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Re: st: Line of equality in scatter plot
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
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Royal College of Surgeons in Ireland
Epidemiology Department,
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