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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