scheduling;

• How can the project be visually presented to better represent the activity flow?
• How long does it take to finish the project if there are no delays?
• When is it necessary to begin and complete the individual activities (at the latest) in order to satisfy this project completion time?
• When can individual activities begin and end (as soon as possible) if there are no delays?
• Which critical bottleneck operations should be prevented in order to avoid delays in the completion of the project?
• How much delay can be allowed for other activities without delaying the completion of the project?
• What is the likelihood of completing the project by the deadline provided the uncertainties in the accurate estimation of the duration of the activity?

A project’s network of activities provides the basis for scheduling the project. A network is a visual representation of the project’s content and goals. 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.

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. Activity-on-arrow (AOA) and activity-on-node (AON) demonstrations are the two common models for network drawing. Arrows are used to depict activities in the AOA strategy whereas nodes are the beginning and finishing spots of activities. On the other hand, nodes depict activities in the AON strategy whilst arrows also constitute relationships of precedence. Projections of the time, price, and resource requirements are created during the network planning stage for each activity. The projections can be based on actual records, time norms, forecasts, features of regression, or other quantitative approaches.

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. In this stage, it is also determined how much slack or float is associated with each activity. To decide the critical activities, the activity track with the minimum slack in the network is used, and the durations of these critical activities also define the duration of the entire project. Other processes conducted throughout network scheduling are resource allocation and time-cost trade-offs.

Network control includes monitoring a project’s progress on a network schedule basis and taking corrective action when necessary. An assessment of overall performance versus expected results will determine the project progress deficiencies.

 The components of the network are described below:

• Node: Activities are represented as circular nodes in the network.
• Arrow: An arrow is a line that connects two nodes to one end with an arrowhead. The arrow means the activity at the arrow’s tail precedes the one at the arrow’s head.
• Activity: An activity is a time-consuming action necessary to accomplish a part of the current project. A node in the AON system or an arrow in the AOA system represents an activity. The job represented by the activity can be demonstrated inside the node or on the arrow by a short phrase or symbol.
• Restriction: A restriction is an arrangement of precedence that determines the pattern of activities. The first is said to be a predecessor of the second if one activity has to be finished before another activity can start.
• Dummy: A dummy is used to show one significant event (e.g. milestone). A dashed circle denotes it and treats it as a zero-time activity. The AON method does not require a dummy. It may, however, be included on the project’s progress for simplicity, network clarity, or to serve as a milestone.
• Predecessor activity: An activity of the predecessor is the one that instantly follows it.
• Successor activity: A successor activity follows one activity immediately.
• Descendent activity: Any activity limited by the one under consideration is a descendent activity.
• Antecedent activity: An antecedent activity is any activity that must precede one.
• Merge point: Once two or more tasks are predecessors of a single task, a merging point occurs. Before the merge activity can begin, all activities preceding the merge point should be finished.
• Burst point: A burst point occurs when there is a prevalent predecessor for two or more operations. Until the burst point activity is finished, no activity erupting from the same predecessor activity can begin.
• Precedence diagram: A precedence diagram is a visual representation of a project’s activities and the preceding requirements for completing the project. Time is usually presented to be from left to right, but there is no effort to make the sizes of nodes or arrows proportional to the time.

To describe a project, three kinds of data are require;

• 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.

All this data requires to be conveyed by the project network. A network is a visual representation of the activities of the project and its relations. A network of projects is a collection of arrows and nodes. It is important to ensure that the project has a unique starting and ending point before drawing the network.

when preparing a  Network diagrams draw  each activity, except for the starting and the ending one, must have both preceding and succeeding activities. There are two ways in which a network diagram is generally drawn for a project:

• Activity on Arrow (AOA) representation
• Activity on Node (AON) representation

As both methods have the same scheduling results, which one to use does not make any difference. Most software, however, only generates one of them, and it is generally AON. Some programs, like Primavera, enable you to choose your preferred method.

The arrows depict activities in this technique, while the nodes denote the start and end of an activity . The arrow distance that connects the nodes is meaningless and arrows may be direct, bent, or curved. If an activity relies on another, the two arrows with a shared node occur on the network diagram.

The basic patterns of AOA diagrams

This technique is also called the method of the preceding diagram. The nodes depict activities in this technique and the arrows represent logical interactions between the activities. If the arrow begins at the end of an activity (activity A) and finishes at the beginning of another activity (activity B), then A is a predecessor to B.

Basic Patterns of AON Diagrams

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
• Use dummy activities when more than one arrow exits the same node and enters at another node. The dummy activity is a zero-duration activity that uses up no resources, plotted as dashed lines, and is used to modify the diagram of the network. When one activity relies on two previous activities and another activity depends only on one of the two previous activities, a dummy activity must be used.  

Use of  dummy activities.

It is possible to use both networks to represent a project network. However, there are some differences between them:

• 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. It indicates quite likely that they are becoming the traditional form of network diagram.

The soul of network-based scheduling methods is a visual representation of the plan for accomplishing the program; such a chart shows the precedence interactions i.e. the dependencies of the activities leading to the underlying goal. This chart is called as the network. This concept has evolved from the bar charts through the years. The paper on forecasting and planning with probability-based tolerances can be accepted as the pioneer paper for development of the PERT.

The application area  of PERT are;

• New plant construction
• New product research and development (R&D)
• Space projects
• Aircraft production
• Development of new missile systems
• Nuclear reactor maintenance
• Management information systems applications,

It is useful to recognize that there are basically two types of project activities: Activities that may have been carried out before, containing a considerable number of elements of chance which are called variable activities, and deterministic type of activity, i.e. one with appropriately known average value and minimal variance in time. Variable activities have comparatively large variance. Development of a new system or research and development projects can be counted as a good example to variable activities. On the other hand, maintenance activities, which have a minimal variance, can be a good example to deterministic activities.

Calculations of mean and variance in Beta distribution are utilized to calculate the mean time and variance in PERT. The formula for the mean is a simple weighted average of the three estimates, assuming that the optimistic and pessimistic estimates are equally possible and that the modal is four times more likely than these two estimates. The estimation equation for mean is depended on the acceptance that most of the observations will fall into three standard deviations minus or plus or in other words, distributed in six standard deviations. This also means that standard deviation is one-sixth of the range between optimistic and pessimistic time. Note that, for these approximation approaches there is no mathematical verification.

The formulas for calculating the mean and standard deviation in PERT;

In these formulas:

“a” is the optimistic performance time. It is the time that would only be exceeded once in twenty trial if the activity could be continuously carried out under the same circumstances.

“m” is the most likely time. It is the distribution’s modal value, or the duration that is more likely to arise than any other duration.

“b” is the pessimistic performance time. It is the time that would only be succeeded once in twenty trial if the activity could be continuously carried out under the same circumstances.

“t_e” is the mean duration of the activity and can be calculated by using the above given formula.

“s²” is the variance of the duration of the activity. Standard deviation is the square root of the

variance and can be symbolized as “s”.

The major steps in PERT analysis can be summarized as follows:

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.
5. Similar to expected duration, calculate the variance of the project as the sum of individual variance of activities on the critical path.
6. In addition to expected project duration, the probability of finishing the project within specified time limits can be calculated, which does not exist in CPM.

Most important inputs of the PERT are the three estimates: optimistic, pessimistic and the modal duration of each activity in the project network diagram( shown as a, b, and m, respectively). It is possible to use several approaches to determine the duration estimates necessary for PERT. The following are some of the strategies.

• 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;

• 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;

• 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
• 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,
• It is not appropriate to use PERT for fairly simple and repetitive projects such as fixed-sequence works.