Latest Finish Time Rule If an activity is an immediate predecessor for just a

# Latest finish time rule if an activity is an

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Latest Finish Time Rule: If an activity is an immediate predecessor for just a single activity, its LF equals the LS of the activity that immediately follows it If an activity is an immediate predecessor to more than one activity, its LF is the minimum of all LS values of all activities that immediately follow it LF = Min {LS of all immediate following activities} Latest Start Time Rule: The latest start time (LS) of an activity is the difference of its latest finish time (LF) and its activity time LS = LF – Activity time Computing Slack Time After computing the ES, EF, LS, and LF times for all activities, compute the slack or free time for each activity Slack is the length of time an activity can be delayed without delaying the entire project Slack = LS – ES or Slack = LF – EF Variability in Activity Times CPM assumes we know a fixed time estimate for each activity and there is no variability in activity times PERT uses a probability distribution for activity times to allow for variability Three time estimates are required Optimistic time (a) – if everything goes according to plan Pessimistic time (b) – assuming very unfavorable conditions Most likely time (m) – most realistic estimate Estimate follows beta distribution Expected Time: t = (a + 4m + b)/6 Variance of times: v = [(b - a)/6]^2 Probability of Project Completion Project variance is computed by summing the variances of critical activities op 2 = Project variance = (Sigma)(variances of activities on critical path) PERT makes two more assumptions: Total project completion times follow a normal probability distribution Activity times are statistically independent What is the probability this project can be completed on or before the 16 week deadline? Z = /s p = (16 wks – 15 wks)/1.76 = 0.57 Where Z is the number of standard deviations the due date or target date lies from the mean or expected date

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Variability of Completion Time for Noncritical Paths Variability of times for activities on non critical paths must be considered when finding the probability of finishing in a specified time
• Spring '08
• levi
• Project Management, Critical path method, Activity Times, specific activities

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