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Re: weibull plot on weibull scaled paper

• To: mathgroup at smc.vnet.net
• Subject: [mg116722] Re: weibull plot on weibull scaled paper
• From: DrMajorBob <btreat1 at austin.rr.com>
• Date: Fri, 25 Feb 2011 06:34:42 -0500 (EST)

You're probably right.

Bobby

On Thu, 24 Feb 2011 05:21:14 -0600, Darren Glosemeyer  
<darreng at wolfram.com> wrote:

> On 2/23/2011 4:24 AM, Bill Rowe wrote:
>> On 2/22/11 at 1:12 PM, btreat1 at austin.rr.com (DrMajorBob) wrote:
>>
>>> This gave the FindRoot:jsing error on the first try just now... and
>>> also the third try.
>>> dist = WeibullDistribution[7, 200];
>>> data = RandomVariate[dist, 300];
>>> ProbabilityScalePlot[data, "Weibull"]
>>> QuantilePlot[data, dist]
>>> So the error is not unusual.
>> Hmmm... If I copy and paste just
>>
>> dist = WeibullDistribution[7, 200];
>> data = RandomVariate[dist, 300];
>> ProbabilityScalePlot[data, "Weibull"]
>>
>> into a single cell then execute it, I see the error you
>> reported. Alternatively, if I type the above by hand into a
>> single cell I still see the error you reported consistently. Or
>> I can copy each line one by one pasting each into a cell then
>> execute each line before pasting the next and still get the error.
>>
>> But if I enter each line by hand into individual cells, I don't
>> see the error. So, there appears to be a bug somewhere. That is,
>> the various ways I've described above of doing the computation
>> should all behave identically.
>>
>>
>
> ProbabilityScalePlot and its friends call the internal parameter
> estimation code for FindDistributionParameters/EstimatedDistribution to
> get the estimates. The reason for the message is that the equations in
> the estimation code for WeibullDistribution were less numerically stable
> than they could have been and so convergence fails more often than it
> should when parameter values are somewhat large. This has been fixed for
> the next release.
>
> I think the fact that the example worked without error when you typed
> the inputs in separate cells is a red herring. The example will converge
> for some data sets and not for others. I suspect the random values you
> got in the case that worked just happened to give a convergent result
> while the random values you'd gotten in the other attempts did not.
>
> Darren Glosemeyer
> Wolfram Research
>


-- 
DrMajorBob at yahoo.com