I have two variables - spline variable of height and another continuous variable. How do I write contrast to get difference in spline effects of height with respect to a reference at height=60 for interacting effect of weight? I have written the code but the ESTIMATE results do not coincide with the manual calculation. Any tip would be greatly appreciated. Thanks.
**sample data;
data mydata;
call streaminit(12345);
/* Generate 2,000 matched case-control strata (1 case, 3 controls per stratum) */
do stratum_id = 1 to 2000;
/* Assign a random Zip code AC percentage to the stratum (0% to 100%) */
ac_percent = rand("Uniform", 0.10, 0.90);
do obs = 1 to 4;
/* Temperature range: 5°C to 35°C */
temp = rand("Uniform", 5, 35);
/* First observation in stratum is the case (Y=1), others are controls (Y=0) */
if obs = 1 then Y = 1;
else Y = 0;
output;
end;
end;
run;
proc phreg data=mydata;
strata stratum_id;
/* Define B-spline for temperature */
effect temp_bs = spline(temp / degree=3 knotmethod=equal(4));
/* Y=0 as censoring variable simulates case-control outcome */
model obs*Y(0) = temp_bs temp_bs*ac_percent / ties=discrete;
ESTIMATE 'Log-OR (Temp=5 vs 20) at AC=0.10' temp_bs [1, 5] [-1, 20] temp_bs*ac_percent [1, 0.10 5] [-1, 0.10 20];
store work.spline_model;
run;
***this does not coincide with the following code;
data grid_data;
do temp = 5 to 35 by 0.5;
do ac_percent = 0.10 to 0.90 by 0.05;
output;
end;
end;
run;
/* Set Reference Temperature T0 = 20°C */
data ref_data;
temp = 20;
/* AC percentage value here cancels out when taking differences */
ac_percent = 0;
run;
/* Score grid points */
proc plm restore=work.spline_model;
score data=grid_data out=grid_scored pred=link_grid stderr=se_grid; *pred=xbeta;
run;
/* Score reference point (T0 = 20°C) */
proc plm restore=work.spline_model;
score data=ref_data out=ref_scored pred=link_ref stderr=se_ref;; *pred=xbeta;
run;
/* Merge reference value to grid and compute Odds Ratio relative to 20°C */
data plot_data;
if _N_ = 1 then set ref_scored(keep=link_ref);
set grid_scored;
* Log Odds Ratio = Log-hazard(T) - Log-hazard(T0=20°C);
log_or = link_grid - link_ref;
or_val = exp(log_or);
run;
