A Problem with the NonlinearFit?

*To*: mathgroup at smc.vnet.net*Subject*: [mg61757] A Problem with the NonlinearFit?*From*: "Axel Ligon" <ligonap at web.de>*Date*: Fri, 28 Oct 2005 03:25:41 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

Deas Sirs, concerning the NonlinearFit procedure I have a problem. I used the following inputs: A) *Raw datas {x, y}* dat1={{0,319.9},{1,196.7},{3,140.2},{7,99.0},{14,56.1},{31,32.8},{90,1.9}} FindFit[dat1,a*Exp[-b*x],{a,b},x] a ->280.376, b->0.175479 <<Statistics`NonlinearFit` f=NonlinearFit[dat1,a*Exp[-b*x],x,{a,b}] 280.376 * Exp(-0.175479x) B) *Convert datas {x, ln[y]}* dat2={{0,5.7680},{1,5.2817},{3,4.9430},{7,4.5951},{14,4.0271},{31,3.4904},{90, 0.6419}} FindFit[dat2,-b*x+a,{a,b},x] a->5.18756, b->0.0518199 <<Statistics`NonlinearFit` f=NonlinearFit[dat2,-b*x+a,x,{a,b}] 5.18756 - 0.0518199 x The difference between A) and B) is only the conversion of y to ln[y] and the equation y = a * exp(-b*x) to ln[y] = ln[a] - b*x. From this it follows that a = exp[ln[a]]. But I get in A) 280.376 / -0.175479 and in B) 179.031 / -0.05182 !! Why do I get these error?? If I use Excel, I don't get these errors. Is it an error in the programming of Mathematica, or not??? I'd be glad if you could help me. With best regards Axel Ligon

**Follow-Ups**:**Re: A Problem with the NonlinearFit?***From:*Chris Chiasson <chris.chiasson@gmail.com>