> Lee Chuntao's original question:
> >
> > In my dataset there is a variable v1, which is a string
> > variable. I try to use l.v1, after i set the time variable (eg,
> > tsset time). I don't know why stata do not permit timeseries
> > operators on string variables, it seems not reasonable for the
> > lag or lead operators, beyond the difference operators
< various answers >
Stata has a number of data types (byte, int, long, float, double, str) but
no explicit syntactical elements for variable types (nominal, ordinal,
interval, ratio). For computational convenience, most of Stata's statistical
and graphing procedures only work if the variable is stored as a non-string
data type, even if some statistical concepts in themselves do not require
numerical values. But some procedures (e.g. -tabulate-) work on strings as
well and you might never suspect there could be a problem.
The example from the current thread: Time series data on the state of a unit
could be any of the four variable types. Events could then be calculated as
a change in state from one time to another, but if you want to do this with
time-series operators in Stata, the variable has to be stored as a numerical
data type.
As long as there is no syntactical way to distinguish the four variable
types, there are other means. Value labels help translate numbers to text
for the reader as a compensation for having to convert textual information
to numerical form. The data management tools for performing these
conversions in Stata abound but perhaps someone could think about drafting
an FAQ or a tutorial on how to use them in the context of variable types (e.
g., "What do I do with my categorical/nominal variables to make them work
and display properly in Stata?"). (There are some already, e.g. on pie
charts.)
-- H?kon
[email protected]
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