You don't really want the diddling, since it gives meaningless values anyway...
For a pragmatic strategy, how about this?: (a) calculate the power at n=2, if bigger than target power, done, else (b) calculate n to reach target power, now guaranteed to have n > 2. Round upwards. Peter D. > On 15 Oct 2019, at 12:07 , Witold E Wolski <wewol...@gmail.com> wrote: > > Dear Peter, > > Yes, It is a technical issue and a matter of diddling around. And I > agree with your comment regarding the 2 observations. > I have several thousands variance estimates for which I need to > compute the sample sizes automatically. Using try statements is > typically the last thing I would like to resort too. > Is there an alternative implementation of power.t.test on CRAN which > could the diddling for me and return plausible sample sizes i.e. > integers. > > best regards > Witek > > On Fri, 4 Oct 2019 at 16:28, peter dalgaard <pda...@gmail.com> wrote: >> >> 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 >> >> >> >> >> >> >> >> >> > > > -- > Witold Eryk Wolski -- 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.
