[R] 'fitdistr' and two views of the same data?

[Gabor Grothendieck]

Note that the gamma distribution seems to work well here.  After running
Andy's code run this to see both superimposed on the same graph:

gparms <- fitdistr(dd, "gamma")[[1]]
yyg <- dgamma(xx, gparms[1], gparms[2])
lines(xx, yyg, col=3)


On 5/18/05, apjaworski at mmm.com <apjaworski at mmm.com> wrote:
> 
> 
> Matt,
> 
> There is nothing wrong here.  I rerun your example and got the same
> parameters.  Your only "problem" is that you let the density plot use the
> default limits on x, which are not reasonable here since your data extends
> to over 200.  Try this:
> 
> hist(dd, freq=FALSE)  #dd is your original data
> xx <- seq(0, 300, by=.1)
> yy <- dweibull(xx, shape=1.5079, scale=60.2139)
> lines(xx, yy, col=2)
> 
> BTW, when you binned your data you also scaled it down by 20, so your scale
> parameter changed accordingly.
> 
> Cheers,
> 
> Andy
> 
> __________________________________
> Andy Jaworski
> 518-1-01
> Process Laboratory
> 3M Corporate Research Laboratory
> -----
> E-mail: apjaworski at mmm.com
> Tel:  (651) 733-6092
> Fax:  (651) 736-3122
> 
>             Matt Jadud
>             <jadudm at gmail.com
>             >                                                          To
>             Sent by:                  R-help at stat.math.ethz.ch
>             r-help-bounces at st                                          cc
>             at.math.ethz.ch
>                                                                   Subject
>                                       [R] 'fitdistr' and two views of the
>             05/18/2005 09:34          same data?
>             AM
> 
>             Please respond to
>                Matt Jadud
>             <jadudm at gmail.com
>                     >
> 
> Hello,
> 
> I have detailed (with pictures and whatnot) my question on my weblog at
> 
> http://www.cs-ed.org/blogs/mjadud/archives/2005/05/a_question_abou.html
> 
> The short version of the question is this:
> 
> When I ask 'fitdistr' to try and fit my distribution as a "weibull"
> distribution, it comes up with some rather wacky parameters.
> 
> If I take the same distribution, and do something like
> 
> newdist <- mapply(function(x) ((x %/% 20) + 1), origdist)
> 
> which effectively forces the data into a histogram, 'fitdist' on
> 'newdist' gives me an entirely different set of parameters.
> Distressingly, the parameters it gives me are, upon inspection, good;
> that is, the parameters reported fit the distribution of the original
> data much better than 'fitdist' of 'origdist'.
> 
> Unfortunately, I'm not savvy enough to tease this out beyond
> "inspection." The weblog entry has the original distribution, plots,
> and whatnot explaining my question in more detail. My question(s) are
> repeated at the bottom of the post, as well as this email address. Any
> help or insights are appreciated; no doubt, I've done something...
> well, wrong.
> 
> Many thanks,
> Matt
> 
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