R help - May 2013 - Chi square 1 d.f.

Dear Fellows,

I am trying to find the exact p-value for some values using chisquare test
with one d.f. So, for example the exact p-value for a calculated chi square
of 14.62 would be 0,0038. I got this data on another software but I would
like to know how to do it on R. Any advice will be really appreciated.

Thanks for your help!

All the best!

	[[alternative HTML version deleted]]

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Luis Fernando Garc?a Hern?ndez
> Sent: Thursday, May 16, 2013 2:41 PM
> To: r-help at r-project.org
> Subject: [R] Chi square 1 d.f.
> 
> Dear Fellows,
> 
> I am trying to find the exact p-value for some values using chisquare
> test
> with one d.f. So, for example the exact p-value for a calculated chi
> square
> of 14.62 would be 0,0038. I got this data on another software but I
> would
> like to know how to do it on R. Any advice will be really appreciated.
> 
> Thanks for your help!
> 
> All the best!
> 
Take a look at 

   ?pchisq

I think what you want is 

   pchisq(14.62, 1, lower.tail=FALSE)


Your other software has it wrong, or you are not interpreting/describing
something correctly.


Hope  this is helpful,

Dan

Daniel J. Nordlund
Washington State Department of Social and Health Services
Planning, Performance, and Accountability
Research and Data Analysis Division
Olympia, WA 98504-5204

On May 16, 2013, at 23:41 , Luis Fernando Garc?a Hern?ndez wrote:
> Dear Fellows,
> 
> I am trying to find the exact p-value for some values using chisquare test
> with one d.f. So, for example the exact p-value for a calculated chi square
> of 14.62 would be 0,0038. I got this data on another software but I would
> like to know how to do it on R. Any advice will be really appreciated.
> 
Hmmm,
> pchisq(14.62, 1, lower=FALSE)
[1] 0.0001315114 

so that doesn't seem to be what you are looking for.

More likely, what your software is doing is to calculate the exact distribution
of the chisquare statistic in the conditional distribution  of a 2x2 table given
both sets of marginals. Notice that this will depend on the particular table
that you are looking at.

I suspect this is in one of the packages, but it shouldn't be too hard to
code up explicitly using the hypergeometric distribution. The simulate.p.value
argument to chisq.test does the same thing but using simulation.

> Thanks for your help!
> 
> All the best!
> 
> 	[[alternative HTML version deleted]]
> 
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