STOCHASTIC SCHEDULING

Transcription

1 STOCHSTIC SCHEDULING Scheduling with Uncertain Durations Some scheduling procedures explicitly consider the uncertainty in activity duration estimates by using the probabilistic distribution of activity durations. That is, the duration of a particular activity is assumed to be a random variable that is distributed in a particular fashion. For example, an activity duration might be assumed to be distributed as a normal or a beta distributed random variable as illustrated in Figure 1. This figure shows the probability or chance of experiencing a particular activity duration based on a probabilistic distribution. The beta distribution is often used to characterize activity durations, since it can have an absolute minimum and an absolute maximum of possible duration times. The normal distribution is a good approximation to the beta distribution in the center of the distribution and is easy to work with, so it is often used as an approximation. Figure 1: Beta and Normally Distributed ctivity Durations If a standard random variable is used to characterize the distribution of activity durations, then only a few parameters are required to calculate the probability of any particular duration. Still, the estimation problem is increased considerably since more than one parameter is required to characterize most of the probabilistic distribution used to represent activity durations. For the beta distribution, three or four parameters are required depending on its generality, whereas the normal distribution requires two parameters. The most common formal approach to incorporate uncertainty in the scheduling process is to apply the critical path scheduling process and then analyze the results from a probabilistic perspective. 1

2 This process is usually referred to as the Program Evaluation and Review Technique (PERT) method. s noted earlier, the duration of the critical path represents the minimum time required to complete the project. Using expected activity durations and critical path scheduling, a critical path of activities can be identified. This critical path is then used to analyze the duration of the project incorporating the uncertainty of the activity durations along the critical path. The expected project duration is equal to the sum of the expected durations of the activities along the critical path. ssuming that activity durations are independent random variables, the variance or variation in the duration of this critical path is calculated as the sum of the variances along the critical path. With the mean and variance of the identified critical path known, the distribution of activity durations can also be computed. Program Evaluation and Review Technique Both CPM and PERT were introduced at approximately the same time and, despite their separate origins, they were very similar. The PERT method shares many similarities with CPM. Both require that a project be broken down into activities that could be presented in the form of a network diagram showing their sequential relationships to one another. Both require time estimates for each activity, which are used in routine calculations to determine project duration and scheduling data for each activity. CPM requires a reasonably accurate knowledge of time and cost for each activity. In many situations, however, the duration of an activity can not be accurately forecasted, and a degree of uncertainty exists. Contrary to CPM, PERT introduces uncertainty into the estimates for activity and project durations. It is well suited for those situations where there is either insufficient background information to specify accurately time and cost or where project activities require research and development. In the original development of PERT approach, O notations are used. However, ON diagramming can be easily used alternatively. The method is based on the well-known central limit theorem. The theorem states that: Where a series of sequential independent activities lie on the critical path of a network, the sum of the individual activity durations will be distributed in approximately normal fashion, regardless of the distribution of the individual activities themselves. The mean of the distribution of the sum of the activity durations will be the sum of the means of the individual activities and its variance will be the sum of the activities variances. The primary assumptions of PERT can be summarized as follows: 1. ny PERT path must have enough activities to make central limit theorem valid.. The mean of the distribution of the path with the greatest duration, from the initial node to a given node, is given by the maximum mean of the duration distribution of the paths entering the node.. PERT critical path is longer enough than any other path in the network. PERT, unlike CPM, uses three time estimates for each activity. These estimates of the activity duration enable the expected mean time, as well as the standard deviation and variance, to be derived mathematically. These duration estimates are: - Optimistic duration (a); an estimate of the minimum time required for an activity if exceptionally good luck is experienced.

3 - Most likely or modal time (m); the time required if the activity is repeated a number of times under essentially the same conditions. - Pessimistic duration (b); an estimate of the maximum time required if unusually bad luck is experienced. These three time estimates become the framework on which the probability distribution curve for the activity is erected. Many authors argue that beta distribution is mostly fit construction activities. The use of these optimistic, most likely, and pessimistic estimates stems from the fact that these are thought to be easier for managers to estimate subjectively. The formulas for calculating the mean and variance are derived by assuming that the activity durations follow a probabilistic beta distribution under a restrictive condition. The probability density function of a beta distributions for a random variable x is given by: α β f(x) = k(x - a) (x -b), a x b, α, β > 1 (1) where k is a constant which can be expressed in terms of α and β. Several beta distributions for different sets of values of α and β are shown in Figure. Figure : Illustration of Several Beta Distributions Using beta distribution, simple approximations are made for the activities mean time and its standard deviation. Using the three times estimates, the expected mean time (t e ) is derived using Eq.. Then, t e is used as the best available time approximation for the activity in question. The standard deviation is given by Eq., and hence the variance (ν) can be determined as ν = σ.

