[R] nls weights warning message

[Prof Brian Ripley]

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
>
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>

-- 
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