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MathGroup Archive 2002

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Re: Accuracy and Precision

  • To: mathgroup at smc.vnet.net
  • Subject: [mg37058] Re: Accuracy and Precision
  • From: pkosta2002 at yahoo.com (Peter Kosta)
  • Date: Mon, 7 Oct 2002 05:26:16 -0400 (EDT)
  • References: <anp065$qtb$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Andrzej Kozlowski <andrzej at tuins.ac.jp> wrote in message news:<anp065$qtb$1 at smc.vnet.net>...
> On Friday, October 4, 2002, at 06:01 PM, DrBob wrote:
> 
>[...]
> 
> I would say this is correct and show that SetPrecision is very useful 
> indeed. It tells you (what of course you ought to already know in this 
> case anyway) that machine precision will not give you a realiable 
> answer in this case. If you know your numbers with a great deal of 
> accuracy you can get an accurate answer:
> 
> In[24]:=
> f = SetAccuracy[333.75*b^6 + a^2*(11*a^2*b^2 - b^6 -
>          121*b^4 - 2) + 5.5*b^8 + a/(2*b), 100];
> a=SetPrecision[77617.,100];  b = SetPrecision[33096.,100];
> 
> 
> In[26]:=
> {f, Precision[f]}
> 
> Out[26]=
> {-0.82739605994682136814116509547981629199903311578438481991\
> 781484167246798617832`61.2597, 61}
> 

Congratulations! You just requested accuracy of 100 for f and got 61 (
to convince yourself add Accuracy[f] to In[26]). If In[24] one
replaces SetAccuracy by SetPrecision the result is similar.

PK

> Again you can be pretty sure that you got an accurate answer, provided 
> of course your original setting of precision was valid.
> 
> Honestly, to say that SetPrecision and SetAccuaracy are useless is one 
> of the silliest thing I have read on this list in years.
> 
> 
> >
> Andrzej Kozlowski
> Yokohama, Japan
> http://www.mimuw.edu.pl/~akoz/
> http://platon.c.u-tokyo.ac.jp/andrzej/


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