Schedule Accuracy Dragan Z. Milosevic, Peerasit Patanakul, and Sabin Srivannaboon Being old friends, project managers Mark and Jerry came to a meeting organized by the engineering vice president, Barb. In addition to the three of them, there were twenty - one project managers sitting in the room. According to the invitation, the meeting was supposed to be a 45 minute session. So Mark and Jerry were hoping they could get back to work shortly after. Barb: Thanks to everyone for coming. As already mentioned in the invitation, the purpose of this meeting is to define a strategy for our group. Last week we were trying to have a meeting about this, but many couldn’t attend. So we moved it to this week, and this is the best time for most of us. To save time, let’s cut to the chase. Our business strategy is time - to - market. Therefore, accuracy of schedule is one of the key elements. But we never actually define what accuracy means. So people often claim that their project schedules are accurate. But to what extent? And based on what? We never know! THE ACCURACY Barb proceeded to give a theoretical explanation of “accuracy” as it relates to the business. She then continued with a specific explanation of what accuracy meant to her particular business unit. Barb: The accuracy is viewed as a percentage to which our project schedules are likely to be accomplished. All our schedules will be in a network form. Specifically, we use the Time Scaled Arrow Diagram, or TAD. Barb continued her speech by showing the three steps in coping with schedule accuracy: First step : The project team must develop a TAD of the project. We require team members to participate in the development as they are expected to discuss the length of each activity. In particular, they must give us three points of estimate of each activity in which they are involved. Typically, the three point estimates are the best case, the most likely case, and the worst case. Speaking mathematically, the range from optimistic to pessimistic should cover six standard deviations, and the estimates should be made at the 99 percent precision level in order to use the regular Beta formulas. Second step : This three point estimate will be used to calculate the likelihood of the schedule. Generally, we assume the project activity durations fit a Beta distribution and are statistically independent, so we can calculate the activities’ means and variances accordingly (based on the Beta formulas). Then, we find the longest path in time of the project, or the critical path, and assume that the overall project duration has a normal distribution. So, we can calculate the Z value and look at the Z - table to find the probability of completing the project ’ s “ critical path ” (note that this is not the probability of completing the “ entire ” project on time). Third step : Generally speaking, those who are responsible for the schedules are project managers. So they have to make sure that all the resources are available when needed in the right quantity. After identifying the probability of accomplishing the project’s critical path, they will meet with the clients to further discuss the project deadline. After the brief explanation of the new approach to the schedule accuracy, everyone in the room looked at each other. They were thinking they needed a short statistics class to truly understand what Barb just said. But before they said anything, Barb continued, “Don’t worry if you don’t understand what I just said. I have arranged the tutorial session for you. And the instructor is already here; ready to start in five minutes. ” At that time, Mark and Jerry knew the meeting wouldn’t end in just 45 minutes. Discussion items 1. Define schedule accuracy. What are major assumptions? 2. Did the approach offer schedule accuracy or buffer against inaccuracy? 3. From your point of view, how important is the probability of completion to the customers?
