[R] a bug with Student t-value??

[(Ted Harding)]

On 07-Nov-09 09:02:38, huaru wang wrote:
> Hello, every one,
> 
> Using the qt() function in R, I got a different Student t-value
> with the Student t table.
> e.g. the return value of function qt(0.95,9) is 1.833, while in
> table it is 2.262.
> 
> I do not know why the difference exists. I guess it may be a bug
> in R.
> -- 

Not at all. The reason is that your table gives the t-value for a
two-sided test (with equal tails), i.e. 0.025 in each tail.
The function qt() in R returns the t-value for a one-sided test,
i.e. 0.05 in the (by default) upper tail. Compare:

  qt(0.95,9)
# [1] 1.833113
  qt(0.975,9)
# [1] 2.262157

The second value agrees with your table. Many printed tables give
the two-sided values, rather than the one-sided.

Hoping this helps,
Ted.

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Date: 07-Nov-09                                       Time: 09:55:50
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