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slawson83
05-21-2007, 11:11 PM
Glenn,

This is a good illustration of the disparity between a statistically
significant difference and a functionally (or clinically) significant
difference. Your t-test may show a statistical difference in the
populations, but the difference is still negligibly small.

Statistics, even means, cannot create accuracy. If the accuracy of your
recording is to one decimal place, then your means etc will not be any
more accurate than that.

Best regards,
Sian

Dr. Sian E. M. Lawson
Centre for Rehabilitation and Engineering Studies (CREST)
School of Mechanical and Systems Engineering
University of Newcastle upon Tyne, U.K.
CREST: +44 (0)191 222 6170
Direct: +44 (0)191 222 8224
Fax: +44 (0)191 222 8600


> -----Original Message-----
> From: * Biomechanics and Movement Science listserver
> [mailto:BIOMCH-L@NIC.SURFNET.NL] On Behalf Of Glenn Fleisig
> Sent: 21 May 2007 15:34
> To: BIOMCH-L@NIC.SURFNET.NL
> Subject: [BIOMCH-L] Significant digits - a Quiz for our community
>
>
> Hi everyone,
>
>
>
> This is general question relevant to a lot of our research.
> It should be simple, but I don't recall the answer.
>
>
>
> How many digits should be used in reporting data?
>
>
>
> Here is an example. A biomechanist is trying to show a
> difference between Technique A and Technique B. He/she
> recruits 36 subjects and records each one of them performing
> both techniques, using equipment that measures each person to
> the nearest 0.1 unit. How many decimal places should be used
> in reporting the data? I can make a "common sense"
> determination, but is there some accepted procedure or guideline?
>
>
>
> Example data:
>
>
>
> TECHNIQUE A
>
> Subject 1: 13.3 units
>
> Subject 2: 35.0 units
>
> Subject 3: 22.2 units
>
> .
>
> .
>
> .
>
> Subject 36: 18.4 units
>
>
>
>
>
> TECHNIQUE B
>
> Subject 1: 12.9 units
>
> Subject 2: 35.0 units
>
> Subject 3: 21.9 units
>
> .
>
> .
>
> .
>
> Subject 36: 18.5 units
>
>
>
>
>
> Even though the mean within-subject difference is less than
> 0.1 units, a paired t-test reveals a statistically
> significant difference.
>
> Technique A mean: 25.33333 units
>
> Technique B mean: 25.36170 units
>
> Average difference : 0.02837 units
>
> Paired t-test p-value: 0.035
>
>
>
> How many decimal places would you use in reporting these mean
> values and difference in your paper? Why?
>
>
>
> Thanks in advance,
>
> - Glenn S. Fleisig, Ph.D.
>
> Glenn S. Fleisig, Ph.D., Smith & Nephew Chair of Research
> American Sports Medicine Institute
> 833 St. Vincent's Drive, Suite 100
> Birmingham, AL 35205
> (email) glennf@asmi.org
> (tel) 205-918-2139
> www.asmi.org
>
>
>
>
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