# [Numpy-discussion] QQ Plot in Python and Noncentral Chisquare

Yi, Chuang chuang.yi at bankofamerica.com
Thu Aug 16 09:05:22 EDT 2012

```Thank you Josef, df can be less than 1 as long as it is nonnegative:

http://stat.ethz.ch/R-manual/R-patched/library/stats/html/Chisquare.html

Here is an example in R:

> Z0<-rchisq(2,df=0.5,ncp =2)
> Z0
[1] 0.5056454 2.0427540

Thanks, Chuang

-----Original Message-----
From: numpy-discussion-bounces at scipy.org [mailto:numpy-discussion-bounces at scipy.org] On Behalf Of josef.pktd at gmail.com
Sent: Wednesday, August 15, 2012 9:59 PM
To: Discussion of Numerical Python
Subject: Re: [Numpy-discussion] QQ Plot in Python and Noncentral Chisquare

On Wed, Aug 15, 2012 at 10:59 AM, Yi, Chuang
<chuang.yi at bankofamerica.com> wrote:
> Hello,
>
>
>
> I am a new user of Python. I have a couple of questions that would
>
>
>
> QQ Plot in Python: I could not find any functions in either Numpy or Scipy
> to do QQ Plot of two vectors of data. For example, in Matlab, one could just
> call qqplot(X,Y), which will generate the QQ plot of vector X against vector
> Y. Would you please let me know which python package has similar
> functionality?
> Noncentral Chisquare: it looks to me that in Numpy and Scipy, the noncentral
> chisquare distribution only allows Integer degree of freedom. For example,
> the following code will produce an error message. In R, however, it also
> allows non-integer degree of freedom. Would you please let me know which
> python package has similar functionality?
>
> data = numpy.random.noncentral_chisquare(0.5, 2, 100)
> ValueError: df <= 0

restriction is >1 not integer

>>> np.random.noncentral_chisquare(1, 2, 100)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "mtrand.pyx", line 1957, in
mtrand.RandomState.noncentral_chisquare
(numpy\random\mtrand\mtrand.c:9847)
ValueError: df <= 0

>>> np.random.noncentral_chisquare(1.0001, 2, 100)
array([  5.75083221,   1.08050491,   0.69267684,   1.37806056,
2.39899   ,   0.31415666,   2.9202386

I don't know if the >1 restriction is justified or could be dropped.

Josef

>
>
>
> Thank you very much,
>
>
>
> Regards,
>
> Chuang
>
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