Question

sigma:
level of confidence

Answer 1

I assume a 99 ss user should know to post the actual question. :P

Answer 2

http://newsoffice.mit.edu/2012/explained-sigma-0209

http://blogs.scientificamerican.com/observations/2012/07/17/five-sigmawhats-that/

http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-041-probabilistic-systems-analysis-and-applied-probability-fall-2010/