Large-scale one-time projects have been around since ancient times; this fact is attested by the construction of the pyramids of Egypt and the aqueducts of Rome. But only recently have operational researchers been scrutinizing the managerial problems associated with such projects.
The project management problem arose with the Polaris armaments project, beginning in 1958. With so many components and sub-components together produced by various manufacturers, a new tool was needed to program and control the project. The PERT (Program Evaluation and Review Technique) was developed by scientists from the Naval Office of Special Projects. Booz, Allen and Hamilton and the Weapons Systems Division of the Lockheed Aircraft Corporation. The technique proved so useful that it has gained wide acceptance in both the government and the private sector.
Around the same time, the DuPont Company, in conjunction with Remington Rand's UNIVAC Division, developed the Critical Path Method (CPM) to control the maintenance of DuPont chemical plant projects. CPM is identical to PERT in concept and methodology.
The PERT / CPM was designed to provide several useful pieces of information for project managers. First, PERT / CPM exposes the "critical path" of a project. These are the activities that limit the duration of the project. In other words, to get the project done early, the critical path activities must be done early. On the other hand, if an activity on the critical path is delayed, the project as a whole is delayed by the same amount. Activities that are not on the critical path have a certain amount of slack; that is, they can be started later, and allow the project as a whole to stay on schedule. The PERT / CPM identifies these activities and the amount of time available for delays.
The PERT / CPM also considers the resources required to complete the activities. In many projects, limitations in manpower and equipment make scheduling difficult. The PERT / CPM identifies the moments of the project in which these restrictions will cause problems and according to the flexibility allowed by the slack times of the non-critical activities, allows the manager to manipulate certain activities to alleviate these problems.
Finally, PERT / CPM provides a tool to control and monitor the progress of the project. Each activity has its own role in it and its importance in the completion of the project is immediately apparent to the project manager. The activities of the critical path, therefore, allow to receive most of the attention, since the completion of the project, depends heavily on them. Non-critical activities will be manipulated and replaced in response to the availability of resources.
CONTENT
HISTORICAL REVIEW
The planning and scheduling of complex projects, especially large non-repetitive unit projects, began to receive special attention at the end of World War II, when the Gantt Chart was released. Until the late 1950s this was the only tool available; at this time, the Office of Special Projects of the Navy of the United States of America, as previously mentioned, in collaboration with Lockheed (manufacturers of ballistic projectiles) and Booz, Allen & Hamilton (consulting engineers), are considering a new method to solve the problem of planning, scheduling and control of the project to build atomic submarines armed with "Polaris" projectiles, where they would have to coordinate and control, over a period of five years, 250 companies,9000 subcontractors and numerous government agencies. In July 1958 the first report of the program was published, which they called Proqramme Evaluation and Review Technique(PERT - Program Evaluation and Technical Review), deciding its application in October of the same year and achieving a two-year advance over the five planned.
By 1960 the first submarines were built in the United States that
they transported and launched solid-propellant submarine-launched ballistic missiles (SLBM). These warhead missiles (Polaris missiles) can hit targets 4,000 km from a submerged submarine. In the mid-1960s, the US Navy developed a powerful inertia-guided anti-submarine missile. This missile could be fired from the torpedo cannons of any submarine. In the late 1960s, Polaris missiles were partly replaced by a new, longer-range type of SLBM: the Poseidon missile, which can carry up to ten warheads.
PERT is a PLANNING, REPLANIFICATION and EVALUATION method designed to exercise proper control over major research and development programs.
PERT is not a passing methodology and its diffusion has been enormous throughout the world. In the United States, the Public Administration only considers offers from private companies that are presented designed following this technique; the Apollo project, which allowed man to set foot on the Moon, was also programmed using PERT. This method begins by decomposing the project into a series of activities, understanding by activity the execution of a task that requires the use of one or more resources (labor, machinery, materials, time, etc.), considering as a fundamental characteristic its duration.
