determining variables that are constant within each id (stacked dataset) - sas

Question

I inherited a poorly documented person-month dataset that does not have a matching person-level dataset. I want to determine which of the variables in the person-month dataset are actually person-level variables (constant for all observations with a particular id), such as you would expect for date of birth. Simplistic example:
id month dob race tx weight
1 1 4058 1 1 105
1 2 4058 1 1 107
1 3 4058 1 2 108
2 1 1622 2 1 153
2 2 1622 2 3 153
2 3 1622 2 2 153
In this example, dob and race are fixed within an individual but tx and weight vary by month within an individual.
I have come up with a clumsy solution: use proc means to calculate the standard deviation of all numeric variables BY id, and then take the maximum of those standard deviations. If the maximum of the std of a variable is 0, there is no variance of that column within any individual, and I can flag that variable as being fixed (or person-level).
I feel like I'm missing a simpler statistical test to determine which of my hundreds of variables are fixed within each individuals and which vary within an individual's observations. Any suggestions?
pT

Answer 1

I would use the NLEVELS option in PROC FREQ. This gives you the number of unique values for each variable, so you're looking for variables with a unique value (nlevels) of 1.
Here's the code, you'll need to sort the data by id beforehand if not done already.
data have;
input id month dob race tx weight;
cards;
1 1 4058 1 1 105
1 2 4058 1 1 107
1 3 4058 1 2 108
2 1 1622 2 1 153
2 2 1622 2 3 153
2 3 1622 2 2 153
;
run;
ods select nlevels;
ods output nlevels=want;
ods noresults;
proc freq data=have nlevels;
by id;
run;
ods results;

Answer 2

I don't think there's a 'simple statistical test' beyond what you have worked out - standard deviation, or even MIN/MAX (which is about the same). I'd probably just do it in PROC SQL, unless there are a huge number of variables; this allows you to use character variables also.
%macro comparetype(var);
max(&var.) = min(&var.) as &var.
%mend comparetype;
proc sql;
select min(origin) as origin, min(type) as type, min(drivetrain) as drivetrain,
min(msrp) as msrp,min(invoice) as invoice,min(enginesize) as enginesize from (
select make,
%comparetype(origin),
%comparetype(type),
%comparetype(drivetrain),
%comparetype(msrp),
%comparetype(invoice),
%comparetype(enginesize)
from sashelp.cars
group by make
);
quit;