This is mainly a technical issue with uniroot trying to go outside of its interval: (2, 1e7)
It is fairly easy to find an approximate solution by diddling a little by hand: > power.t.test(delta = 0.5849625, sd=0.01, n=1.04, sig.level=0.05)$power [1] 0.8023375 Notice, however, that 1.04 observations in each group makes no sense at all. In order to actually do a t-test you need at least 2 observations per group (since we assume equal group sizes) or you have no variance estimate. Already at sd=0.1, you are crossing the n=2 border, so for any smaller sd, you will just get higher power with n=2. (Also, anything with single-digit degrees of freedom for variance is probably expecting rather much regarding to Gaussian distribution of your data.) -pd > On 4 Oct 2019, at 14:30 , Witold E Wolski <wewol...@gmail.com> wrote: > > Hi, > > power.t.test works for some range of input parameters but fails otherwise. > >> power.t.test(delta = 0.5849625, sd=0.1, power=0.8, sig.level=0.05)$n > [1] 1.971668 >> power.t.test(delta = 0.5849625, sd=0.05, power=0.8, sig.level=0.05)$n > [1] 1.620328 >> power.t.test(delta = 0.5849625, sd=0.01, power=0.8, sig.level=0.05)$n > Error in uniroot(function(n) eval(p.body) - power, c(2, 1e+07), tol = tol, : > did not succeed extending the interval endpoints for f(lower) * f(upper) <= 0 > In addition: Warning message: > In qt(sig.level/tside, nu, lower.tail = FALSE) : NaNs produced > > I guessing that sd is very small compared with delta, hence the > problem. But what are allowed values (ratios) of delta and sd? > > Best > Witek > > > > > > > -- > Witold Eryk Wolski > > ______________________________________________ > R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. -- Peter Dalgaard, Professor, Center for Statistics, Copenhagen Business School Solbjerg Plads 3, 2000 Frederiksberg, Denmark Phone: (+45)38153501 Office: A 4.23 Email: pd....@cbs.dk Priv: pda...@gmail.com ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.