R help - Aug 2007 - in cor.test, difference between exact=FALSE and exact=NULL

Pardon my ignorance, but is there a difference in cor.test between
exact=FALSE and exact=NULL when method=spearman?

Take for example:

x<-c(1,2,2,3,4,5)
y<-c(1,2,2,10,11,12)
cor.test(x,y, method="spearman", exact=NULL)

This gives an error message,
Warning message:  Cannot compute exact p-values with ties in:
cor.test.default(x, y, method = "spearman", exact = NULL)

However, when exact is changed to FALSE, this seems to run okay.

cor.test(x,y, method="spearman", exact=FALSE)

Question:  should this be exact = FALSE in the documentation and/or the code?

Thanks,
Andrew
MGH Cancer Center

Andrew Yee wrote:
> Pardon my ignorance, but is there a difference in cor.test between
> exact=FALSE and exact=NULL when method=spearman?
>
> Take for example:
>
> x<-c(1,2,2,3,4,5)
> y<-c(1,2,2,10,11,12)
> cor.test(x,y, method="spearman", exact=NULL)
>
> This gives an error message,
> Warning message:  Cannot compute exact p-values with ties in:
> cor.test.default(x, y, method = "spearman", exact = NULL)
>
> However, when exact is changed to FALSE, this seems to run okay.
>
> cor.test(x,y, method="spearman", exact=FALSE)
>
> Question:  should this be exact = FALSE in the documentation and/or the
code?
>
>   
No. The default is indeed NULL.

This implies that calculation of exact p-values will be attempted, and
when there are ties you get a warning (NB: not error) message.  Setting
exact=FALSE, no attempt is made and no warning is given.
> Thanks,
> Andrew
> MGH Cancer Center
>
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>   

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