The Program (or project ) evaluation and review techniques , usually abbreviated PERT , is a statistical tool used in management project, designed to analyze and represent the tasks involved in completing a given project.
First developed by the United States Navy in the 1950s, it is commonly used in conjunction with the critical path method ( CPM ).
Video Program evaluation and review technique
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Project Evaluation and Engineering Reviews are usually abbreviated as PERT. PERT is a method of analyzing the tasks involved in completing a given project, especially the time it takes to complete each task, and to identify the minimum time required to complete the total project. It combines uncertainty by allowing it to schedule projects while not knowing exactly the details and duration of all activities. This is more an event-oriented technique than initially oriented and completion, and is more widely used in projects where time is a major factor than cost. It is applied to infrastructure projects and Research and Development is very large, one time, complicated, not routine, and not routine.
The Review Review Technique (PERT) program offers management tools, which rely on "arrows and activity and event events : arrows representing activity is required to achieve an event or a node that indicates each completion phase of the entire project. "
PERT and CPM are complementary tools, because "CPM uses one time estimate and one cost estimate for each activity • PERT can use three time estimates (optimistic, expected, and pessimistic) and there is no cost for each activity.Although this is a clear distinction , the PERT term is applied increasingly to all critical path scheduling. "
Maps Program evaluation and review technique
History
"PERT" was developed primarily to simplify the planning and scheduling of large and complex projects. It was developed for the US Naval Special Project Office in 1957 to support the US Navy Polar Submarine Naval Submarine Project. It finds applications throughout the industry. The first example was that it was used for the 1968 Winter Olympics in Grenoble which PERT applied from 1965 until the opening of the 1968 Olympics. This project model was the first of its kind, a revival for scientific management, founded by Frederick Taylor (Taylorism) and later refined by Henry Ford (Fordism). The DuPont critical path method is found at almost the same time as PERT.
Initially PERT stood for Task Evaluation Research Program, but in 1959 it was renamed. It was published in 1958 in two publications of the US Department of the Navy, entitled "Research Assessment Program Evaluation, Summary Report, Phase 1. and Phase 2. In the 1959 article on < i> The American Statistician Mainly Willard Fazar, Head of Program Evaluation Branch, Office of Special Projects, US Navy, provides a detailed explanation of key concepts from PERT. He explained:
Through electronic computers, PERT techniques process data representing large, limited achievements (events) essential to achieve the ultimate goal; the interdependence of those events; and the approximate time and timeframe required to complete each activity between two consecutive events. Time expectations include estimates of "probability of time", "optimistic time", and "pessimistic time" for each activity. This technique is a management control tool that measures prospects for achieving goals on time; highlights danger signals requiring management decisions; discloses and defines both methodical and slack in the flow plan or network of sequential activities to be performed to fulfill the objectives; compare the current expectations with the scheduled completion date and calculate the likelihood of meeting the scheduled date; and simulate the effect of options for decisions - before the decision.
The PERT concept was developed by an operations research team working with representatives from Booz Operations Research Department, Allen and Hamilton; Office of Evaluation of Lockheed Missile Systems Division; and Program Evaluation Branch, Office of Special Projects, of the Department of the Navy.
Ten years after the introduction of PERT in 1958, American librarian Maribeth Brennan published a selected bibliography with about 150 publications on PERT and CPM, published between 1958 and 1968. Origin and development are summarized as follows:
PERT originated in 1958 with... Polaris missile design and construction scheduling. Since then, it has been used extensively not only by the aerospace industry but also in many situations where management wants to achieve goals or complete tasks within scheduled times and expenses; it becomes popular when the algorithm for calculating the maximum value path is contained. PERT and CPM can be calculated manually or by computer, but usually they require major computer support for detailed projects. A number of colleges and universities now offer instructional courses in both.
For the division of work units in PERT other tools developed: Work Damage Structure. The Job Damage Structure provides "a complete networking framework, Work Damage Structure officially introduced as the first analysis item in implementing GROWTH basis."
