R help - May 2009 - anyone know how to calculate chi square value from P value?

On 5/11/2009 3:36 PM, Anyuan Guo wrote:
> Dear all,
>     I have P value of a list of markers. Now I need the chi square value 
> with degrees of freedom 2.
>     I noticed there are several Chisquare functions (dchisq, pchisq, 
> qchisq, and rchisq), but it seems all are not for my purpose.
>     In microsoft excel, there is a function CHINV to do this, such as 
> CHINV(0.184, 2) is 3.386, that means the chi square value for P value 
> 0.184, degree 2 is 3.386.
>     Does the R package has some function to do this?
qchisq(0.184, df=2, lower=FALSE)

Duncan Murdoch

On Mon, 2009-05-11 at 12:36 -0700, Anyuan Guo wrote:
> Dear all,
>     I have P value of a list of markers. Now I need the chi square value 
> with degrees of freedom 2.
>     I noticed there are several Chisquare functions (dchisq, pchisq, 
> qchisq, and rchisq), but it seems all are not for my purpose.
>     In microsoft excel, there is a function CHINV to do this, such as 
> CHINV(0.184, 2) is 3.386, that means the chi square value for P value 
> 0.184, degree 2 is 3.386.
>     Does the R package has some function to do this?
> 
> Thanks
> 
> Anyuan
It never hurts to read the documentation:

?qchisq

for example. Here "q" stands for "quantile." You want the
Chi-square
value that has 0.184 area to the left. To compute this:
> qchisq(1-0.184,2)
[1] 3.385639

You have to subtract 0.184 from 1, because qchisq finds the value with
the given area below it. To double check the result:
> 1-pchisq(3.385639,2)
[1] 0.184


Rick B.

On 11-May-09 19:36:00, Anyuan Guo wrote:
> Dear all,
>     I have P value of a list of markers. Now I need the chi square
> value with degrees of freedom 2.
>     I noticed there are several Chisquare functions (dchisq, pchisq, 
> qchisq, and rchisq), but it seems all are not for my purpose.
>     In microsoft excel, there is a function CHINV to do this, such as 
> CHINV(0.184, 2) is 3.386, that means the chi square value for P value 
> 0.184, degree 2 is 3.386.
>     Does the R package has some function to do this?
> 
> Thanks
> Anyuan
Yes, and you already looked at it but apparently did not recognise it!

Either:

  qchisq(1-0.184, 2)  ## (note "1-0.184", since 0.184 is the upper
tail)
# [1] 3.385639

Or:

  qchisq(0.184, 2, lower.tail=FALSE) ## Default for lower.tail is TRUE
# [1] 3.385639

Enter ?qchisq for more informatio0n on this and related function.
Ted.

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Date: 11-May-09                                       Time: 18:15:34
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Dear all,
    I have P value of a list of markers. Now I need the chi square value 
with degrees of freedom 2.
    I noticed there are several Chisquare functions (dchisq, pchisq, 
qchisq, and rchisq), but it seems all are not for my purpose.
    In microsoft excel, there is a function CHINV to do this, such as 
CHINV(0.184, 2) is 3.386, that means the chi square value for P value 
0.184, degree 2 is 3.386.
    Does the R package has some function to do this?

Thanks

Anyuan