New Contributor
Posts: 2

Quantile for chi square

Hello,

I am trying to use SAS for the first time and I am stuck. My first homework problem is : Find the quantile for chi squared with 4 df at a probablity of .47.

How can I write this program?

sthudson0

Super User
Posts: 23,700

Re: Quantile for chi square

My stats is getting rusty these days, but I think you're looking for one of the following:

cumulative distribution function for chi square (CDF) - not likely

probability distribution function for chi square (PDF) - possible

Cumulative inverse - quantile (CINV) - likely

``````data want;
prob=0.47;
df=4;
y=probchi(0.47, 4);
z=cinv(0.47, 4); *<-most likely this number;
run;``````

New Contributor
Posts: 2

Re: Quantile for chi square

Thank you very much.  I wil lgive this a try.

Posts: 5,526

Re: Quantile for chi square

quantile('CHISQ', .47, 4); /* definitely */

PG
Posts: 1,246

Re: Quantile for chi square

Just a side note: It seems that the two functions are implemented differently. In this particular case the results differ in the last bit (i.e. 2^-51) on my Windows machine. (Irrelevant for practical purposes, of course.)

``````data _null_;
c=cinv(.47, 4);
q=quantile('CHISQ', .47, 4);
if c ne q then do;
put 'Not exactly equal.';
d=c-q;
put 'Difference: ' d;
put (c q) (hex16. /);
end;
run;``````

So, which one is closer to the "true" result?

Well, it depends ... For the exact argument of 0.47=47/100 c is closer, but for the internal 64-bit floating-point representation of 0.47 (which equals 0.47 - 0.96*2^-55) q is closer, says my computer algebra software.

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