Dan,

Precision of a measuring device, by definition, is the repeatability you

find using the device (like a tolerance on a milling machine for example).

It is different from the accuracy of the device (how close the measurement

is to the true value). To obtain the precision of something like a measuring

device, I don' t know of any other method than collecting an adequate sample

size on one standard test piece and calculate standard deviation, as you

suggest.

However, to me, when we talk about precision of an experiment, it is

referring to the power (often ignored), defined as 1-beta, where:

Beta = P(type II error)

Thus, power can be interpreted as P(reject null hypothesis|hypothesis is

false)

Beta, however, is also dependent on sample size and standard deviation.

The point: if your experiment is to determine difference within the samples,

it sounds like a randomized complete block design will work well, using

differing samples as blocks. If there are differences in samples, a post-hoc

test can be used to determine which one(s) are significantly different

statistically.

Thus, the precision would be the magnitude of difference you can reject the

null hypothesis with a specified power. This can be done via tables or with

many statistical software packages.

Any other thoughts on the topic out there?

Dave

David Hebert

Research Engineer, Motion Analysis Laboratory

Shriners Hospital For Children

Springfield, MA 01104-2396

413-735-1265

dhebert@shrinenet.org

-----Original Message-----

From: Barker, Dan (RGH) [mailtoan.Barker@RGH.SA.GOV.AU]

Sent: Monday, March 27, 2000 6:04 AM

To: BIOMCH-L@NIC.SURFNET.NL

Subject: Precision of an experiment

Dear Biomech-l subscribers

To calculate the precision of a measuring device or experiment it is usual

to repeat the test a number of times ON ONE SPECIMEN, calculate the mean and

SD of the variable of interest i.e. strain, and then calculate the

coefficient of variation. CoV=standard dev./mean.

Is there however another method to assess precision when (for example 4)

DIFFERENT SAMPLES are tested twice each i.e. 8 measures (4 paired

measurements)? Therefore instead of multiple repeated tests of one specimen,

I have multiple specimens with only one repeated measure of each.

Thanks in advance

Dan Barker

Dan.Barker@rgh.sa.gov.au

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