Terminology
Events and events
In the PERT diagram, the main building block is a event, with connections to its famous predecessor events and successor events.
- PERT event : a point that marks the beginning or completion of one or more activities. It does not waste time and does not use resources. When marking the completion of one or more activities, it is not "achieved" (not occurring) until all events leading to the event have been completed.
- predecessor event : an event that immediately precedes some other event without any other event intervening. An event can have some preliminary events and may be the precursor of some events.
- successor event : an event that immediately follows several other events without other intervening events. An event can have multiple successor events and can be the successor of multiple events.
In addition to events, PERT also knows activities and sub-activities:
- PERT activity : the actual performance of a time-consuming and resource-consuming task (such as labor, materials, space, machinery). This can be understood as representing the time, effort, and resources needed to move from one event to another. PERT activity can not be done until the previous event occurred.
- PERT sub-activity : PERT activities can be further described into a subset of activities. For example, activity A1 can be decomposed into A1.1, A1.2 and A1.3. Sub-activity has all the nature of the activity; in particular, sub events have predecessor events or successors such as activity. Sub-activities can be decomposed into sub-activities more subtle.
Time
PERT has set four types of time required to complete an activity:
- optimistic time : the minimum time required to complete an activity (o) or path (O), assuming everything is going better than expected
- pessimistic time : the maximum time it may take to complete an activity (p) or path (P), assuming everything goes wrong (but not including major disaster).
- most likely time : the best estimate of the time required to complete an activity (m) or path (M), assuming everything works as usual.
- expected time : the best estimate of the time required to complete an activity (te) or path (TE), explains the fact that things do not always go as usual (the implication is that the expected time is the average time the task takes if the task is repeated on a number of occasions over a long period of time).
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- te = ( o 4m p ) ÃÆ' Â · 6
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- time deviation standard : time variability to complete an activity (? te ) or path (? TE )
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- ? te = ( p - o ) ÃÆ' Â · 6
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Management tools
PERT provides a number of tools for management with concept determination, such as:
- float or slack is a measure of the excess time and resources available to complete the task. This is the amount of time that project tasks can delay without causing delays in subsequent tasks ( free float ) or the entire project ( float total ). Positive looseness will show ahead of schedule ; negative slack will show behind schedule ; and zero slack will show on schedule .
- critical path : the longest sustainable path taken from the initial event to the terminal event. This determines the total time required for the project; and, therefore, any time delays along the critical path will delay the attainment of terminal events at least by the same amount.
- critical activity : Activities that have a total buoy equal to zero. Activity with zero float does not have to be on a critical path because the track may not be the longest.
- Lead time : the time when the introduction event should be completed to allow sufficient time for the activities that must pass before a particular PERT event reaches completion.
- pause time : the earliest time for which a successor event can follow a particular PERT event.
- quick tracking : do more important activities in parallel
- crash into critical paths : Shorten the duration of important events
Implementation
The first step for project scheduling is to determine the tasks required by the project and the order in which they must be completed. The sequence may be easily recorded for some tasks, for example when building houses, soil should be assessed before the foundation can be placed) while difficult for others (there are two areas that need to be assessed, but there are just enough bulldozers to do one). In addition, the estimated time usually reflects the normal time, not in a hurry. Often, the time required to perform the task can be reduced by an additional cost or a reduction in quality.
Example
In the following example there are seven tasks, labeled A via G . Some tasks can be performed simultaneously ( A and B ) while others can not be performed until their predecessor tasks are completed ( C can not start until > A done). In addition, each task has three time estimates: an optimistic time estimate ( o ), the most probable or normal time estimate ( m ), and pessimistic time estimates ( p ). Expected time ( te ) is calculated using the formula ( o 4 m p ) ÃÆ' Â · 6.
After this step is completed, one can draw a Gantt chart or network diagram.