In parallel with the PERT research works, another system was also developed, correcting certain defects of the first, simplifying the presentation and culminating in a methodology called CPM - CRITICAL PATH METHOD - CRITICAL PATH METED. It was in 1957 that the research team of The Du Pont company, led by JE Kelley and MR Walker, created a technique, similar to PERT, which they call Critical Path Melhod (CPM,Critical Path Method), which was used for scheduling maintenance closures of chemical processing plants, with which they achieve spectacular results in the plants. This method is very similar to PERT, its fundamental difference is the nomenclature (logical if one takes into account that they are the results of independent research) and that, later, JE Kelley introduced a relationship between the cost and the duration of the activities, which the PERT did not take into account, when estimating the duration of activities for a given cost level. On the other hand, while CPMworks with deterministic durations for tasks the PERT, more focused on temporal aspects, uses probabilistic estimates for them. However, both methods are very similar and are often presented in a combined way.
The main problem of the DIRECTION nowadays consists not only in how to make a decision, or which decision to choose, but how to justify the decision and how to communicate it to the people who are linked to the project.
The PERT-CPM methodology gives us the answer; as we are faced with such a technique that is applicable to all types of management functions and programs.
When used conscientiously, the results can be highly beneficial.
It is a valid management instrument to obtain security in planning and control and is applicable at all levels of complexity, from simple and short-term problems, to the most complicated and long-range ones.
Simply speaking, PERT-CPM is a planning technique and management control instrument that uses the theory of the "Network". Once the various activities that make up the project have been defined, the “Network” is formed with them, showing the succession of activities in logical sequence and the degree of interdependence between them. The duration time associated with each activity is estimated, and the critical parts of the project are determined. The "Network" is the map, the graphic representation of the internal organization of the project. As can be seen in figure No. 1.
Nowadays it is very common to use packages for microcomputers which allows to better manage and administer the resources for the realization of a project, as is the case of Microsoft Project, which just by entering the information through the list of activities, displays the project by means of a table of bars or networks. The names of the tasks or activities, the duration, start date for the first activity and the immediate processes of each one are written as shown in figure No. 2 and 3. The program automatically builds the rest of the tasks according to the information is provided to you.
The preset duration units can be days, weeks, months, or years. These changes can be made quickly and easily without having to reclassify and reorganize the entire project.
Figure No. 2 is known as a Gantt chart. This type of diagram is widely used in practice to show the schedule of a project, since the bars show the start and finish times of activities.
Figure No. 2
You can choose between various types of views with the toolbar on the left of the screen. The default is the Gantt chart.
The PERT diagram shows the project network. At the beginning the activity boxes are aligned from left to right, but can be moved as desired. Figure No. 3 shows the project grid after placing the boxes in the same vertical way as in the figure. Note that each table provides relevant information on the activity.
Figure No. 3
DEFINITIONS AND CONCEPTS
PERT and CPM are based on substantially the same concepts, although they represent some fundamental differences. First, as originally developed, PERT methods were based on probabilistic estimates of the duration of activities, which resulted in a probabilistic route through a network of activities and a probabilistic time to completion of the project. CPM methods, on the other hand, assume constant or deterministic activity times.
The network plan summarizes, in compact form, a large amount of important information: the activities required, their precedence ratios, and the slack from the schedule. From the basic plan, we can easily deduce the fundamental data about the nearest and most remote start and finish times, the allowable slack in the activity schedule and the critical path. Figure No. 4
The essence of PERT planning is based on a representation in the form of a network of the activities that are required as indicated in figure No. 4 In said graph, the arrows represent the necessary activities represented by letters, indicating next to the arrows the estimated times for its realization. In network planning, the length of the arrows is generally meaningless. The numbered circles represent the starting and ending points of the activities and are called events or nodes. The direction of the arrows indicates the flow, in the sense that node 2 marks the end of activity A and the initiation of activities B, C and D; node 3 marks the end of activity B and the beginning of activity E. Therefore,the network also represents the precedence relationships of all activities. For example, activities B, C and D cannot be started until activity A has been completed; but activities B, C and D can go on simultaneously.
