A simple calculation
Monday, March 15, 2010
10:08 AM
A simple calculation: 
4-sigma site: down for one hour a year. 
6-sigma site: down for half a minute per year. 
Worst case: both down at different times: 6-sigma doesn’t' add much downtime. 
But what if a 4-sigma site subscribes to 4-sigma service? 
Worst case: down two hours a year. 
So you lose one more hour of work. 
And you lose one more hour of revenue. 
Is this downtime cost worth paying 100x  for the service?
Usually, no. Because you could afford to lose one hour already, and making your service more efficient than that wasn't worth it already!