PERT. Technique for the review and evaluation of programs
INTRODUCTION
This work deals with the PERT Analysis which is one of the many tools of the Administration to have a better efficiency and effectiveness when developing the appropriate processes to develop certain strategies or events that you plan to do in the future, but let's talk of some history PERT has very interesting origins since in the past this technique was used for project programming, it was developed by the Navy of the United States of America in 1958 for planning and the Polaris project. It has been commented in the scientific literature that in this case with the presence of about 3000 contractors, the use of PERTthe completion date for the construction of a high-powered nuclear submarine advanced 2 years, but today, PERT is widely applied in Government and Industry. The Department of Defense of the United States of America, NASA and other government agencies, require companies now to work with this tool on special projects using PERT Analysis.
We hope that this project will be very useful for all those students or teachers who give the most convenient use to their needs.
ORIGINS
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.
There are two origins of the critical path method: the PERT method (Program Evaluation and Review Technique) developed by the United States Navy in 1957 to control the execution times of the various activities that make up space projects, due to the need to finish each one of them within the available time intervals. It was originally used by the Polaris project time control and is currently used throughout the space program.
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.
PERT'S STORY
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 (ballistic missile manufacturers) and La 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 an advance of two years on the five foreseen..
By 1960, the first submarines to transport and launch solid-propellant submarine-launched ballistic missiles (SLBM) were built in the United States. 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. With this method, the project 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 its duration as a fundamental characteristic.
In parallel with the PERT research work, another system was also developed, correcting certain defects of the first, simplifying the presentation and culminating in a methodology called CPM - CRITICAL PATH METED - CRITICAL PATH METHOD. It was in 1957 that the team of Research by the Du Pont company, led by JE Kelley and MR Walker, created a technique, similar to PERT, which they called Critical Path Melhod (CPM, Critical Path Method), which was used for the Programming of 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, JEKelley introduced a relationship between cost and duration of activities, which PERT did not take into account, when estimating the duration of activities for a given cost level. On the other hand, while CPM works with deterministic durations for tasks, PERT, more focused on temporal aspects, uses probabilistic estimates for them. However, both methods are very similar and are usually presented in a combined way.both methods are very similar and are usually presented in a combined form.both methods are very similar and are usually presented in a combined form.
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, non-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 cost of lowest possible operation 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.
Critical Path Programming techniques present a project graphically and relate its individual tasks in a way that allows attention to be focused on those tasks that are critical to project completion. For critical path scheduling techniques to be applied, a project must have the following characteristics:
1. It must consist of a set of well-defined jobs or tasks whose completion means the total completion of the project.
2. The works or tasks must be independent; that is, they can be started, stopped and executed separately within a certain sequence.
3. Jobs or tasks must be performed in a certain order; some must precede the others in a given sequence.
The PERT also served to program the Apollo project, with this magnificent method we must understand what an activity (1) is such as the execution of a task that requires the use of one or more resources (Labor, Machinery, materials etc.), considering its duration as a fundamental characteristic.
Good management of large-scale projects requires careful planning, scheduling, and coordination of many interrelated activities. In the early 1950s, formal procedures based on the use of networks and networking techniques were developed to assist in these tasks. Among the most outstanding procedures are PERT (program evaluation and review technique) and CPM (critical path method).Although originally PERT-type systems were applied to evaluate the programming of a research and development project, they also they are used to control the progress of other special project types. Examples include building programs, computer programming, preparing proposals and budgets,In planning maintenance and installing computer systems, this type of technique has been applied even to film production, political campaigns, and complex surgical operations.
The goal of PERT-like systems is to aid planning and control, so it doesn't involve much direct optimization. Sometimes the primary objective is to determine the probability of meeting specific due dates. It also identifies those activities that are most likely to become bottlenecks and therefore indicates at which points the greatest effort should be made to avoid delays. A third objective is to evaluate the effect of program changes. For example, the effect of a possible change in resource allocation from less critical activities to those identified with bottlenecks can be assessed. Another important application is the evaluation of the effect of deviating from schedule.
All PERT-type systems use a project network to graphically visualize the interrelationship between their elements. This representation of a project plan shows all the relationships of origin, regarding the order in which the activities should be carried out.
Another very fundamental concept is the event (2) (also known as a stage, node or event), which represents a point in time that does not consume resources and only indicates the beginning and end of an activity (or of a set of activities).
(1) DIRECTORATE OF OPERATIONS, José Antonio Domínguez Machuca, Subject of the Basic Principles of PERT, Page 332.
(2) DIRECTORATE OF OPERATIONS, José Antonio Domínguez Machuca, Subject of the Basic Principles of PERT, Page 333.
GENERALITIES
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, meanwhile, assume constant or deterministic activity time.
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 the activities, their closest and most distant starting dates, and the sequence required in production..
1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. Fixed cost: a cost that remains constant, regardless of the duration of the task or the work performed by the resource.
7. Allowable delay: is the amount of time a task can be postponed before it causes a delay for another task.
8. Outline: a hierarchical structure for a project that shows how some tasks fit into larger groups.
9. 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.
10. 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.
11. Slack - 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.
12. Total Slack: This is the amount of time a task can be postponed before it delays the project's finish date.
13. 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.
