BASIC TIME ANALYSIS (PERT ANALYSIS)
In many situations, activity times can be estimated accurately. For example, in projects such as construction or maintenance, manager may have sufficient experience or historical data to provide activity time estimates that are fairly accurate. In addition, the nature of these activities may have low variability and thus time would be relatively constant. In other cases, activity times are uncertain and perhaps best described by a range of possible values. In these instances the uncertain activity times are treated as random variables with associated probability distributions.
The most common method of dealing with uncertain activity times is to obtain three time estimates for each activity. The three estimates are :
(1) Optimistic Time (a or to) :
This is the shortest time an activity can take to complete. For this estimate, no provision for daily as or setbacks are made. It represents an ideal estimate.
(2) Most Likely Time (m or tm) :
This refers to the time that would be expected to occur most often if the activity were frequently repeated under exactly the same conditions. It assumes that things go in a normal way with few setbacks. It is the modal time.
(3) Pessimistic Time (b or tp) :
This is the longest time the activity could take to finish. If everything went wrong and abnormal situations prevailed, this would be the time estimate.
Using the values of a, b and m, the expected time of various activities and their standard deviations are calculated as follows. The three time estimates are reduced into a single expected time (Tei) with the weighted average formula :
tei = a+4m+b/6
Once the expected time of the activities are obtained, the critical path of the project network is determined using these time estimates. Having found the critical path, the PERT methodology assumes that the sum of the mean times and the summation of the variances of critical jobs would yield the expected project duration and its variance.
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