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st: St: Re: graph over


From   "Allan Reese (Cefas)" <allan.reese@cefas.co.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: St: Re: graph over
Date   Thu, 2 Apr 2009 10:23:18 +0100

There's a generic answer to this that I prefer to call lateral thinking
rather than a kludge.

The data comprises three variables:
   ESL percentage        - the Y var
   time of observation   - the X var
   school (categorical)  - the group var

Scatterplot Y against X shows the overall pattern, and Stata has an
option to label each point with its group.
However, it's not immediately obvious how to plot the groups with
different symbols: msymbol(varname) is not available [HINT to StataCorp
- it would be nice.]  Nor how to join the points for each group.

The solution is to restructure the data, and one option would be reshape
long->wide, making ESL+school variables.  This might be more appropriate
if the time points of observations in each school were equivalent, and
cross-correlations would be calculated.

My usual solution is to keep the original cases but divide the
observations for each group into separate variables.
You can do this by a series of

   gen ESL1 = ESL if school==1   // etc
or
   xi , noomit i.school|ESL
   recode _I* (0=.)
except that (in my Stata 9.2) generates the dummy main effect variables
as well, contrary to Manual [HINT to StataCorp - bug? Known?]

Then 
   scatter _IschoolX* time
plots each school with a different symbol (within system limits for no
of groups!).  Any other twoway plot with a varlist would be similar.

There are lots of graph problems that are best tackled by splitting one
variable into groups. 

R Allan Reese
Senior statistician, Cefas
The Nothe, Weymouth DT4 8UB 

Tel: +44 (0)1305 206614 -direct
Fax: +44 (0)1305 206601 

www.cefas.co.uk 



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