Re: Exponetial Fit

*To*: mathgroup at christensen.cybernetics.net*Subject*: [mg1802] Re: Exponetial Fit*From*: rjfrey at rentec.com (Robert J. Frey)*Date*: Mon, 31 Jul 1995 23:07:42 -0400*Organization*: Renaissance Technologies Corp.

John L. White (crunch at s-cwis.unomaha.edu) wrote: : I have tried to do a exponetial fit to a list of data from a physics : experiment, but Mathematica does a terrible job when I do the following : command: : Exp[Fit[Log(data),{1,t},t]] I assume this is Exp[Fit[Log[data], {1,t}, t]] : Does anyone know of a better way? Thanx in advance, John : -- I assume data is a vector of observations:{{x1,y1}, {x2,y2}, ...} In what follows a and b are parameters to be estimated, t is the independent variable and y the dependent variable. Generally, if the term "exponential model" is used, the model one wants to fit is: y == a Exp[b t] if you take the Log of both sides: Log[y] == Log[a] + b t The data pairs {t, Log[y]} can then be subject to a linear regression, yielding estimates for Log[a] and b. This isn't what you did. Now, there is the power model: y == a t^b if you take the Log of both sides: Log[y] == Log[a] + b Log[t] The data pairs {Log[t], Log[y]} can then be subject to a linear regression, yielding estimates for Log[a] and b. This does appear to be what you did in the Fit, but the final Exp is still wrong. -- Regards, Robert (rjfrey at rentec.com