Flow through the network. If it is necessary to carry out certain activities, attention can be paid to the individual ones and the moment in which each one should start, in order to accommodate them within a general program.
The conceptualization of the activity system as a network came to constitute an important step in the analysis of large-scale production systems. The concept of flow through the network focuses on important scheduling factors, such as the interaction between the respective duration of activities, their earliest and most distant starting dates, and the sequence required in production.
The slack. Another concept that arises when viewing the set of activities as a network is slack. It is defined by the flexibility available in scheduling activities. Through the effective use of slack, management can find alternatives to leverage resources in the most effective way.
Critical activity. Knowledge of the operations that are critical, that is, those that appear on the critical path, indicates the points on which management must focus its attention in order to complete a project on time.
Critical Route There are activities that, if delayed, cause a delay in the entire project; and if they are advanced, they cause an advance in the conclusion of the project. These types of activities are called Critical Activities, which integrated make up the Critical Path (Critical Path), so they must be monitored with greater care by the professionals who manage the project.
The activities that are not part of the Critical Path are called Non-Critical Activities, and have the characteristic that they can accept a certain maximum delay without affecting the total execution time of the project or the execution time of other activities. The maximum allowable delay in an activity is called Total Slack.
Any additional delay to the total slack of the activity will affect the entire project, since once this reservation time is consumed, the activity becomes a critical activity. In these cases it is very common that there is more than one critical path in the project.
During any time during the development of a project there will always be at least one activity that is critical. It is not convenient to have too many critical activities at any one time, as the total control of the project becomes more difficult, and the probability that the project execution deadlines will be missed becomes higher.
The presence of a sufficient number of non-critical activities during the development of the projects allows to overcome temporary limitations of economic, physical and human resources, without affecting the completion date of the projects, through the priority allocation of such resources to the critical activities, and a limited allocation to non-critical activities.
When the set of activities is visualized as a network, the concept of the critical path through the network arises. This concept is fundamental to the administrative problem of allocating resources in the most effective way.
Base calendar: calendar that specifies the working and non-working hours of a project and its resources. A base calendar differs from a resource calendar in that it specifies the working and non-working times for a given resource.
Fixed cost - A cost that remains constant regardless of the duration of the task or the work performed by the resource.
Allowable delay - The amount of time a task can be postponed before it causes a delay for another task.
Outline: a hierarchical structure for a project that shows how some tasks fit into larger groups. In Project for Windows 95, subtasks are indented under summary tasks.
Resource Pool: A series of resources available to be assigned to the tasks of a project. A resource pool can be used exclusively by one project or shared by several.
Milestone: a benchmark that marks important events in a project, and is used to monitor the progress of the project. Any task with zero duration is shown as a milestone.
Slack margin - The amount of time a task can be postponed before it affects the dates of other tasks or the project finish date. Slack is also often referred to as slack.
Total Slack - This is the amount of time a task can be postponed before it delays the project's finish date.
Planning: the process of allocating resources in the most effective way possible. This requires not only defining, but also scheduling tasks taking into account three constraints: time, resources, and money.
Snooze - The amount of time a task has fallen behind its planned schedule. Snooze is the difference between the scheduled start or finish of a task and the planned start or finish schedule. Snooze can occur when a planned schedule is fixed and the effective dates entered later for tasks are later than the baseline dates, or the effective durations are longer than the baseline durations.
Priority: An indication of the availability of a task for redistribution, conflict resolution, or over assignments by delaying certain tasks. The tasks with the lowest priority are the ones that are delayed first. Tasks can also be sorted by priority.
Project: group of related tasks that are performed in a finite period of time and aimed at fulfilling a series of specific objectives.
Resources - The personnel, equipment, and supplies used to complete the tasks in a project.
Essential Resources - The resource that works on a task for the longest period of time. The essential resource determines the duration of work.
