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

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

Subject |
RE: st: RE: FW: graphing ordinal panel data over time |

Date |
Tue, 8 Jun 2010 15:33:24 +0100 |

Maarten's -proprcspline- (SSC) mentioned below seems to have the edge here as directly respecting the constraint that the proportions necessarily add to 1. I don't think that this machinery of Roger's does that, except by approximation. It's probably a fine point for graphics, but doing it properly is always a good idea. Nick n.j.cox@durham.ac.uk Roger Newson If Paul wants to track the proportions with the 3 values over time, with confidence limits, then Paul might define 3 identifier variables identifying the 3 values, and model the progress of the means (proportions) of these with time (possibly using -ci- with -statsby- to make an output dataset with 1 obs per time point), and then use -eclplot- to produce the confidence interval plots, Alternatively, if time is a continuous variable, then Paul might prefer to use a spline regression model of log(p/(1-p)) with respect to time, using the -frencurv- module of the -bspline- package to compute a spline basis whose parameter values are values of the spline at points on the time axis, and then use -logit-, -logistoc- or -glm-, with the -noconst- option, to define confidence intervals for the p/(1-p) values at these time points, and then to use -parmest- to produce an output dataset with 1 observation per parameter and data on estimates and confidence limits, and then to do an end-point transformation of the confidence intervals to the original probability scale before plotting the confidence intervals over time using -eclplot-. The examples of Newson (2001) and Newson (2004) might be helpful here. If Paul is principally interested in measuring the ordinal association of this 3-level ordinal variable with time, then the -somersd- package, downloadable from SSC, might provide a way to do this. I hope this helps. Best wishes Roger Newson R. 2001. Splines with parameters that can be explained in words to non-mathematicians. Presented at the 7th UK Stata User Meeting, 14 May, 2001. Download from http://ideas.repec.org/p/boc/usug01/11.html Newson R. 2004. B-splines and splines parameterized by their values at reference points. (This is a manual for the bspline package, downloadable from the SSC archive site, and is a post-publication update of a Stata Technical Bulletin article.) Download from http://www.imperial.ac.uk/nhli/r.newson/papers.htm#program_manuals On 08/06/2010 13:16, Maarten buis wrote: > --- Seed, Paul forwarding a.hense@jpberlin.de >> I have got an ordered outcome variable with three values >> over a period of 20 years. To describe the data, I want >> to show how the three values of my outcome variable have >> developed over time. > > --- On Tue, 8/6/10, Nick Cox wrote: >> There are several possible graphs here. For example, you >> can -contract- and then just draw line plots of frequencies >> or percents. Another way to do it directly is using >> -catplot- (SSC). > > Yet another way would be look at -proprcspline-, see > -ssc d proprcspline- and > <http://www.maartenbuis.nl/software/proprcspline.html> > > An important difference is that -proprcspline- also applies > certain amount of smoothing, which can for example be helpful > when some the of the periods contain relatively few cases, such > that the changes between periods becomes too eratic. > * * 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: FW: graphing ordinal panel data over time***From:*Maarten buis <maartenbuis@yahoo.co.uk>

**Re: st: RE: FW: graphing ordinal panel data over time***From:*Roger Newson <r.newson@imperial.ac.uk>

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