[R] Fitting a curve to weibull distribution in R using nls

[Aditya Bhatia]

Yes. I do.I'm trying to repeat the methodology of a paper. They have fitted
their data to a weibull curve and so I want to do the same too, but I'm
unable to figure out how..

On Wed, Oct 14, 2015, 9:44 AM David Winsemius <dwinsemius at comcast.net>
wrote:

>
> On Oct 13, 2015, at 2:42 PM, Aditya Bhatia wrote:
>
> > I am trying to fit this data to a weibull distribution:
> >
> > My y variable is:1  1  1  4  7 20  7 14 19 15 18  3  4  1  3  1  1  1
> > 1  1  1  1  1  1
> >
> > and x variable is:1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18
> > 19 20 21 22 23 24
> >
> > The plot looks like this:http://i.stack.imgur.com/FrIKo.png and I want
> > to fit a weibull curve to it. I am using the nls function in R like
> > this: nls(y ~ ((a/b) * ((x/b)^(a-1)) * exp(- (x/b)^a)))
>
> So despite the fact that the Weibull function has a continuous domain, you
> want to fit a set of integers to something like the Weibull with these
> values as case weights with the "x-values" being the position of these
> integers in a sequence?
>
>
> --
> David.
> >
> > This function always throws up an error saying: Error in
> > numericDeriv(form[[3L]], names(ind), env) :
> > Missing value or an infinity produced when evaluating the model
> > In addition: Warning message:
> > In nls(y ~ ((a/b) * ((x/b)^(a - 1)) * exp(-(x/b)^a))) :
> >  No starting values specified for some parameters.
> > Initializing ‘a’, ‘b’ to '1.'.
> > Consider specifying 'start' or using a selfStart model
> >
> > So first I tried different starting values without any success. I
> > cannot understand how to make a "good" guess at the starting values.
> > Then I went with the SSweibull(x, Asym, Drop, lrc, pwr) function which
> > is a selfStart function. Now the SSWeibull function expects values for
> > Asym,Drop,lrc and pwr and I don't have any clue as to what those
> > values might be.
> >
> > I would appreciate if someone could help me figure out how to proceed.
> >
> > Background of the data: I have taken some data from bugzilla and my
> > "y" variable is number of bugs reported in a particular month and "x"
> > variable is the month number after release.
> >
> > ______________________________________________
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> > 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.
>
> David Winsemius
> Alameda, CA, USA
>
>

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