Redistribution: resolution of resource or allocation conflicts by delaying certain tasks.
Subproject: a project used within another, in which it is represented as a single task. Subprojects can be used to divide projects into more manageable units and thus reduce memory usage.
Subtask: a task that is part of a summary task. Information about the subtask is included in the summary task. You can designate subtasks by using the schema levels feature of Project for Windows 95.
Summary Task - A task that is made up of subtasks and summarizes those subtasks. You can use the Outline Leveling feature of Project for Windows 95 to create summary tasks. Project for Windows 95 automatically determines the summary task information (duration, cost, and so on) using the information from the subtasks.
Load leveling: The concept of load leveling is very important to establish a good deployment of resources. By leveling out the use of resources, it is possible to minimize the costs associated with fluctuations in activity levels.
Slack Absorption: Multiply the scheduled execution time and therefore by one of the amount of work that remains to be done. The result is the time it takes to finish the activity normally. The time available is subtracted from the previous time and the difference represents the delay, which must be absorbed by the total slack. If this is not possible, proceed as follows:
Absorption by Comprehension: The optimal time is multiplied or therefore by one of the volume of work pending to be executed. The product represents the time required to complete the activity under optimal conditions, that is, with maximum acceleration. If this time is less than the available time, it means that the project will not be delayed, but if it is greater, the difference will be the amount of time that the project will delay, except that an activity after the delayed activity can be compressed within the process.
APPLICATIONS
The field of action of this method is very wide, given its great flexibility and adaptability to any large or small project. To obtain the best results, it should be applied to projects that have the following characteristics:
- That the project is unique, not repetitive, in some parts or in its entirety.
- That all or part of the project must be executed in a minimum time, without variations, that is, in critical time. That the lowest possible operating cost is desired within an available time.
Within the scope of application, the method has been used for the planning and control of various activities, such as construction of dams, opening of roads, paving, construction of houses and buildings, repair of ships, market research, settlement movements, regional economic studies, audits, university career planning, distribution of operating rooms times, factory extensions, planning of itineraries for collections, sales plans, population censuses, etc., etc.
DIFFERENCES BETWEEN PERT AND CPM
As stated before, the main difference between PERT and CPM is the way the time estimates are made. E1 PERT assumes that the time to perform each of the activities is a random variable described by a probability distribution. CPM, on the other hand, infers that the times of the activities are known in a deterministic way and can be varied by changing the level of resources used.
The time distribution that PERT assumes for an activity is a beta distribution. The distribution for any activity is defined by three estimates:
- the most probable time estimate, m;
- the most optimistic time estimate, a; and the most pessimistic time estimate, b.
DEFINITION OF THE PROJECT
In every activity to be carried out, precise and clear knowledge of what is going to be carried out, its purpose, feasibility, available elements, financial capacity, etc. is required. This stage, although essential for the execution of the project, is not part of the method. It is a previous stage that must be developed separately and for which the Critical Path Method can also be used. It is an investigation of viable and available objectives, methods and elements.
CONSTRUCTION OF PERT - CPM DIAGRAMS
List of activities that make up a diagram.
The first thing to do to constitute a PERT diagram is to organize a list, as complete as possible, of all the activities that make up the work or project. For this, it is necessary that the person who is going to do the PERT / CPM carefully study the project and make use of the information of all the other people who are related to them, such as engineers, technicians, material manufacturers, assemblers, teachers. and any other assistants who can provide information.
The degree of subdivision that is adopted depends, among other things, on the degree of precision that is being given to the future control of the project. The more detailed the list, the lower the chances of large errors. Errors become details only. Add at the end anything that has been forgotten. Activity numbering is for reference only. It is of no importance in the construction and interpretation of a PERT / CPM diagram.
Event numbering.
After performing a PERT diagram, we must number the events. The most correct way to do it is as follows: Each event is numbered, jumping from one to another in the direction of the arrows representing the activities, taking care not to number any of them, without all the others preceding it in the diagram having been already numbered. Thus, before we must list an event, we will check how many arrows reach it. Following these arrows in reverse, we will verify if the events where they originate have already been numbered. Only after we have verified this will we give that event the number following the last one used.
