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Re: st: RE: FW: graphing ordinal panel data over time

From   Roger Newson <>
To   "" <>
Subject   Re: st: RE: FW: graphing ordinal panel data over time
Date   Tue, 8 Jun 2010 15:26:15 +0100

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


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

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

Roger B Newson BSc MSc DPhil
Lecturer in Medical Statistics
Respiratory Epidemiology and Public Health Group
National Heart and Lung Institute
Imperial College London
Royal Brompton Campus
Room 33, Emmanuel Kaye Building
1B Manresa Road
London SW3 6LR
Tel: +44 (0)20 7352 8121 ext 3381
Fax: +44 (0)20 7351 8322
Web page:
Departmental Web page:

Opinions expressed are those of the author, not of the institution.

On 08/06/2010 13:16, Maarten buis wrote:
--- Seed, Paul forwarding
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

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.

Hope this helps,

Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen

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