six sigma in relation to process duration

Would it be possible to elaborate a little on the relation between six sigma quality requirement and process duration?

I am looking at the question that has 13 days of the PERT task duration estimate and 1 day as a standard deviation. The correct answer reads that the task can be completed from 7 to 19 days period. I don't really understand where 6 days duration deviation came from...


Any input is greatly appreciated.

admin's picture

Question is not clear

Just to clarify the question I am struggling with...

The activity has an optimistic estimate of 10 days, pessimistic estimate of 16 days and most likely estimate of 13 days. If your company has a quality requirement of Six Sigma what is the duration within which task must be completed?

I calculated 13 days mean.. and standard deviation = 1. Correct answer is 7 - 19 days. I don’t understand why it is +- 6 days from the mean.


I will make an attempt. Will request the experts on this forum to review once.

Your Estiated Duration (ED) for this will be (O+4M+P)/6 = 13

Your Standard Deviation (SD) will be (P-O)/6 = 1

6 six sigma will be 6SD which is 6*1 = 6

Range = ED + SD and ED - SD

So you get 13-6 to 13+6 which is 7 to 9 days

I think that I still don't understand the part "6Sigma will be 6 SD". Why would you translate level of quality Six Sigma into broader effort period to perform the task?

How does it make sense ?


6*SD for 6 Sigma , and 3*Sd for 3 Sigma. Isnt 6 Sigma supposed to be more closer to the estimate.

this is a good question that I would like to know also, if it's Standard deviation * 6  for six sigma and Standard Deviation *3 for three Sigma,  3 sigma will be more acurate, which sounds weird to me ?

It is a range and 6 Sigma will be more accurate than 3 sigma. If you go  by numbers the confusion is bound to be there, since you just see one number as 3 times and second as 6 times.

Consider the same example, Estimated duration is 13 days, So what is the probablity of completing the activity in the range of  10-16 days which is +-3 sigma. Obviously the confidence level of completing the activity within 10-16 days is less as compared to completing in 7 to 19 days.

This is the precise reason why 6 sigma is more accurate than 3 sigma.

For 6 sigma, range will be 7-19 days. For complete explanation on PERT and SD, you can read following articles
* PERT Formula and Use of Standard Deviation 
* How to use PERT, CPM and Standard Deviation Together?