Let's look at the key chain schedule.

Note: The content of this article comes from the learning and summary of the book "key chain"

Task duration hypothesis
If you are dependent on two AB tasks, the duration of task a is 10 days, and that of Task B is 5 days. When task a actually takes only 6 days to complete, Task B will still start in 11 days. On the one hand, the reason is that the team or individual who completes the task ahead of schedule does not report the facts. On the other hand, the team or individual assigns the task to be completed in stages within 10 days due to the completion of the task in advance, additional time is used to process other tasks encountered every day.

Conclusion
1. The delay of one step will be passed to the next step, and the time earned by the early completion is often consumed.
2. The delay of each step in the sequence step will accumulate, but the time earned by the completion in advance cannot be accumulated.

Path aggregation risks
In the network diagram of the project schedule, path aggregation points often have the greatest risk. If there are currently three project tasks A, B, and C simultaneously, all three tasks must be completed before task D can be performed, any delay of the three ABC tasks will lead to the delay of the d task. If the probability of completion of A, B, and C is 50%, the probability of completion of task D is only 50% * 50% * 50% = about 13%.

Conclusion
1. The path aggregation point in the Progress network diagram has the maximum progress risk.
2. If the security time is not placed at the end of the parallel sequence, it does not have any effect on reducing the risk of aggregation.

Risk probability
We use a simple three-point estimation to estimate the probability of task completion. The most likely time for a task to be completed is 12 days, the most optimistic time is 6 days, and the most pessimistic time is 24 days. At this time, the mean value is 13 and the standard deviation is 3. Due to the large pessimistic estimation time, this probability distribution is not a typical normal distribution, but a distribution with a long right tail, for example. In this figure, we can see that to increase the project completion probability by 30%, we need to invest a lot of Security Time. Early completion of a project is often not rewarded, but delayed delivery is criticized. Therefore, the project estimates adopt the rightmost security value rather than the adjusted median value.

Conclusion
1. Murphy's Law: all possible troubles will inevitably happen.
2.200% of the security time is often only used to increase the security time by 20%. When the estimated risk does not occur, the security time will be wasted.

Secondary Key Path
The secondary key path in the project is the path sequence that must be considered. If all non-critical paths start at the latest, all paths will become key paths without any buffer space, project risks will multiply. If all the project tasks start as early as possible, the Project Manager will be exhausted and unable to ensure the focus on the work and tasks.

Conclusion
1. When we make non-critical path tasks start at the latest to balance resources, project risks are often increased.
2. Tracking the project's secondary key paths at all times. They may all be upgraded to the project's critical path at all times.

1. Project Buffering
Shorten the estimated time of each task on the key route, and place all the summarized Security time at the end of the key route. In this way, the total release time forms a project buffer.

2. Connection Buffer
Our attention should not be simply placed on key paths. All connection paths on a critical path must be considered. Any delay in a connection path will affect the delay in the Key Path. Therefore, you should consider the connection paths related to the key paths and set the connection buffer.

3. Integrated key routes of Project buffering and connection Buffering
As shown in the figure, all the tasks marked with X require resource X. Although connection buffering and project buffering are considered in the network diagram. However, project tasks are not restricted by resources. When considering resource constraints, it is not simply balanced by buffering, but the key path is no longer simply calculated based on the longest duration.

 
4. Key Chain
The key chain is the longest task line in the project considering resource constraints. The real cycle of a project is determined by the length of the key chain. The key chain should not only consider connection buffering, but also project buffering. You also need to consider the resource constraints on specific types of tasks. In order to discharge the optimal key chain, there will also be the task Sorting Problem between multiple tasks that require the same resource. When you do not consider resource sorting, the key chain is relatively simple. However, after you consider the resource sorting, the key chain schedule method will be very complex.

 
From: http://blog.sina.com.cn/s/blog_493a8455010008v4.html