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From |
Steve Samuels <sjsamuels@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Re: xsampsi produces inconsistent results |

Date |
Sun, 6 Jun 2010 06:34:45 -0400 |

In other words, the effect size (3.651 = delta/sd) is unrealistic; the program is computing n = 1 per group, with df = 0. That the program fails here is reasonable, although the error message could be better. Steve On Sun, Jun 6, 2010 at 12:00 AM, Joseph Coveney <jcoveney@bigplanet.com> wrote: > Michael McCulloch wrote: > > I'm using -xsampsi- to estimate sample sizes for a crossover experiment. > I compared two examples, each with alpha=0.25, and beta=0.20. > > Experiment 1 (where mean difference =30, and its standard deviation = > 45): > xsampsi, alpha(0.025) beta(0.20) delta(30) stddev(45) > > Experiment 2 (where mean difference =2.3, and its standard deviation > = 0.63): > xsampsi, alpha(0.025) beta(0.20) delta(2.3) stddev(.63) > > The first example produces a result, but the second leads to error > message: > "Need to install package 'nct' to compute non-central t distribution" > > However, I have installed -nct-. > > Any feedback would be appreciated! > > -------------------------------------------------------------------------------- > > The complete output of your second case is shown below. > > You can see that it's trying to compute a t statistic of an N of 1, which gives > rise to a negative degrees of freedom for the quantile computation, which gives > rise to a missing value for the t quantile, which when fed to -nct- (set trace > on to see the call to -nct-) gives rise to a nonzero return code, which is > trapped with the omnibus (and in this case erroneous) error message. > > -xsampsi- is user-written, not official. I recommend contacting the author > with the bug report. > > In the meantime, you can use simulation to do your power analysis. > > Joseph Coveney > > . xsampsi, alpha(0.025) beta(0.20) delta(2.3) stddev(.63) > Sample size for cross-over trial (AB/BA) > according to Senn 1991: 'Cross-over trials in clinical research' > Parameters below expressed in terms of the basic estimator, the treatment > contrast. > --------------- > > Alpha level = 0.025 Power = 0.800 > Zalpha = 2.241 Zbeta = 0.842 > > Detectable effect size = 2.300 > Std.dev. of basic estimator = 0.630 > > Approximate N (normal) = 1 > Critical value of t(df=-1,0.013) = . > Noncentrality parameter of t = 3.651 > Need to install package 'nct' to compute non-central t distribution > r(999); > > > * > * For searches and help try: > * http://www.stata.com/help.cgi?search > * http://www.stata.com/support/statalist/faq > * http://www.ats.ucla.edu/stat/stata/ > -- Steven Samuels sjsamuels@gmail.com 18 Cantine's Island Saugerties NY 12477 USA Voice: 845-246-0774 Fax: 206-202-4783 * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: Re: xsampsi produces inconsistent results***From:*Michael McCulloch <mm@pinestreetfoundation.org>

**References**:**st: xsampsi produces inconsistent results***From:*Michael McCulloch <mm@pinestreetfoundation.org>

**st: Re: xsampsi produces inconsistent results***From:*"Joseph Coveney" <jcoveney@bigplanet.com>

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