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From |
"Nick Cox" <n.j.cox@durham.ac.uk> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
RE: st: Line of equality in scatter plot |

Date |
Thu, 9 Oct 2008 15:43:30 +0100 |

-concord- is a user-written program whose latest public version was released through the Stata Journal in 2007. A even later version with tweaked dialog will be released with Stata Journal 8(4) later this year. See the help for -concord- for many further details, including references. Please note also that -concord- has been written up and revised intermittently over a period of 10 years now by Thomas Steichen and myself. Almost all the publications on -concord- are visible to all under the 3-year moving window policy of the Stata Journal (here including the Stata Technical Bulletin). Also in this domain note -pairplot- from SSC. This includes a variety of graphical handles. For example, as Ronan rightly says it is a good idea to plot difference versus mean. On the other hand, if it's a better approximation that two (strictly positive) variables vary multiplicatively then the more natural plot is the logarithmic equivalent, i.e. log of ratio vs geometric mean. See the help for -pairplot- for more details. Nick n.j.cox@durham.ac.uk Ronan Conroy 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. * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: Line of equality in scatter plot***From:*"LL Miller, Avon Longitudinal Study Parents and Children" <L.L.Miller@bristol.ac.uk>

**Re: st: Line of equality in scatter plot***From:*Ronan Conroy <rconroy@rcsi.ie>

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