Dear Dr. Marta,

Many thanks for the prompt help.

I have certain questions with respect to article 1 , are the standard deviation of repeated measurements on the same subject is the measurement error of that subject?

What is the meaning of the following phrase?

measurement error depends on the size of the measurement

And finally , if there is no relation between the sd and the mean , could we said that the device is reliable?

Many thanks in advance.

Omar.

Marta García-Granero <

[hidden email]> wrote: Hi Omar

OF> My colleague would like to examine accuracy or the

OF> reliability of a device , the device measuring teeth's strength.

OF> He has 50 teeth , he will measure the teeths strength

OF> today and after 2 or 3 days he will do the same process by using

OF> the same device , to examine the accuracy of the device. Could

OF> we use Paired-Samples T Test if the data follow the normal

OF> distribution or we should use Two-Related-Samples Tests if the

OF> data non normal?

Neither test you mention will answer your question (measurement

error), since both focus on whether there is a bias (systematic

difference between the two days). I suppose you want statistics to

verify the repeatability of a device (the ammount of random

variation). A p-value (obtained by any test) will not answer the

question. Take a look at these links:

http://bmj.bmjjournals.com/cgi/content/full/313/7059/744http://bmj.bmjjournals.com/cgi/content/full/313/7048/41http://www-users.york.ac.uk/~mb55/meas/Time ago I wrote some code for the methods discussed in those appers.

Let me know if you are interested in having it.

--

Regards,

Dr. Marta García-Granero,PhD mailto:

[hidden email]
Statistician

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"It is unwise to use a statistical procedure whose use one does

not understand. SPSS syntax guide cannot supply this knowledge, and it

is certainly no substitute for the basic understanding of statistics

and statistical thinking that is essential for the wise choice of

methods and the correct interpretation of their results".

(Adapted from WinPepi manual - I'm sure Joe Abrahmson will not mind)

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