Next step, create a network diagram by hand or by using software diagram
Network diagrams can be made by hand or by using a software diagram. There are two types of network diagrams, activity on arrows (AOA) and activity on the node (AON). Activity on the node diagram is generally easier to create and interpret. To create an AON diagram, it is recommended (but not necessary) to start with a node named initial . This "activity" has a duration of zero (0). Then you draw any activity that has no previous activity ( a and b in this example) and connect it with the arrow from start to each node. Furthermore, since both c and d have a as predecessor activity, their vertices are drawn with arrows coming from a . The e activity is listed with b and c as the predecessor activity, so the e node is drawn with arrows coming from both < i> b and c , indicating that e can not start until both b and c have have been completed. The f activity has d as its predecessor activity, so the arrow is drawn to connect the activity. Likewise, the arrow is taken from e to g . Since no activity comes after f or g , it is recommended (but again not necessary) to connect it to the node labeled done .
By itself, the network diagram illustrated above does not provide more information than the Gantt chart; however, it can be expanded to show more information. The most common information shown is:
- Activity name
- Expected duration
- Initial start time (ES)
- Initial finish time (EF)
- Late start time (LS)
- End of end time (LF)
- Allowance
To determine this information it is assumed that the activity and normal duration time are given. The first step is to define ES and EF. ES is defined as the maximum EF of all predecessor activities, unless the activity in question is the first activity, in which ES is zero (0). EF is ES plus task duration (duration EF = ES).
- ES for start is zero because this is the first activity. Since the duration is zero, EF is also zero. This EF is used as ES for a and b .
- ES for a is zero. Duration (4 working days) added to ES to get EF of four. This EF is used as ES for c and d .
- ES for b is zero. Duration (5.33 working days) added to ES to get EF of 5.33.
- ES for c is four. Duration (5.17 business days) added to ES to get EF of 9.17.
- The ES for d is four. Duration (6.33 business days) added to ES to get EF from 10.33. This EF is used as ES for f .
- The ES for e is the largest EF of its predecessor activity ( b and c ). Since b has EF 5.33 and c has EF 9.17, ES e is 9.17. Duration (5.17 business days) added to ES to get EF 14.34. This EF is used as ES for g .
- The ES for f is 10.33. Duration (4.5 working days) added to ES to get EF of 14.83.
- The ES for g is 14.34. Duration (5.17 business days) added to ES to get EF of 19.51.
- ES for done is the largest EF of its predecessor activity ( f and g ). Since f has EF 14,83 and g has EF 19,51, ES of finish is 19,51. Complete is a milestone (and therefore has a zero duration), so EF is also 19.51.
If there are no unexpected events, the project should take 19.51 business days. The next step is to determine the late start (LS) and final completion (LF) of each activity. This will ultimately indicate whether there are activities that have leeway. LF is defined as the minimum LS of all successor activities, unless the activity is the last activity, in which LF equals EF. LS is LF minus the duty duration (LS = LF - duration).
- LF for complete is equal to EF (19.51 days) as this is the last activity in the project. Because the duration is zero, LS is also 19.51 working days. This will be used as LF for f and g .
- The LF for g is 19.51 working days. Duration (5.17 business days) is subtracted from LF to get LS 14.34 working days. This will be used as LF for e .
- The LF for f is 19.51 business days. Duration (4.5 working days) is deducted from LF to get LS of 15.01 business days. This will be used as LF for d .
- The LF for e is 14.34 business days. Duration (5.17 business days) is subtracted from LF to get LS of 9.17 business days. This will be used as LF for b and c .
- LF for days is 15.01 business days. Duration (6.33 working days) is subtracted from LF to get LS of 8.68 business days.
- The LF for c is 9.17 business days. Duration (5.17 business days) is subtracted from LF to earn LS from 4 business days.
- LF for b is 9.17 business days. Duration (5.33 working days) is subtracted from LF to obtain LS from 3.84 business days.
- The LF for a is the minimum LS of the replacement activity. Since c has LS of 4 working days and d has LS of 8.68 work days, LF for a is 4 working days. Duration (4 working days) is deducted from LF to get LS from 0 working days.
