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Re: st: Line of equality in scatter plot

From   Ronan Conroy <[email protected]>
To   "[email protected]" <[email protected]>
Subject   Re: st: Line of equality in scatter plot
Date   Thu, 9 Oct 2008 10:19:35 +0100

On 8 Oct 2008, at 15:59, LL Miller, Avon Longitudinal Study Parents and Children wrote:

I'm trying to assess the reliability of a continuous variable with the same variable assessed by a second person. I have used alpha var1 var2 to get Cronbachs alpha but wanted to see what it is doing visually. I created a scatter plot but can't find any option that will allow me to draw a line on the plot through the diagonal i.e. where the points should fall if there is
no difference in the two assessments.

Now that you have been told how to do it, I wonder if you would consider why you are doing it!

A Limits of Agreement plot is a better way of expressing the agreement between two raters, neither of whom is the gold standard. It shows the average of the two raters for each observation (the best estimate of the observation's true value) against the difference between the raters. Your method does not display the differences but instead leaves it to the person to judge them by using an angled line - something that most people find hard.

It allows you to detect situations in which there is more agreement in some scale regions than others (for example, examiners may agree pretty well on a really bad and a really good student but may disagree more about mid-range student performance).

This is available by installing -concord- which will also give you a concordance coefficient between the raters.

Ronan Conroy

[email protected]
Royal College of Surgeons in Ireland
Epidemiology Department,
Beaux Lane House, Dublin 2, Ireland
+353 (0)1 402 2431
+353 (0)87 799 97 95
+353 (0)1 402 2764 (Fax - remember them?)

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