Re: [R] nls - convergence problem

Spencer Graves Fri, 07 Jan 2005 11:52:25 -0800

Hi, Doug:

How would you diagnose something like this? For example, might the following (from ?nlsModel) help:

    DNase1 <- DNase[ DNase$Run == 1, ]
    mod <-
     nlsModel(density ~ SSlogis( log(conc), Asym, xmid, scal ),
              DNase1, start=list( Asym = 3, xmid = 0, scal = 1 ))
    mod$Rmat()        # R matrix from the QR decomposition of the gradient

Wouldn't the first zero or nearly diagonal element of Rmat identify a variable to which the model was insensitive and could therefore be fixed or removed from the model? Thanks, Spencer Graves

Douglas Bates wrote:

[EMAIL PROTECTED] wrote:
Dear list,
I do have a problem with nls. I use the following data:
test
    time  conc dose
   0.50  5.40    1
   0.75 11.10    1
   1.00  8.40    1
   1.25 13.80    1
   1.50 15.50    1
   1.75 18.00    1
   2.00 17.00    1
   2.50 13.90    1
   3.00 11.20    1
  3.50  9.90    1
  4.00  4.70    1
  5.00  5.00    1
  6.00  1.90    1
  7.00  1.90    1
  9.00  1.10    1
12.00  0.95    1
14.00  0.46    1
24.00    NA    1
30.00    NA    1
36.00    NA    1
I use the self-starting function SSfol:
nls(conc~SSfol(dose,time,lKe,lKa,lCl),data=test,trace=T,control=nls.control(maxiter=13,tol=0.001,minFactor=1.E-500),na.action=na.omit)
This gives the following output:
99.15824 : -1.2061792 0.1296157 -4.3020997 86.07567 : -0.7053265 -0.3873204 -4.1278009 85.19743 : -0.5548499 -0.5333776 -4.1173627 85.19246 : -0.5466376 -0.5415731 -4.1173247 85.1922 : -0.5444637 -0.5437461 -4.1173223 85.1922 : -0.5442240 -0.5439857 -4.1173223 85.1922 : -0.5441337 -0.5440760 -4.1173223 85.1922 : -0.5441104 -0.5440993 -4.1173223 85.1922 : -0.5440984 -0.5441113 -4.1173223 85.1922 : -0.5441089 -0.5441008 -4.1173223 85.1922 : -0.5441006 -0.5441091 -4.1173223 85.1922 : -0.5441051 -0.5441046 -4.1173223 85.1922 : -0.5441051 -0.5441046 -4.1173223 85.1922 : -0.5441051 -0.5441046 -4.1173223 Error in nls(conc ~ SSfol(dose, time, lKe, lKa, lCl), data = test, trace = T, : number of iterations exceeded maximum of 13
I do not understand this error message for two reasons:
1. Apparently the algorithm has converged, at the end of the output the
change of parameter estimates seems to be less than 0.001
2. If the maximum number of iterations is reached I would expect a
warning in the case that the algortihm did not converge. However, here
the algorithm apparently has converged. Thus I do not understand what
happened.
But it hasn't converged. The nls function uses a relative offset convergence criterion that is not being satisfied here. By setting the minimum step factor to a very small number you are more small steps than you would otherwise but you are not converging. It appears that the parameter estimates are not defined for this model/data set combination.

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Re: [R] nls - convergence problem

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