1. 1. Six sigma manual from by Mr.RamNarayana Consultant World class manufacturing from Amara raja Batteries ltd.
   2. Grant, Eugene Lodewick, Leavenworth, Richard S., Statistical Quality Control, McGraw-Hill Book Company, New York, 1988, pp. 574-575, ISBN 0-07- 024117-1
   3. Stamatis, D. H., TQM Engineering Handbook, CRC Press, 1997, pp. 240-241 ISBN 0-8247-0083-X
   4. Bhote, Keki .R., The Ultimate Six Sigma: Beyond Quality Excellence to Total Business, Amacom, 2001, p. 176,ISBN0-8144-0677-7