- The LF for getting started is the minimum LS of the replacement activity. Because the a has latitude of 0 weekdays and b has LS of 3.84 work days, the LS is 0 weekdays.
The next step, the determination of the critical path and the possibility of slack
The next step is to determine the critical path and if there is loose activity. The critical path is the path that requires the longest to complete. To specify a track time, add the task duration to all available paths. Activities that have leeway can be delayed without changing the overall time of the project. Slack is calculated in one of two ways, slack = LF - EF or slack = LS - ES. The activity on the critical path has a zero vacancy (0).
- The path length of adf is 14.83 business days.
- The path length of aceg is 19.51 business days.
- The path length is please is 15.67 business days.
The critical path is aceg and the critical time is 19.51 working days. It is important to note that there may be more than one critical path (in a project more complex than this example) or that the critical path may change. For example, let's say that activities d and f take pessimistic time (b) to complete instead of their expected time (T E ). The critical path is now adf and the critical time is 22 working days. On the other hand, if the c activity can be reduced to one business day, the road time for aceg is reduced to 15.34 business days, which is slightly less than the time of the new critical path, < i> please (15.67 business days).
Assuming this scenario does not happen, slack for each activity can now be determined.
- Start and complete is a milestone and by definition has no duration, therefore they can not slack off (0 working days).
- Activities on the critical path by definition have zero clearances; However, it is always a good idea to examine math when drawing by hand.
- LF a - EF a = 4 - 4 = 0
- LF c - EF c = 9.17 - 9.17 = 0
- LF e - EF e = 14,34 - 14,34 = 0
- LF g - EF g = 19.51 - 19,51 = 0
- Activities b have LF 9.17 and EF 5.33, so leeway is 3.84 business days.
- The activity d has LF 15.01 and EF 10.33, so leeway is 4.68 business days.
- Activity f has LF 19.51 and EF 14,83, so slack is 4.68 business days.
Therefore, the b activity can be delayed by almost 4 business days without delaying the project. Likewise, activity d or activity f may be delayed 4.68 business days without delaying the project (alternatively, d and f can be delayed by 2.34 working days respectively).
As the project scheduling tool
Benefits
- The PERT chart explicitly defines and creates visible dependencies (precedence relation) between work-breaking elements (usually WBS).
- PERT facilitates the identification of critical paths and makes this visible.
- PERT facilitates initial identification, start end, and looseness for each activity.
- PERT provides a possible reduction in project duration due to a better understanding of dependencies that lead to increased overlapping activity and tasks where possible.
- The large amount of project data can be organized and presented in the diagram for use in decision making.
- PERT can provide a probability of completion before a certain time.
Losses
- There is the potential for hundreds or thousands of individual activity and dependence relationships.
- PERT is not easily scalable for smaller projects.
- Network graphics tend to be large and heavy that require multiple pages to print and require custom-size paper.
- The lack of time period on most PERT/CPM charts makes it difficult to show status even if colors can help (for example, certain colors for completed nodes).
Uncertainty in project scheduling
During project implementation, however, real life projects will never perform exactly as planned due to uncertainty. This may be due to ambiguity resulting from subjective estimates that are susceptible to human error or may result from variability arising from unexpected events or risks. The main reason that PERT can provide inaccurate information about project completion time is due to the uncertainty of this schedule. This inaccuracy may be large enough to make such estimates unhelpful.
One possible method to maximize the durability of a solution is to include safety in the basic schedule to absorb anticipated disorders. This is called proactive scheduling . Pure proactive scheduling is utopia; insert safety in a basic schedule that allows for any disruption that may lead to a basic schedule with a very large wedge range. The second approach, called reactive scheduling , consists of defining procedures for reacting to interruptions that can not be absorbed by the baseline schedule.
See also
References
Further reading
External links
- Media related to PERT stairs on Wikimedia Commons
Source of the article : Wikipedia
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