PERT

Chart

Terminology

• PERT action: it is the moment which designates by itself(himself) the beginning or end of one or several tasks. The action does not use either time, or resources. It designates end of one or several tasks, but it is not considered "executed" until all actions conducting to this event are not complete.
• Predecessor of an event: an event (or events) which directly precede other actions without any intermediate tasks. Such action can be a consequence of accomplishment more than one task.
• Successor of an event: an event (or events) which follow at once other action without any intermediate actions. It can be a consequence of accomplishment more than one task.
• Action of PERT: this actual task performance. It absorbs time, and for its accomplishment resources (such as work, materials, the premises, the equipment) are required, and it is possible to understand it as costs of time, the efforts and resources necessary for transition from one action to another. Action of PERT cannot be considered as complete until all events preceding it are not complete.
• Optimistic calculation of time (O): minimum possible time necessary for accomplishment of a task, that everything goes better, than was expected.
• Pessimistic calculation of time (O): the greatest possible time necessary for accomplishment of a task, considering that everything will go not so (except for the largest accidents).
• The most probable time (N): a sober time estimate, necessary for accomplishment of a task, proceeding from the fact that everything will pass as usual.
• The expected time (O V): a probabilistic time estimate, necessary for accomplishment of a task provided that everything will go the turn (in the sense that the average expected time is calculated, on the basis of many times the performed task during the long time frame). This time on the following formula is calculated: About In = (O + 4H + O) ÷ 6
• Floating or reserve time (P R) is the amount of time necessary for accomplishment of a task within the project which will not cause a delay of accomplishment of the subsequent tasks  — (loose stock) or end of all project  — (the general stock).
• Critical path: the longest possible continuous way accepted from the beginning of an event before its full completion. This way defines amount of the calendar time necessary for project implementation, and, therefore, any delays on a critical path will delay achievement of a final event, at least, on the same amount of time.
• Critical activity: activity on which accomplishment it is not released reserve time. If this action has no reserve time, it does not mean yet that it is on a critical path.
• Input time: time during which the previous action should be fully completed to provide normal accomplishment of the next event of PERT.
• Delay time: the earliest term to which the next event of PERT should be executed.
• Float time: admissible float time and resources necessary for task performance. The positive reserve (+) indicates advancing of the diagram; the negative reserve (-) indicates lag from the diagram, and the zero reserve indicates normal schedule performance.
• Fast tracking: side-by-side execution of the most critical actions.
• Violation of a critical path: reduction of duration of the major actions.

Accomplishment of PERT

The first step in project planning  is determination of tasks, necessary for accomplishment of an action, and an order in which they should be executed. For some tasks the order can be easily defined (for example, at construction of the house, the earth should be measured to a base tab), for others to make it quite difficult (for example, there are two sections which should be prepared, and the bulldozer for work only one). Besides, settlement the estimate reflects normal not critical time. Very often, time necessary for task performance can be reduced at additional expenses or quality degradation.

The chart PERT represents the tool which facilitates decision making. The first draft of the PERT chart represents events with a step of 10 units (10, 20, 30, etc.) that further there was an opportunity to include additional actions. Two consecutive events in the PERT chart are connected among themselves by actions which are usually presented in the form of arrows on the scheme. All actions of the chart are provided in the logical sequence and any event, cannot begin until the action which is directly preceding it is complete. The scheduler decides what stages should be to the chart of actions and also sets their sequence. The PERT chart can consist of several pages with a large number of subtasks. The value of PERT consists that using this method it is possible to solve many problems at the same time, at the same time significantly reducing time and resources.

Management of PERT

• Planning and creation of model (chart) in parts. Each contractor makes the list of necessary actions. Then all actions communicate and built in the logical sequence according to the principle "the action can begin only when …."
• Time estimate and analysis of ways of accomplishment. Runtime of each action is defined. Appropriated to each action several settlement times of type (H)), pessimistic (O), optimistic (O) earlier. Over time refused this practice as it only tightened time of discussion and complicated calculations in general. The analysis of a way is the most responsible stage. Here calculation of runtime of each action and the project in general are analyzed and critical points of accomplishment are defined. Importance represents not calculation as such, but start time of project implementation and an opportunity to optimize it. The repeated analysis of each action and a possibility of their accomplishment on the basis of new technologies is for this purpose carried out.
• Operational planning. Based on the results received during the analysis determination of necessary resources begins. During this phase of optimization priorities on a basis the price - accomplishment quality-speed are defined.
• Control of actions at the runtime works. After the beginning of the project it is necessary to control the course of accomplishment of actions in respect of the sequence, time and the used resources. Any changes in a work progress (advancing of the diagram, unforeseen delays  , etc.) also should be under control of PERT.

Components

The main components of the chart PERT are actions, communications, plans and the diagram of accomplishment. Actions  is a set of works which need to be executed for project completion. The main attribute of an action, except the description, is estimated time. Some actions representing the key moments (milestones) in project implementation received the name Milestones (interim statement).

Communications represent the relations between actions. The most frequent communication is Nachalo-Zaversheniye, it means that the following event cannot begin if previous is not complete. Other types of communications are the Beginning Beginning, End End, Zaversheniye-Nachalo. Except the Beginning and End the task performance percent can act as communication. Communications can also have the duration if between accomplishment of two actions there should pass certain time, but between them it is impossible to insert other action. Data of type the fixed date, not earlier than, not after  , etc. can be attributed to many actions accurately to define structure of accomplishment.

It is necessary to enter all working and non-working days, holidays, possible dates to the diagram of accomplishment overtime and the non-working periods. As a result the diagram and actions should be strictly connected.