Another unsolicited testimonial came in from a satisfied customer yesterday evening. Jerry said: Thank you! What more do I need to say! I have been in test engineering for years now and have been dodging to learn statistical maths for along time now! I have worked with various maths including differential equations and some FFT equations. My previous company was low volume wafer (Research and only program managers who tried to save their programs would do statistical maths) I could simply depend on trouble shooting on a real time bases, one by one real time failures. Now that I am in the Market, Test Engineers like myself have to know statistical maths and bell curve troubleshooting. Even though I could make an argument that they need to depend on common logic (circuit debugging and practical knowledge) for failure analysis. Test engineers in today's world need to analyze failure by measured percentages and how to rectify the numbers. I give in and here is my thanks to you. After reading the simple version on Mr. Niles I ran across your web site and found your problems. Ye old familiar symbol of sigma and found the maths not to complex. It seemed very similar in averaging Impedances in complex conjugates. Anyhow I simulated my own data sets of Currents for applied voltages of VBEE simulating some failures and BAM. I am ready to trouble shoot in series now. Just wanted to thank you for easy explanation. Aw, I've come over all embarrassed and shy! DW