In the diagram case, the vertices will be the events and the arcs the activities, and a series of conditions must be met:
- The diagram will only have one start and one end event. All activities, except those that exit the start event or reach the end event, will have at least one preceding and one following activity. As seen in the logical network chart.
- Every activity ij will reach an event of a higher order than the one from which it exits (i <j). There cannot be two activities that, having the same initial event, have the same final event, or vice versa.
The first condition requires that both the beginning of the project and the end of the project are unique; Thus, for example, if a project can start with the performance of several activities simultaneously, all of them will come out of the initial event. The second, once the first has been completed, implies that any activity represented in the diagram will be part of a path that begins at the initial event and ends at the end. In these paths there will be no returns, since, implicitly, that is the condition imposed in the third place. The fourth prevents two different activities from having the same name.
Sometimes, compliance with the aforementioned rules may prevent raising the priority relations of some activities. When this happens, they resort to the use of fictitious activities ; These do not consume time or any type of resource, their sole purpose being to solve the aforementioned dependency problems.
To begin to build the diagram, we start from the knowledge of all the activities that make up the project, as well as their priority relationships. It is very convenient to collect this information in a systematic way, as this will greatly help to build the diagram. There are basically two formats for this, the chaining matrix and the precedence table.
PROJECT SCHEDULING
Once the diagram has been drawn up, the sequence of activities is clear and you can proceed to scheduling them. For this, it is necessary to know the durations of the different activities. Generally, these cannot be fixed exactly, as many random factors are related to them. An example is the activity «write a report»: could you tell us how long it took you? We assume that the answer would be something like "it depends." The PERT addresses this problem by evaluating the duration of an activity based on three estimates:
- Optimistic duration: representing the minimum time the activity could be executed if everything went exceptionally well, with no setbacks occurring during the execution phase. It is considered that the probability of being able to finish the activity in this duration is not greater than 1%. Most probable duration, or modal estimate, which is the time that, normally, will be used in executing the activity; In the event that said task had been carried out several times, it would be the duration with the greatest frequency of appearance Pessimistic duration, which represents the maximum time in which the activity could be carried out if all the circumstances that influence its duration were totally unfavorable. The probability is considered, at most, 1 per 100.
Generally, the network plan similar to the one in Figure 4 is the one that is used and not the one based on the time scale. The reason is simply that the entire system is usually computer-based. Once the activities and their precedence relationships are determined, the standard computer programs will provide all the information necessary for the scheduling of each activity (the nearest and furthest dates of initiation and completion, as well as the allowable clearances), indicating which activities are on the critical path, Thus, as already mentioned, the length of the arrows in the network plan need not matter, since the network itself is only one element for the calculation of other important data in the network. Program. Of course,From the information provided by the computer, the network based on the time scale can be elaborated, if it is assumed that the visualization of the program by graphic means can represent some advantage.
Again it is noted that the interdependence of the series of activities, the importance of the project completion date and the unique nature of the latter require that the planning of what is going to be done and the corresponding programming be intimately linked. Planning for the use of resources, human and otherwise, is discussed later in this chapter, which is equally important.
Bearing in mind the generalities of the PERT method, a relatively simple example, that of the construction of a house, will be used to illustrate the methods used to generate the network representation of a project. The development phases can be divided as follows: activity analysis, arrow diagram, and node numbering.
- Activity analysis : The analysis of each activity is functionally comparable to the procedures that the production engineer or the programmer follow when specifying operations, work methods and tools necessary for the manufacture of parts and products. However, in large projects a certain degree of complexity is imposed due to the very high number of components and activities, to the extent that some of them can be overlooked. Thus, although planning professionals are generally available, the list of activities is often formulated partially in meetings and roundtables that include management and operating personnel. Arrow diagramsTo develop an arrow diagram it is necessary to take into account the precedence relationships between the activities that are required. This should be based on a complete, confirmed and approved list of activities. The information needed for the arrow diagram comes from the answers to the following questions: What activities must be completed before a particular one can be started? What activities can be carried out simultaneously? What activities must come immediately afterwards? of each one in particular?
