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View Full Version : Re: Stats Power. Report Confidence Limits - p values

Mauricio Barrero
01-26-2005, 03:24 AM
Statisticians correct if I'm wrong but in this example you cannot try to
establish a significant difference with an 80% of confidence. In a student
test you compare average values of two groups. Depending on the differences
between this values, their standard deviation and the sample size you can
establish a statistically significant difference (usually people set alpha
to 5% meaning that there is only 5% probability that if a difference is
found is just due to chance alone).

In this example you have to establish first the groups you want to establish
a significant difference: My guess TMJ through plastic splint and non
treatment (since this 80% is solely based on success using this method in
relation to failure, and not related to other methods)

Even if TMJ through plastic splint was only 1% successful for a large enough
sample size it would be statistically significant to non treatment 0%
(success) with basically a 100% confidence (this assuming that the variation
of these percentages is 0 or close to 0, if not then to get closer to 100%
confidence you'll need a larger sample size)

Same thing if you are comparing two methods with say 60% and 80% success
rate and having a large enough sample size (that accurately describe this
percentages) you can also establish a statistically significant difference.
Again if the variation of these percentages is small and the sample size
large enough you can achieve a percentage of confidence close to 100%.

Mauricio

----- Original Message -----
From: "Bruce Etnyre, Ph.D., P.T."
To:
Sent: Wednesday, January 26, 2005 11:34 AM
Subject: Re: [BIOMCH-L] Stats Power. Report Confidence Limits - p values

> Would anyone like to comment on clinical significance?
>
> For example, my dentist can completely cure temporomandibular joint
> dysfunction (TMJ) in 80% of patients by applying a plastic splint to
> the teeth. Scientifically, this is not statistically significant. It
> does not even approach statistical significance. So statistically we
> would accept the null hypothesis that there is no difference in the
> treatment of TMJ by using this device or not because the level of
> confidence is only 80%.
>
> Statistically, my dentist is a failure, but clinically, a world-beater!
>
> BE
>
> At 9:58 AM -0500 1/26/05, Dr. Chris Kirtley wrote:
> >This brings up an interesting issue. I spoke to a statistician last year
who
> >told me that the only reason Fischer came up with these tables, and
indeed the
> >whole concept of significance testing, was because there were no
computers
> >available at that time. He considered T-tests, ANOVAs etc. to be stopgap
> >methods that could to be used to get an approximate estimate until better
> >computing power came along (as he expected it would).
> >
> >If Fischer were to be alive today, he would likely be appalled that we
are
> >still using his extremely simplified methods. Apparently any decent real
> >statistician worth his salt these days performs a simulation in order to
> >compute the likelihood of error. I was never able to find out how
> >this is done,
> >but perhaps someone else on the list can enlighten us? It really is
> >time all of
> >us in biomechanics moved into the modern age!
> >
> >Bryan Kirking wrote:
> >
> >> To comment and question some of Dr. Allison's insight:
> >>
> >> >>My understanding of the arbitrary "line in the sand" of 0.05 was
> >> >>originally due to the choice of the original tables (pre computer)
> >
> >--
> >Dr. Chris Kirtley MD PhD
> >Associate Professor
> >Dept. of Biomedical Engineering
> >Catholic University of America
> >Washington DC 20064
> >Alternative email: kirtleymd@yahoo.com
> >
> >-----------------------------------------------------------------
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> >For information and archives: http://isb.ri.ccf.org/biomch-l
> >-----------------------------------------------------------------
>
>
> --
> Bruce Etnyre, Ph.D., P.T.
> Kinesiology Department
> Professor and Chair
> Rice University
> 6100 Main MS 545
> Houston, Texas 77005
> USA
> etnyre@rice.edu
> Phone: (713)348-5936 or 8816
> FAX:(713)348-8808
>
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