50% task time estimate50% task time estimate - A task time estimate that has a 50% chance (or .5 probability) of being achieved.
Illustration: In the figure below the probabilities given represent the area under the curve to that point in time.
Perspective: The distribution of task times is generally viewed as an asymmetrical positive or skewed to the right distribution, which means the right tail is quite long. Under a significantly skewed distribution, a task time estimate that has 80% chance (or .8 probability) of being achieved might be as much as twice as long as an estimate that has a 50% chance (or .5 probability) of being achieved.
In traditional project management, the resources/resource managers are frequently responsible for and measured based on completing their tasks within the time they have estimated. In view of this responsibility, the resources/resource managers have a strong incentive to provide very high estimated task times. As a result, task time estimates with chance of completion in the 80-90% range are frequently seen. This padding, while seemingly increasing the probability that individual tasks will be completed within the time estimated, has two common effects at the individual task level: either the resource falls victim to the student syndrome and delays starting work on the task until the local protection built into the task estimate is gone, or the resource finishes the task early and continues to "improve" or find other things to do until the task time is consumed (Parkinson's Law). In addition, using 80-90% task time estimates causes the project duration to be inflated without providing protection for the whole project because the protection built into individual task estimates is used up task-by-task as each task is completed.
In contrast, with the critical chain approach, resources and resource managers are responsible for providing a "50% task time estimate" and are not held responsible for achieving the estimate. Protection is provided by the project buffer for all critical chain tasks and by feeding buffers for non-critical-chain tasks. Frequently the conversion of the given task estimate to a "50% task time estimate" is used in the implementation of critical chain project management. After resources are comfortable with the critical chain methodology they are asked to estimate their own 50% task times. Time estimates may also be established based on task touch times. No matter how the task times are determined they should be challenging but achievable. While the basis for task time estimates may differ, the overarching rule is that the project buffer is 1/3 of the project duration.
Caution: Some lead times are static or fixed. In those cases, a 50% task time estimate is unrealistic and unachievable.