# Re: [R] chi square table

From: Berwin A Turlach <berwin_at_maths.uwa.edu.au>
Date: Fri, 07 Nov 2008 10:16:28 +0800

G'day Cruz,

On Fri, 7 Nov 2008 09:47:47 +0800

> Hi,
>
> How do we get the value of a chi square as we usually look up on the
> table on our text book?
>
> i.e. Chi-square(0.01, df=8), the text book table gives 20.090
>
> > dchisq(0.01, df=8)
> [1] 1.036471e-08
> > pchisq(0.01, df=8)
> [1] 2.593772e-11
> > qchisq(0.01, df=8)
> [1] 1.646497
> >
>
> nono of them give me 20.090

The value that your textbook denotes, presumably, with chi^2_0.01 (or some similar notatation) is in fact the 0.99 quantile of the chi-square distribution; which R readily calculates:

R> qchisq(0.99, df=8)
[1] 20.09024

<rant on>
That's the problem with introductory textbook whose author think they do the students a favour by using notation as z_alpha, z_0.01, z_(alpha/2) instead of z_(1-alpha), z_0.99, z_(1-alpha/2), respectively. In my opinion this produces in the long run only more confusion and does not help students at all. It just panders to intellectual laziness of (some) students and shows a great deal of confusion on the side of the authors.
I would search another textbook
<rand off>

Cheers,

Berwin

• Full address ============================= Berwin A Turlach Tel.: +65 6516 4416 (secr) Dept of Statistics and Applied Probability +65 6516 6650 (self) Faculty of Science FAX : +65 6872 3919 National University of Singapore 6 Science Drive 2, Blk S16, Level 7 e-mail: statba_at_nus.edu.sg Singapore 117546 http://www.stat.nus.edu.sg/~statba

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