Thomas Hoffmann <[EMAIL PROTECTED]> writes: > Dear R-helpers, dear Andrew > > Andrew C. Ward schrieb: > > >Dear Thomas, > > > > I wonder if you have any NAs in xy$x or xy$y. > > The Dataframe looks like the following: > > xy > x y > 1 3.0 121.50951 > 2 2.0 30.61258 > 3 4.0 323.14837 > 4 8.2 3709.92471 > For testing reasons I calculated y by: > > y <- 2.9 x^3.4
Did you read the help page for nls that emphasizes *Do not use 'nls' on artificial "zero-residual" data.* and explains why? In any case, the best way to fit such a model to these data is to use the plinear algorithm for partially linear models. Either > nls(y ~ x^exp(logpow), data=xy, start = c(logpow=0.1), alg='plinear',trace = TRUE) 2661838 : 0.1000 281.0589 87270.38 : 0.9550582 15.4392528 1573.086 : 1.181199 3.903272 1.645442 : 1.222351 2.929585 2.301169e-06 : 1.223774 2.900035 1.483770e-11 : 1.223775 2.900000 1.483769e-11 : 1.223775 2.900000 Nonlinear regression model model: y ~ x^exp(logpow) data: xy logpow .lin 1.223775 2.900000 residual sum-of-squares: 1.483769e-11 or > nls(y ~ exp(log(x) * pow), data=xy, start = c(pow=1.1), alg = 'plinear', trace = > TRUE) 2684530 : 1.1000 283.5010 408828.2 : 2.045211 47.930710 33333.73 : 2.848788 9.185464 857.465 : 3.293871 3.622750 1.307911 : 3.395685 2.926367 3.723389e-06 : 3.399993 2.900044 1.483772e-11 : 3.4 2.9 1.483769e-11 : 3.4 2.9 Nonlinear regression model model: y ~ exp(log(x) * pow) data: xy pow .lin 3.4 2.9 residual sum-of-squares: 1.483769e-11 > >Also, I think > >you could take logs of your equation and end up with a > >linear expression? > > > if I use: > > plot (x,y) > abline lm ( log(y)~log(x),xy) > > the line does not seem to fit the plotted datapoints at all. While the > fitted exponent seems to be okay, the obtained intercept value is > wrong. That's because your plot is in the x,y scale and the line is in the log(x), log(y) scale. You need to convert back to the original scale to put the line on the plot. -- Douglas Bates [EMAIL PROTECTED] Statistics Department 608/262-2598 University of Wisconsin - Madison http://www.stat.wisc.edu/~bates/ ______________________________________________ [EMAIL PROTECTED] mailing list https://www.stat.math.ethz.ch/mailman/listinfo/r-help