Re: st: Re: xsampsi produces inconsistent results

[Michael McCulloch <mm@pinestreetfoundation.org>]

Thank you Steve for the explanation.

On Jun 6, 2010, at 3:34 AM, Steve Samuels wrote:

> 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
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> > *   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
>
> *
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Best wishes,

Michael McCulloch, LAc MPH PhD
Pine Street Foundation
124 Pine Street
San Anselmo, CA 94960-2674
tel:	415-407-1357
fax: 	206-338-2391

*
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