Common practice is to work backwards through the list of activities, generating the ones that immediately precede each activity, as can be seen in the table for the house construction project. The estimated normal time for each activity is also indicated there. Then it is possible to elaborate the arrow diagram, to represent the logical precedence.
Care must be taken in the arrow diagram to correctly represent the actual precedence requirements. For example, see the activities immediately preceding activity s, “polishing and varnishing floors”, and activity u “finishing electrical installation”. Activity s has as its immediate predecessors ao and t, “finish carpentry” and “painting” respectively, while u is only immediately preceded by activity t. The relationship indicated by the arrow diagram in figure 6a does not correctly represent this situation for n, because it specifies that the initiation of udepends on o and t (which is not true). To properly represent this situation, one has to resort to a fictitious activity whose duration is zero. Figure 6b illustrates what has been said. Activity u, “complete electrical installation”, is made dependent here only on the completion of the painting (activity t). However, the fictional activity, both the "termination of the woodwork 'as the" painting "conjuring must be completed before you can start the activity s, "Polish and varnish floors." The dummy activity provides the logical sequence relationship; But since it has been assigned zero time, it does not alter the programming relationships that will develop later.
Another utility of the dummy activity is to provide a separate and specific node for the beginning and end of each activity, thus avoiding confusion. Figure No.5
- The diagram does not properly reflect the precedence requirements, since u appears to depend on the completion of o and t, although in reality it depends solely on t. Creating two nodes using dummy activity between them provides the appropriate predecessors for the s and u activities. Node numbering: Node numbering is effective in computer programs, in order to establish logical relationships in the network and to avoid the appearance of cycles or closed circuits. In the event that an activity is represented as going back in time, a closed circuit will be produced. This can be seen in figure 6.
Cycles can appear on the network due to an error or when, when setting activity plans, you try to show the repetition of one operation before starting the next one. The repetition of an activity must be represented by separate additional activities, defined by their own number of nodes.
A closed circuit will produce an endless cycle in the computer programs, without there being a built-in routine for stopping and identifying the cycles. Thus a network diagram that is drawn correctly must be non-cyclical.
COSTS AND PENDING
In this step, the costs of each activity carried out in standard time and in optimal time will be requested. Both costs must be provided by the persons responsible for the execution, in accordance with the budgets already supplied by them. These costs should be noted in the information matrix.
| Activities | Normal | Limit | |
| A. From the Plant Engineer | |||
| 1. Project | 600.00 | 800.00 | |
| 2. Cost | 100.00 | 100.00 | |
| 3. Approval | - | - | |
| 4. Unpacking | 200.00 | 200.00 | |
| 5. Placement | 600.00 | 800.00 | |
| 6. Installation | 1,400.00 | 2,800.00 | |
| 7. Testing | 6,100.00 | 6,300.00 | |
| 8. Start | - | - | |
| 9. Review | 2,100.00 | 2,800.00 | |
| 10. Machine Painting | 960.00 | 960.00 | |
| 11. Building Painting | 3,160.00 | 3,520.00 | |
| 15,220.00 | 18,280.00 | ||
| B. From the Electrical Engineer | |||
| 12. Project | 6,000.00 | 6,500.00 | |
| 13. Cost | 100.00 | 100.00 | |
| 14. Approval | - | - | |
| 15. Transformer | 18,600.00 | 19,000.00 | |
| 16. Lighting | 8,900.00 | 9,300.00 | |
| 17. Switches | 4,100.00 | 4,400.00 | |
| 37,700.00 | 39,300.00 | ||
| C. Of the Contractor Engineer | |||
| 18. Project | 4,000.00 | 4,600.00 | |
| 19. Cost | 100.00 | 100.00 | |
| 20. Approval | - | - | |
| 21. Foundation | 3,400.00 | 3,800.00 | |
| 22. Floors | 2,800.00 | 3,200.00 | |
| 23. Windows | 1,900.00 | 2,200.00 | |
| 12,200.00 | 13,900.00 | ||
| Total of the Three Budgets | 65,120.00 | 71,480.00 | |
| Buy New Machinery | 80,000.00 | 80,000.00 | |
| Totals …………………………. | 145,120.00 | 151,480.00 | |
In the previous table we see the budgets with the normal cost for the activities carried out in standard time and the limit cost for the activities carried out on optimal time.
