Exactly one of the parameters `n`

, `delta`

, `power`

,
`sd`

, and `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 `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.