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Message: [QUOTE="alanglloyd, post: 3870245"] You have added a column showing the difference to the previous. Your formula computes the difference to the next. Having calculated the change, now extend that by calculating the change in the change. Extending the example (and calculating the change to the next, and then the change to the change to the next) would provide : 1 115 10 0 2 105 10 0 3 95 10 5 4 85 5 0 5 80 5 0 6 75 5 0 7 70 5 0 8 65 5 0 3 is obviously the breakpoint. The breakpoint is the change of the slope. Or if you've done calculus, the breakpoint is at d2y/dx. Think Babbages's difference engine. <g> Alan Lloyd [/QUOTE]