Re: [R] nls weights warning message

From: Prof Brian Ripley <ripley_at_stats.ox.ac.uk>
Date: Thu, 31 Jul 2008 14:21:50 +0100 (BST)

Somewhat coincidentally this will work if you don't supply 'data=Data'.

On Thu, 31 Jul 2008, Prof Brian Ripley wrote:

> On Thu, 31 Jul 2008, Lisa Readdy wrote:
>
>> The following warning message occurs when running the nls on some data:
>>
>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> Warning message:
>> In is.na(wts) : is.na() applied to non-(list or vector) of type 'NULL'
>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>> and I would like to know what is causing it and how I can fix it.
>>
>> As an example, the following, from Venables and Ripley, which does not
>> produce the warning message and then on the data which does procude the
>> warning message.
>>
>>
>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>> ##Venables and rippley
>>
>> library(MASS)
>> data(wtloss)
>> wtloss.st = c(b0=90, b1=95, th=120)
>> n1<-length(wtloss)
>> Weight=c(1,rep(0.1,n1-1))
>>
>> wtloss.fm = nls(Weight ~ b0 + b1*2^(-Days/th), data=wtloss,
>> start=wtloss.st,
>> trace=T, weights=Weight )
>
> I am not sure what you think that does. It uses the Weight column of the
> data frame as the weights, which is not what you seem to have intended.
>
>> wtloss.fm
>>
>> ##My data
>> y<-c(73,73,70,74,75,115,105,107,124,107,116,125,102,144,178,149,177,124,157,128,
>> 169,165,186,152,181,139,173,151,138,181,152,188,173,196,180,171,188,174,198,172,
>> 176,162,188,182,182,141,191,190,159,170,163,197)
>>
>> x<-c(1,1,1,1,1,2,2,3,3,3,3,3,3,4,4,4,5,5,5,5,6,6,6,6,6,6,7,7,7,7,7,7,7,7,7,8,8,8,
>> 8,8,8,8,8,8,8,9,9,9,9,9,11,12)
>>
>> Data<-data.frame(cbind(y,x))
>>
>> n2<-length(x)
>> weight<-c(1,rep(0.1,n2-1))
>>
>> Ta<-min(Data$x)
>> Tb<-max(Data$x)
>> G.st<-c(k=0.005, g1=50,g2=550)
>>
>>
>> #no weights
>> growthFit<-nls(y~((g1)*exp((log(g2/g1))*(1-exp(-k*(x-Ta)))
>> /(1-exp(-k*(Tb-Ta))))), data=Data, start=G.st, trace=T)
>>
>>
>> #with weights
>> growthFit.wts<-nls(y~((g1)*exp((log(g2/g1))*(1-exp(-k*(x-Ta)))
>> /(1-exp(-k*(Tb-Ta))))), data=Data, start=G.st, trace=T, weights=weight)
>
> The difference here is that you don't have all your variables of the same
> length:
>
> Browse[1]> print(n)
> y x Ta Tb
> 52 52 1 1
>
> and the variable-finding mechanism is not set up to find weights in that
> case. That's a bug, but you can avoid it by substituting the scalars (e.g.
> using substitute()).
>
> --
> Brian D. Ripley, ripley_at_stats.ox.ac.uk
> Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
> University of Oxford, Tel: +44 1865 272861 (self)
> 1 South Parks Road, +44 1865 272866 (PA)
> Oxford OX1 3TG, UK Fax: +44 1865 272595
>
> ______________________________________________
> R-help_at_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.
>

-- 
Brian D. Ripley,                  ripley_at_stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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Received on Thu 31 Jul 2008 - 13:30:24 GMT

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