The totals in the normal cost column indicate the direct costs of the project executed in standard times, however the limit cost totals do not indicate a real cost, since it will not be necessary for all activities to be carried out in optimal time, but just some of them.
GRAPHING A PERT-CPM DIAGRAM
Suppose we take our car to the service station to have the services corresponding to the 15,000 km done.
Washing - Grease - Oil Change - Rotate Rubbers - Change Oil Filter - Polish. As can be seen in figure No 7
First of all, the mechanic first decides to raise the cart with the platform, but before doing so, to be able to rotate the tires, you have to remove the spare, remove the cups and loosen the ring nuts, now if the cart is raised, step To follow would be to lubricate the engine under the cover of the car while the car is raised, for this reason the lubrication is divided into two fractions; lubricate under the car (chassis), lubricate under the engine cover and the oil change is done while the car is raised. At the end, all you need is washing and polishing.
As it was observed, the most important thing for the creation of a PERT - CPM diagram is knowing how to concatenate and make decisions in relation to the activities that determine a project.
ELASTICITY MATRIX
In order to make effective and quick decisions during the execution of the project, it is necessary to have at hand the data of the probabilities of delay or advance of work of each one of the activities, that is, their elasticity.
Let us first examine the procedure for calculating the clearances provided by the possibility of delaying an activity without consequences for other jobs.
Slack is the freedom that an activity has to extend its execution time without harming other activities or the total project. There are three classes of clearances:
- Total clearance; does not affect the completion of the project;
- Free play; does not modify the termination of the process; and Independent clearance; it does not affect the completion of earlier activities or the initiation of subsequent activities.
Total slack is important to the project manager, who is responsible for completing the project on time; free play is of interest to the head of execution of a process because of his responsibility for it; and the independent slack is information that is useful to the person who will coordinate the project work.
To calculate the clearances, the approved network is measured in the forward direction, as the first reading and then in the opposite direction as the last reading. The first reading will be indicated for each event within a circle and the last reading will also be indicated for each event within a square. It starts with the zero time indicated on the initial event and the standard duration of each activity is added, accumulating in each event.
PROJECT CONTROL WITH PERT DIAGRAMS
Once the project execution plan is known, as well as the dates of each of the activities that comprise it, it will be necessary to monitor it. To do this, the information obtained in the periodic controls must be transferred to the diagram. Starting from a diagram without dates. The steps to follow to see the expected gear after the checkpoint are as follows:
- The date of the control is set as the start date of the project. Activities that have ended will be considered with zero duration. Activities in progress will be scheduled with a duration equal to the estimated time for completion. Activities that have not started will continue with the duration initial.
If the control is carried out manually, it is more convenient to cancel the activities that have already ended, making all the activities in progress (with a duration equal to the time left to finish) and those others that may begin at the moment of control, respecting the rest of the diagram. From said diagram, it is reprogrammed in accordance with what is stated in the previous paragraph, observing whether or not the duration of the project has been modified. In the first case, an attempt would be made to take the appropriate measures to make it finish on the desired date. Thus, if the project has been delayed, activities that belong to the critical path will have to be shortened, in order to achieve an advance on the completion date; In doing so, it will be necessary to take into account that new critical paths do not appear or,if so, shorten them all together, so that the project can be completed on the desired date.