14. 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.
15. 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.
16. Project: group of related tasks that are performed in a finite period of time and aimed at fulfilling a series of specific objectives.
17. Resources: the personnel, equipment and supplies used to complete the tasks of a project.
18. Essential Resources - The resource that works on a task for the longest period of time. The essential resource determines the duration of work.
19. Redistribution: resolution of resource or allocation conflicts by delaying certain tasks.
20. 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.
21. Subtask: a task that is part of a summary task. Information about the subtask is included in the summary task.
22. Summary Task - A task that is made up of subtasks and summarizes those subtasks.
HOW TO BUILD A PERT
In the case of PERT, the vertices will be the events and the arcs the activities, and a series of conditions must be met:
- The Graph will have only one initial event and one final event. All activity, except for the one that exits the initial event or reaches the final event, will have at least one preceding and one following activity. All ij activity will have an event of a higher order than that of the what comes out (i
The first condition requires that both the beginning of the project and the end itself be 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 is fulfilled, implies that any activity represented in the graph will be part of a path that will begin in that, implicitly, that is the condition imposed in 3rd place. The fourth prevents 2 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, fictitious activities are used, these consume time or any type of resource, their sole purpose being to solve the aforementioned dependency problems.
To start building a PERT mode graph we will give the nomenclature of this graph below:
PERT NOMENCLATURE (3)
(3) Table taken from the book, MANAGEMENT OF PRODUCTION AND OPERATIONS, Everrett E. Adam, JR, p. 478.
WORKING IN PERT
How does PERT work?
PERT works by following the steps listed below:
1. All project activities must be clearly identified.
2. The sequencing requirements between activities must be determined.
3. A diagram must be constructed that reflects the sequence relationships.
4. Estimates of time must be obtained to carry out each activity.
5. The network is evaluated by calculating the critical path and other similar decision variables. Evaluation is made up of the program and plan for subsequent monitoring.
6. As time passes and experience accumulates, the program is reviewed and reevaluated.
Analysis of Each:
Step 1 is important because it forces the production / operations manager to plan. The moment in which the activities list is made is often the first opportunity in which the managers in which the managers acquire a clear conscience of the complexity of the project.
Steps 2 and 3 also require planning as precedence relationships must be established and recorded. The administrator is forced, in step 4, to estimate the time that each project activity will require, we must pay close attention to the precedence relationships that are typically used in PERT.
The time estimates are obtained either from historical data or from the experience of those who are responsible for successfully completing a particular activity. In some circumstances, the times will simply be polite hunches to management. Optimistic times (to), pessimistic times (tp) and most probable times ™ must be estimated so that the expected time (average) of each activity can be calculated with the following equation.
Example
In the following example you will not be given sequences or names, just find the critical path and the clearances corresponding to the critical path.
First we will calculate the TE, which is the accumulated expected time which will allow us to know the maximum allowable time that can elapse for the event to take place.
Then we will analyze we will determine the critical path which is simply calculated from the previous points, then it is presented as follows:
So then we will determine the different TL
Now let's calculate the clearances corresponding to the critical path are presented below:
TH = TL - TP
TH1 = 0 - 0
TH1 = 0
TH2 = 5 - 5
TH2 = 0
TH4 = 13 - 13
TH4 = 0
TH5 = 20 - 20
TH5 = 0
TH6 = 28 - 28
TH6 = 0
TH8 = 36 - 36
TH8 = 0
And if we also want to know the Clearances of different points, for example the Clearance of a point AB, we can determine it as follows:
HT = TLj - TEi - te (i, j)
Ht (2,1) = 5 - 0 - 5
Ht (2,1) = 0
Ht (4,2) = 13 - 5 - 8
Ht (4,2) = 0
Ht (5,4) = 20 - 13 - 7
Ht (5,4) = 0
Ht (6.5) = 28 - 20 - 8
Ht (6.5) = 0
Ht (8.6) = 36 - 28 - 8
Ht (8,6) = 0
And just as the exercise is concluded and we could also calculate the other clearances of the other points and they will give us results of which we can leave that time on hold, also from there we can define that we can leave some activities that we can leave a little time for Do the activities that are critical to do.
When we notice that they give us all zero we can define that these activities must be done in the longest possible time and they have to be the main priority of our programming.
CONCLUSIONS
1. Network planning techniques are unique in their form, especially when it comes 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.
2. It is interesting that 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.
3. PERT and CPM techniques are in such a similar way that they have not withstood the innumerable attempts to diversify them and keep them in opposite fields.
BIBLIOGRAPHY
- PRODUCTION AND OPERATIONS MANAGEMENT, Everett E. Adam, JR & Ronald J. Ebert, PRISMA Editorial Board, prepared by PHH Prentice Hall, this edition contains 791 Pages OPERATIONS MANAGEMENT FOUNDATIONS, Mark M. Davis & Nicholas J. Aquilano, Richard B. Chase, Third Edition, McGraw Hill, Printed in Spain, this edition contains 598 Pages DEPARTMENT OF OPERATIONS, José Antonio Domínguez Machuca & Santiago García González 1 Edition, McGraw Hill Version 1995, this issue has 503 Pages.
