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Project Evaluation and Review Technique (PERT) is one of the main project management methods used in electrical engineering construction. Due to indeterminacy of the criteria of the time parameter estimation and neglect of the project resource constraints, scheduling error often occurs. This paper introduces a new PERT schedule planning method based on the improvement of the time parameter estimation. Given that the planned duration of a single activity obeys β distribution, the principle that time parameter estimation should follow is discussed, and then a method how to instead the most optimistic time by the least time with resource constraints is proposed, therefore the criteria of time parameter estimation are determined. Next, the β distribution is fitted according to the given time parameters. Finally, cost-time linear optimization model is established, including probabilistic constraint and the buffer constraint of the β distribution. The model is solved, results in the optimal planned schedule. Simulation analysis verifies the effectiveness of the improved method. Introduction In the management of power construction projects, progress, quality and cost are the three major management objectives, they have a relation of unity of opposites and the optimal of schedule is very important [1]. Project Evaluation and Review Technique (PERT) has been widely used in the progress management of power construction projects [2][3]. However, since the PERT was put forward, there are lots of discussion about its rationality, which mainly focus on whether the hypothesis of the distribution model is reasonable and the impact of the estimated results caused by the choice of the three time parameters (the shortest time, the longest time and the most reasonable time). In [4], it was proposed that the PERT duration distribution model could be one of the four different probability distribution including βdistribution, Г distribution, triangular distribution and exponential distribution, in which β distribution was the suggested option. Reference [5] emphasized that it is more reasonable to choose the β distribution as the model of the duration in PRET, the main reason was that distributions such as triangular distribution and Г distribution, can be simulated by changing the parameters of the β distribution. The choice of three time parameters in PERT will greatly influence the estimation of construction period. Reference [6] came up with six kinds of fixed expectation and variance estimation formula of boundary value probabilities, considering the difficulty to confirm the extreme probability of the shortest and longest time, which could result in a large deviation of the estimated result of the duration. In [7], the variance estimation formula was improved, considering the influence of most reasonable time. When the difference between the longest time and the shortest time is the same, the variance estimation result will be determined by the most reasonable time, which solves the problem of the conflict between the variance estimation result and the reality. In [8], the answer was given to the question how the three time parameters should be chosen to be closer to the mean value. It was pointed that the probability of the shortest, most probable 21 and longest time was more satisfied to the mean estimate when they were close to 1/6, 4/6 and 5/6. In addition, reference [9] had already proposed that progress schedule expectation of PERT is much smaller than the actual statistical value, which had not been effectively resolved yet. On the other hand, the resource constraints of the construction project also affect the three time values. Reference [10] introduced the activity criticality indicators and importance degree indicators to assess the risk of activities in the PERT, and used them as risk coefficients in resource allocation decision, meanwhile the expected value of time was estimated by the look-up table method. But the method which corrects results directly according to the experience does not really reflect the impact of resource constraints on the value of time, whose reasonability worth more discussion. In view of the above problems, this paper discusses the scale principle of three time parameters given that PERT planning duration is subject to the β distribution, adding the consideration of resource constraints to it. On the basis of the simulation of β distribution, as well as the situation that expected value is too small, we propose a new PERT schedule planning method to obtain a more reasonable schedule value. Simulation results show that the proposed method is effective. PERT and Its Problems PERT PERT is a technique considering the uncertainty of the duration of the network planning and evaluation. Its basic processes applied to the construction process progress management are as follows: Draw P3 network diagram according to the logic relationship between working procedures of the construction project; Estimate the duration of the process reasonably after clearing the power grid project duration target, in general, the schedule assumption is subject to the β distribution, which can be obtained using the three-time estimation method; Finally, we get the schedule by determining the critical path based on the expected duration or the total time difference between processes is equal to 0. It is possible to further calculate the flexible time schedule of each process on the critical path to cope with the progress deviation in the subsequent progress of the project. It can be seen that the most critical link of PERT applied to the project schedule development, is the estimation of three time parameters, so as to determine the β distribution of the planned duration. The section headings are in boldface capital and lowercase letters. Second level headings are typed as part of the succeeding paragraph (like the subsection heading of this paragraph). Problem Analysis The β distribution of the PERT plan duration is related to three times, namely, the shortest time a, the longest time b, the most reasonable time m. The β distribution function is expressed as

A PERT Schedule Planning Method Based on the Improved Time Parameter Estimation