[R] errors in help("TDist")?

Emmanuel Paradis paradis at isem.univ-montp2.fr
Thu Nov 29 15:55:24 CET 2001

Dear all,

The help page on the t distribution says:

     The most used applications are power calculations for t-tests:
     Let T= (mX - m0) / (S/sqrt(n)) where mX is the `mean' and S the
     sample standard deviation (`sd') of X_1,X_2,...,X_n which are
     i.i.d. N(mu,sigma^2). Then T is distributed as non-centrally t
     with `df'= n-1 degrees of freedom and non-centrality parameter
     `ncp'= mu - m0.

1) It seems that power.t.test() uses ncp = sqrt(n) * (mu - m0)/sigma. Is
the above incorrect, or am I missing something?

2) I thought the paramaters of the normal distribution were mean and
standard-deviation: should one read "N(mu, sigma)" above?

3) I understand that the non-central t is obtained from the t by "sliding"
its distribution by 'ncp': the density of the former could thus be obtained
with dt(x, nu) + ncp. If so, is it worth adding this to the help?

Emmanuel Paradis
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