Proc Logistic categorical variable

[TJ87]

Hello,

I'm running Proc Logistic for a continuous variable that I have categorized as quartiles. I seem to get two different answers for the odds ratio estimate of Quartile 4 vs Quartile 1 depending on how I code the variable. The first (standard) program is:

proc logistic data=have;
class var_quartile (ref="1") /param=ref;
model outcome (event="1")= var_quartile;
run;

In a second program, I re-coded the predictor as a binary indicator:

data new;

set have;

if var_quartile=4 then var_new=1;

else if var_quartile=1 then var_new=0;

else var_new=.;

run;

proc logistic data=new;
class var_new (ref="0") /param=ref;
model outcome (event="1")= var_new;
run;

Could someone please tell me why SAS gives me two different answers for the same comparion? Thanks.

[Reeza]

What happens if you run your model as:

proc logistic data=have;
where var_quartile in (1, 4);
class var_quartile (ref="1") /param=ref;
model outcome (event="1")= var_quartile; 
run;

My guess - a rusty one - is the second model totally excludes the var_new with missing and that's affecting the variance estimates. 

[TJ87]

Hi Reeza,

That syntax gives me the same wrong answer as if I had used the indicator variable. I've used dummy variables (Quartile2, Quartile3, Quartile4) to calculate the correct answer, but I'd like to be able to use a binary variable for the comparison of interest. Thanks. 

[Reeza · Accepted solution]

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@TJ87 wrote:

Hi Reeza,

but I'd like to be able to use a binary variable for the comparison of interest. 

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The OddsRatio doesn't give you the correct comparison?

proc logistic data=have;
class var_quartile (ref="1") /param=ref;
model outcome (event="1")= var_quartile; 
oddsratio var_quartile;
run;