Based on a simple scatterplot of pourcma vs transat, a 4 parameter logistic looks like wild overfitting, and that may be the source of your problems. Given the huge scatter, a straight line is about as much as would seem sensible. I think this falls into the "Why ever would you want to do such a thing?" category.
-- Bert Bert Gunter Genentech Nonclinical Biostatistics 650-467-7374 -----Original Message----- From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On Behalf Of Patrick Giraudoux Sent: Friday, March 27, 2009 12:39 PM To: r-h...@stat.math.ethz.ch Cc: Francis Raoul Subject: [R] nls, convergence and starting values "in non linear modelling finding appropriate starting values is something like an art"... (maybe from somewhere in Crawley , 2007) Here a colleague and I just want to compare different response models to a null model. This has worked OK for almost all the other data sets except that one (dumped below). Whatever our trials and algorithms, even subsetting data (to check if some singular point was the cause of the mess), we do not reach convergence... or screw up with singular gradients (?) etc... eg: nls(pourcma~SSlogis(transat, Asym, xmid, scal), start=c(Asym=30, xmid=0.07, scal=0.02),data=bdd, weights=sqrt(nbfeces),trace=T,alg="plinear") As anyone a hint about an alternate approach to fit a model ? Or an idea to get evidence that such model cannot be fitted to the data.... bdd <- structure(list(transat = c(0.0697, 0.13079, 0.314265, 0.241613, 0.039319, 0, 0, 0, 0, 0, 0.0805, 0.41, 0.30585, 0.27465, 0.06085, 0.09114, 0.05766, 0.036983, 0.093186, 0.046624, 0, 0, 0, 0, 0.000616, 0, 0.0025, 0.0325, 0.03125, 0.04599, 0.38398, 0.524505, 0.450337, 0.061831, 0.133926, 0.091806, 0.00928, 0.25114, 0.3074, 0.431056, 0.026158), transma = c(0.04141, 0.01599, 0.101803, 0.002378, 0.039319, 0.00472459016393443, 0.0031016393442623, 0.000178524590163934, 0.00255704918032787, 0.000346229508196721, 0.0665, 0.012, 0.0553, 0.0045, 0.0056, 0.00155, 0.00124, 0.011966, 0.001736, 0.004712, 3.62903225806452e-05, 9.79838709677419e-05, 2.20161290322581e-05, 0.00462, 0.0100644444444444, 0.00213111111111111, 0.046, 0.005, 0.01195, 0.07154, 0.08468, 0.141182, 0.086578, 0.027959, 0.003159, 0.003081, 0.13862, 0.00754, 0.078648, 0.068324, 0.025288), nbfeces = c(22L, 26L, 43L, 30L, 35L, 25L, 21L, 36L, 34L, 37L, 23L, 32L, 40L, 35L, 30L, 16L, 25L, 37L, 37L, 34L, 31L, 35L, 41L, 31L, 34L, 39L, 5L, 14L, 31L, 13L, 21L, 34L, 32L, 36L, 36L, 40L, 31L, 35L, 39L, 29L, 32L), pourcma = c(50, 34.6153846153846, 27.9069767441860, 43.3333333333333, 65.7142857142857, 32, 28.5714285714286, 22.2222222222222, 50, 10.8108108108108, 26.0869565217391, 40.625, 12.5, 22.8571428571429, 43.3333333333333, 6.25, 4, 10.8108108108108, 16.2162162162162, 23.5294117647059, 25.8064516129032, 45.7142857142857, 39.0243902439024, 25.8064516129032, 41.6666666666667, 27.5, 20, 14.2857142857143, 22.5806451612903, 15.3846153846154, 38.0952380952381, 17.6470588235294, 78.125, 61.1111111111111, 25, 37.5, 22.5806451612903, 40, 17.9487179487179, 41.3793103448276, 50), pourcat = c(22.7272727272727, 30.7692307692308, 41.8604651162791, 56.6666666666667, 5.71428571428571, 0, 0, 0, 0, 0, 30.4347826086957, 15.625, 45, 74.2857142857143, 13.3333333333333, 50, 12, 18.9189189189189, 27.0270270270270, 20.5882352941176, 0, 0, 0, 0, 0, 5, 40, 0, 0, 7.69230769230769, 9.52380952380952, 38.2352941176471, 59.375, 5.55555555555556, 41.6666666666667, 42.5, 9.67741935483871, 14.2857142857143, 51.2820512820513, 79.3103448275862, 6.25)), .Names = c("transat", "transma", "nbfeces", "pourcma", "pourcat"), class = "data.frame", row.names = c(NA, -41L)) ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
