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

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

Subject |
RE: st: RE: New package -wridit- on SSC |

Date |
Tue, 28 Feb 2012 17:39:53 +0000 |

Paul's post makes me suspect that my using the name -ridit()- for the -egen- function in -egenmore- (SSC) was a bad idea. That said, I don't feel energetic enough to change it, not least because I suspect that the original sense of Bross is no longer widely used. I do respect history, however. That interesting detail aside, I have used ridits, or the same beast by any other name, in this way. When plotting cumulative probabilities for an ordinal variable, using the operator < produces a lower probability of 0 and using the operator <= produces an upper probability of 1. Neither treats categories symmetrically and both are inconvenient if you feel tempted to plot on a logit scale, as I often am. So, using centred cumulative probabilities, meaning probability of values below + (1/2) probability of this value, are useful graphically. The point is discussed, indeed laboured, in Section 5 of SJ-4-2 gr0004 . Speaking Stata: Graphing categorical and compositional data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N. J. Cox Q2/04 SJ 4(2):190--215 (no commands) discusses graphical possibilities for categorical and compositional data and in the help file of -distplot- (latest public version downloadable from SJ 10-1 files). A grandparent of all this is possibly Galton, F. 1907. Grades and deviates. Biometrika 5: 400-406. In a now widely used notation and terminology Galton proposed plotting positions (i - 1/2) / n. Nick n.j.cox@durham.ac.uk -----Original Message----- From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Seed, Paul Sent: 28 February 2012 16:41 To: statalist@hsphsun2.harvard.edu Subject: Re: st: RE: New package -wridit- on SSC Thanks as ever to Roger & Nick for their work on this. I went back to the original article by Bross to try to understand what ridits were: Bross, I. D. J. 1958. How to use ridit analysis. Biometrics 14: 38-58. http://www.jstor.org/stable/2527727 The name ridit actually derives by analogy from probit and logit, and is in part "Relative to an Identified Distribution". Neither -wridit- not -egen , ridit()- have options to specify the identified distribution, as Bross intended. However, it is possible to fudge it: Assume I want ridits for rep78 in the auto dataset, and want to use the US subsample as my Identified Distribution (or reference distribution). ************************** * Begin example analysis * ************************** sysuse auto, clear egen ridit_rep78_usa = ridit( rep78) if foreign == 0 bys rep78 (foreign) : replace ridit_rep78_usa = ridit_rep78_usa[1] if foreign[1] == 0 bys foreign : summ ridit_rep78_usa * Contrast this with using all the data as the reference group: egen ridit_rep78 = ridit( rep78) bys foreign : summ ridit_rep78 * The means have shifted by a fixed amount, and the SD have changed slightly. ************************** * End example analysis * ************************** > Roger B. Newson > > Thanks as always to Kit Baum, a new package -wridit- is now available > for download from SSC. In Stata, use the -ssc- command to do this. > > The -wridit- package is described as below on my website, and calculayes > weighted ridits for a variable. Zero weights are allowed, in which case > the ridits for the observations with zero weights are relative to the > weight distribution in the observations with non-zero weights. Ridits, > and the left, right and central inverse ridits, are important in rank > statistics, which, strictly speaking, are really ridit statistics. They > are also potentially useful in spline statistics, where the user might > want to define a spline in the ridit of an X-variable, instead of in the > X-variable itself. > > I would like to thank Nick Cox for writing the -ridit()- function of the > -egenmore- package, which generates unweighted ridits, and from which I > borrowed a few ideas for -wridit-. I slightly revised the algorithm for > -wridit-, in order to avoid the small numerical accuracy issues > associated with adding a small probability to a large probability. > > --------------------------------------------------------------------------- > package wridit from http://www.imperial.ac.uk/nhli/r.newson/stata10 > --------------------------------------------------------------------------- > > TITLE > wridit: Generate weighted ridits > > DESCRIPTION/AUTHOR(S) > wridit inputs a variable and generates its weighted ridits. > If no weights are provided, then all weights are assumed > equal to 1, so unweighted ridits are generated. > > Author: Roger Newson > Distribution-Date: 22february2012 > Stata-Version: 10 > > INSTALLATION FILES (click here to install) > wridit.ado > wridit.sthlp > --------------------------------------------------------------------------- > (click here to return to the previous screen) > * * 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/ * * 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**:**Re: st: RE: New package -wridit- on SSC***From:*"Seed, Paul" <paul.seed@kcl.ac.uk>

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