【正文】 en.A network does not only graphically display the relationships among various activities, it provides also information on the critical path. The critical path indicates which activities are the most critical to be performed in time to avoid delaying the whole project. Higgins, James: The Management Challenge Planning by network is based on the Theory of Graphs. Heinrich, Lutz: Systemplanung MPM uses nodes to represent activities and arrows for their relationships as this is easier to follow. One must know that SAP supports several types of relationships and not only the standard relationship. The following relationships are supported:Finish StartThe preceding activity must finish before the following activity may start.Start StartOne activity cannot start until the other one has started.Finish FinishOne activity cannot be finished until the other one is finished.Start FinishOne activity cannot be finished until the other one has startedFor some activities, the earliest possible start and the latest possible start differs. The difference between the earliest possible start and the latest possible start is called slack time. A slack time greater than zero means that the duration of the activity can be longer than planned without delaying the pletion date of the whole project. The sequences of activities having a slack time of zero are called the critical path. In this context critical means that, if the duration of one of these activities is longer than expected, the pletion date of the whole project is delayed by at least that period of time. One must also realize that there can be several critical paths. Moreover, the critical paths can change during the project as a result of unexpected delays of certain activities. The change of the critical path due to excessive consumption of the slack time is referred to as Parkinson Phenomena. Schierenbeck, Henner. Einf252。hrung in die Allgemeine Betriebswirtschaftslehre, The slack time for an activity cannot be used without affecting the slack time for other activities. As a rule of thumb, the total slack time can be used only once within a noncritical path without changing the particular path into a critical one. Thus, the MPM (Metra Potential Method) method splits the slack time into four different parts. Schierenbeck, Henner. Einf252。hrung in die Allgemeine Betriebswirtschaftslehre, p. 159ff The total slack time is calculated as the difference between the earliest possible start and earliest possible end. This is equal to the difference between the latest possible start and the latest possible end. The free slack time at the end is calculated assuming that all predecessors have begun at the earliest possible date. Thus, the free slack time at the end calculates, provided the assumption holds, the time the activity can be delayed in order to enable the successor of the activity in question to start at the earliest possible date, too. The free slack time in the beginning is the counterpart of the free slack time at the end. It is calculated as the time at the beginning of a noncritical path if all successors start at the latest possible time and the predecessor has started at the latest possible time, too. The independent slack time is calculated by assuming that all successors are going to start at the earliest possible time and all predecessors have ended at the latest possible time. Thus, this type of slack time can be used to the full extent without resulting neither in a reduction of slack times of other activities nor in a delay of the whole project.Note that the free slack time at the end and the free slack time at the beginning affect each other. If one part is consumed, the counterpart is not available to its full extent.In project planning, planning is a continuous dynamic process. Thus, replanning is unavoidable. Moreover, for different situations different sequences of actions are optimal. Hence, planning with revisions is referred to as rolling or revolving planning. Laux, Helmut. Entscheidungstheorie, Grundlagen p..273By taking into account the dependencies among activities and their estimated duration, activities that are most likely to bee bottlenecks can be identified. Last, but not least, planning by network is useful to evaluate possible effects of deviations from schedule. Originally, only the effect on time could be evaluated. However, in an integrated system like SAP R/3 effects on other areas such as, ., production capacities can be simulated in real time. Chase R./Aquilano N./Jacobs R. Production and Operations Management: Manufacturing and Services , Hillier F./Liebermann G. Introduction to Operations Research, Sixth Edition After having set up a network, make sure that you perform a final check: Which important activity may have been overlooked? Are the sequential relationships among activities correct?Note that time estimates are just that estimates – and, therefore, subject to the problems associated with estimation. Higgins, James: The Management Challenge Basic calculationsSummary of calculations Schierenbeck, Henner. Einf252。hrung in die Allgemeine Betriebswirtschaftslehre. p. 163CalculationEarliest possible startMax (earliest possible end) of all predecessorsEarliest possible endEarliest possible start + duration (activity)Latest possible endMin (earliest possible start) of all successorsLatest possible startLatest possible end duration (activity)Total slack time for activityLatest possible end Earliest possible start DurationFree slack time at the endMin (Earliest possible start(Successor)) of all successorsFree slack time in the beginningLatest possible start Max (Latest possible end (Predecessor)) of all predecessorsIndependent slack timeMin (Earliest possible start (Successor)) Max (Latest possible end (Predecessor) duration (activ