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

[David Winsemius]

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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David Winsemius
Alameda, CA, USA