Hi David
In general you should average the variances in order to arrive at an
overall estimate of variability when the sample sizes are the same.
Is the membrane produced in a roll like cling film and are you taking a
series of measurements across the roll at various locations across its
length? If the answer is yes then control charting has something to
offer.
Best Wishes
Robin
G Robin Henderson
Stroke Audit Coordinator
Royal Infirmary of Edinburgh
0131 242 6934
-----Original Message-----
From: A UK-based worldwide e-mail broadcast system mailing list
[mailto:[log in to unmask]] On Behalf Of David D
Sent: 10 November 2009 10:03
To: [log in to unmask]
Subject: Averaging standard deviations
Hello,
we estimate width of a membrane by measuring electrical conductivity at
various points. The measuring equipment passes three electrical pulses
at each point and reports the mean conductivity the standard deviation
of the conductivity. This process is repeated in several points of the
membrane surface. Our task is to estimate its average width, as well as
the stdev. The former is a simple average of the measurements.
How do you estimate the stdev of the width?
Thank you
David
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