Hello Somboon
There are many ways in which you can use Six Sigma techniques in Project Management. Here is one that you can consider using while planning a project:
Most common way of project planning is to identify the tasks required to complete the project, estimate their completion time, and determine their interdependencies followed by development of the network diagram (or graph) to compute project completion time along with other parameters. This methodology works well if (point) estimates of task completion time are highly accurate. In other words the actual completion time experiences negligible variation.
In real world that is full of variations, PERT methodology of project planning comes to our rescue. The 2 key differences in this methodology are (a) understanding the distribution of task completion time (from past data) and (b) application of Central Limit Theorem (CLT) to compute the project completion time with a defined confidence level.
For a quick reference, CLT states that irrespective of the distribution of the population, the sampling distribution of mean will tend to be distributed normally, as long as your sample size is big enough (about 30 or more). Note sampling distribution is the distribution pattern of a given statistic found by repeated sampling of the population.
The distribution of completion times provides us the minimum, maximum, and the most likely (mode) completion time, which allows us to compute the mean and variance of completion time. On the critical path of the project, the mean and variance are added to find out the mean project completion time and the corresponding variance. And this follows a normal distribution according to the CLT. With this information, we can now easily compute the project completion time with required confidence level. Imagine in case of point estimates equal to average task completion time value, the project completion time will have a confidence level of just about 50% only.
Hope it gives you an idea.
Warm regards,
SKS