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On Wed, Nov 11, 2009 at 10:25 AM, Henderson, Robin <
[log in to unmask]> wrote:
> 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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