\HeaderA{power.t.test}{Power calculations for one and two sample t tests}{power.t.test}
\keyword{htest}{power.t.test}
\begin{Description}\relax
Compute power of test, or determine parameters to obtain target power.
\end{Description}
\begin{Usage}
\begin{verbatim}
power.t.test(n = NULL, delta = NULL, sd = 1, sig.level = 0.05,
             power = NULL,
             type = c("two.sample", "one.sample", "paired"),
             alternative = c("two.sided", "one.sided"),
             strict = FALSE)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{n}] Number of observations (per group)
\item[\code{delta}] True difference in means
\item[\code{sd}] Standard deviation
\item[\code{sig.level}] Significance level (Type I error probability)
\item[\code{power}] Power of test (1 minus Type II error probability)
\item[\code{type}] Type of t test
\item[\code{alternative}] One- or two-sided test
\item[\code{strict}] Use strict interpretation in two-sided case
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
Exactly one of the parameters \code{n}, \code{delta}, \code{power},
\code{sd}, and \code{sig.level} must be passed as NULL, and that
parameter is determined from the others. Notice that the last two have
non-NULL defaults so NULL must be explicitly passed if you want to
compute them.

If \code{strict = TRUE} is used, the power will include the probability of
rejection in the opposite direction of the true effect, in the two-sided 
case. Without this the power will be half the significance level if the 
true difference is zero.
\end{Details}
\begin{Value}
Object of class \code{"power.htest"}, a list of the arguments
(including the computed one) augmented with \code{method} and
\code{note} elements.
\end{Value}
\begin{Note}\relax
\code{uniroot} is used to solve power equation for unknowns, so
you may see errors from it, notably about inability to bracket the
root when invalid arguments are given.
\end{Note}
\begin{Author}\relax
Peter Dalgaard.  Based on previous work by Claus
Ekstrøm
\end{Author}
\begin{SeeAlso}\relax
\code{\LinkA{t.test}{t.test}}, \code{\LinkA{uniroot}{uniroot}}
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
 power.t.test(n = 20, delta = 1)
 power.t.test(power = .90, delta = 1)
 power.t.test(power = .90, delta = 1, alt = "one.sided")
\end{ExampleCode}
\end{Examples}