PERT and CPM ADVANTAGES
one. - Teaches a logical discipline to plan and organize a long-range detailed program.
2.- Provides a standard methodology for communicating project plans through a three-dimensional table (time, personnel, cost).
3.- Identify the most critical elements (segments) of the plan, in which potential problems could harm the fulfillment of the proposed program.
4.- It offers the possibility of simulating the effects of alternative decisions or unforeseen situations and an opportunity to study their consequences in relation to the deadlines for compliance with the programs.
5.- Provides the probability of successfully meeting the proposed deadlines.
In other words: CPM is a dynamic system, which moves with the progress of the project, reflecting at any moment the present STATUS of the action plan.
CONCLUSION
Network planning techniques are unique in their form, especially with regard to critical path concepts. The concepts of load balancing, minimum cost, and limited resource scheduling have provided a rational basis for project management that relies on carefully crafted comprehensive plans. It can be said that the plans are derived from the analysis of several outstanding alternatives. Being computer-based, they can be applied to very large systems. They are flexible, so they can be modified when experience so advises.
Interestingly, the independent application of PERT and CPM in two different environments has produced essentially similar methodologies. CPM grew out of maintenance engineering operations, where you had a lot of experience and uptime were relatively well known; so it evolved as a deterministic model. Rather, PERT arose in a research and development environment, where there is great uncertainty regarding uptime, resulting from a probabilistic model.
The PERT and CPM techniques are in such a way that they have not withstood the innumerable attempts to diversify them and keep them in opposite fields. The CPM, which originated in private companies, emphasized deterministic evaluations and the cost factor, while the PERT, at least initially, only emphasized the time factor and probabilistic techniques to estimate it. Currently, with the great dissemination of PERT / COST, the two systems are integrated in such a way that it is common to designate them with the joint acronym PERT / CPM, as a single system, whose differences are not important.
Once the network of activities, the critical path and the statistical data of the program have been established, a project plan is established. From the information, additional data can be extracted regarding the demand for resources of the initial program; It is possible to formulate alternative programs, in order to level the loads. The distribution of the time that is assumed for the activity is defined by three estimates, (probable time estimate, optimistic time, pessimistic time) taking into account that the project completion time is the sum of all the expected times of the activities on the critical path, thus it is known that the time distributions of the activities are independent and the variance of the project is the sum of the variances of the activities on the critical path.
PERT and CPM have been applied to numerous projects. Starting with their initial application to the Polaris project and to the maintenance of chemical plants, today they (and their variants) are applied to the construction of roads and buildings, and to the development and production of high-tech items such as airplanes, space vehicles, ships and computers.
The CPM was developed to handle repetitive or similar projects (eg, chemical plant maintenance). Obviously, a great deal of experience is gained over time in such circumstances, even though two projects may not be the same. This experience led to the analysis of collision techniques used in CPM networks.
While CPM and PER'I 'are essentially the same, their nuances make each more applicable than the other in different situations. In both methods the essential information desired is the critical path and the clearances. These allow the project manager to make informed decisions, based on the principle of management by exception, about the plans and projects of the current work and to monitor the progress of the project.
BIBLIOGRAPHY
- BUFFA, Elwood. Technical Direction and Administration of production. First Edition, Editorial Solano. 1982. Colombia.KELEMEN, Frank. Pert-Cpm Critical path manual technique. Fourth Edition, Editocial Iberoamericana. 1978. Mexico, DFDOMÍNGUEZ, Jose. Operations Management tactical and operational aspects in production and services. Second Edition, Prestice HallHILLIER, Frederick S. Operations Research. Seventh edition. Editoral Mac Graw Hill. Mexico, DFhttp: //www.geocities.com/jdssystems/Archivo/Diagrama.htmhttp: //www.gestiopolis.com/recursos/documentos/fulldocs/ger/pertcpm.htm Title: "PROYECTO POLARIS, PERT Y CPM"
Contributed by: María Alejandra Hinojosa - [email protected]
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