4 a + m + b t e = () b a σ = () By adopting activity expected mean time, the critical path calculations proceed as CPM. ssociated with each duration in PERT, however, is its standard deviation or its variance. The project duration is determined by summing up the activity expected mean time along the critical path and thus will be an expected mean duration. Since the activities on the critical path are independent of each other, central limit theory gives the variance of the project duration as the sum of the individual variances of these critical path activities. Once the expected mean time for project duration (T X ) and its standard deviation (σ X ) are determined, it is possible to calculate the chance of meeting a specific project duration (T S ). Then normal probability tables are used to determine such chance using Equation. T T S T Z = X () σ S TX + Z * X = σ is an equivalent form of Equation.1, which enables the scheduled time for an X event to be determined based on a given risk level. The procedure for hand probability computations using PERT can be summarized in the following steps: 1. Make the usual forward and backward pass computations based on a single estimate (mean) for each activity.. Obtain estimates for a, m, and b for only critical activities. If necessary, adjust the length of the critical path as dictated by the new t e values based on a, m, and b.. Compute the variance for event x (ν X ) by summing the variances for the critical activities leading to event x.. Compute Z using Equation and find the corresponding normal probability. Consider the nine activity example project shown in Table 1. Suppose that the project have very uncertain activity time durations. s a result, project scheduling considering this uncertainty is desired. Table 1: Precedence Relations and Durations for a 9-ctivity Project Example ctivity Description Predecessors Duration B C D E F G H I Site clearing Removal of trees General excavation Grading general area Excavation for trenches Placing formwork and RFT for concrete Installing sewer lines Installing other utilities Pouring concrete B, C B, C D, E D, E F, G 9 1

5 Table shows the estimated optimistic, most likely and pessimistic durations for the nine activities. From these estimates, the mean, variance and standard deviation are calculated. In Figure, PERT calculations are performed very similar to that of CPM, considering the mean duration of each activity. Table : ctivity Duration Estimates for the 9-ctivity Project ctivity Duration Standard Mean a m b Deviation B C D E F G H I D H C E G..0 I B. 1.0 F Figure : PERT Calculations for 9-ctivity Example The critical path for this project ignoring uncertainty in activity durations consists of activities, C, F and I. pplying the PERT analysis procedure suggests that the duration of the project would be approximately normally distributed. The sum of the means for the critical activities is = 0.0 days, and the sum of the variances is (0.) + (0.) + (0.) + (0.) = 1. leading to a standard deviation of 1. days. With a normally distributed project duration, the probability of meeting a project deadline can be computed using Equation (). For example, the probability of project completion within days is: 0 Z = =.1 1.

6 where z is the standard normal distribution tabulated value of the cumulative standard distribution, which can be determined form standard tables of normal distribution. From Table, the probability of completing the project in days is 100%. -σ -σ -σ 0 -σ -σ -σ Figure : Normal Distribution Curve Table : rea under the Normal Curve Measured from the Center SD rea % from the center SD rea % from the center 0.1σ Example: Suppose that a network has been developed for a particular project with non-deterministic durations for the activities and the completion time for that network is 0 days and the sum of the standard deviation for the activities on the critical path is 10. Find the probability that the project will be completed in 00 days. Solution:

7 First, convert the normal random variable to the standard normal random variable. T S T Z = X = (00 0) /. = - 0. σ X From Table, the corresponding probability = 1.% Then, the probability to complete the project in 00 days = 0 1. =.%. Example: Given the information from the previous example, what is the duration that you can give with 90 percent assurance? Solution: From tables find the value of z corresponding to probability of 0%, thud yields z = 1. the, apply z into equation : 1. = (t 0) /. or t =. x = 0 days..10 Criticisms to Program Evaluation and Review Technique While the PERT method has been made widely available, it suffers from three major problems. First, the procedure focuses upon a single critical path, when many paths might become critical due to random fluctuations. For example, suppose that the critical path with longest expected time happened to be completed early. Unfortunately, this does not necessarily mean that the project is completed early since another path or sequence of activities might take longer. Similarly, a longer than expected duration for an activity not on the critical path might result in that activity suddenly becoming critical. s a result of the focus on only a single path, the PERT method typically underestimates the actual project duration. s a second problem with the PERT procedure, it is incorrect to assume that most construction activity durations are independent random variables. In practice, durations are correlated with one another. For example, if problems are encountered in the delivery of concrete for a project, this problem is likely to influence the expected duration of numerous activities involving concrete pours on a project. Positive correlations of this type between activity durations imply that the PERT method underestimates the variance of the critical path and thereby produces over-optimistic expectations of the probability of meeting a particular project completion deadline. Finally, the PERT method requires three duration estimates for each activity rather than the single estimate developed for critical path scheduling. Thus, the difficulty and labor of estimating activity characteristics is multiplied threefold.

8 MONTE CRLO SIMULTION Monte Carlo Simulation Characteristics: - Replaces analytic solution with raw computing power. - voids need to simplify to get analytic solution - No need to assume functional form of activity/project distributions. - Used by Van Slyke (19). - llows determining the criticality index of an activity (Proportion of runs in which the activity was in the critical path). - Hundreds to thousands of simulations needed. Monte Carlo Simulation Process: - Set the duration distribution for each activity. - No functional form of distribution assumed. - Could be joint distribution for multiple activities. - Iterate: for each trial ( realization ) - Sample random duration from each distributions - Find critical path & durations with standard CPM; Record these results - Report recorded results Report recorded results. - Duration distribution - Per--node criticality index (% runs where critical) Network Example ctivity Optimistic time, a Most likely time, m Pessimistic time, b Expected value, d Standard deviation, s B C D E F G

9 Summary of Simulation Runs for Example Project Run ctivity Duration Number B C D E F G Project Duration Distribution Critical Path -D-F-G -D-F-G Completion Time Then the probability that a project ends in a specific time (t) equals number of times the project finished in less than or Equal to t divided by the total number of replications. For example, the probability that the project ends in 0 weeks or less is: P( 0 ) = 1 / 0 = %. Criticality Index Criticality index is defined as the proportion of runs in which the activity was in the critical path. PERT assumes binary (either 100% or 0%). 9