Hi everybody,

I have a question related to this topic:

I want to analyze paired data using regression analysis to adjust for covariates. I used PROC MIXED statement in SAS and I modeled the difference as the Y in the model with my covariates as Xs. I have a problem regarding the interpretation of the output. So, basically, I want to know the difference in response adjusted for my covariates. However, the output gives me the parameter estimate for each predictor (for example, the difference in response between males and females for the gender covariates). How can I know the value of the actual difference adjusted for the covariates in the model?

So, basically, in independent data, I had a "Drug" column in my data to compare the difference in response between the 2 drugs (some patients took drug A and others took drug B). I did multiple linear regression using PROC GLM statement in SAS and I put the difference in response as Y and "Drug" as one of the Xs in my model beside other covariates.

Regarding the paired data, I don't have the "Drug" column because every patient took the 2 drugs. I want to compare the difference in response between the 2 drugs in each patient. So, here is my SAS code:

PROC MIXED DATA=PRA.PAIRED_1;

MODEL DIFF_PAIRED = SEX pre_CSBP_BB pre_CSBP_TD / SOLUTION CLPARM;

RUN;

QUIT;

DIFF_PAIRED is the difference in response between the 2 drugs.

The output:

Covariance Parameter Estimates

Cov Parm Estimate

Residual 323.59

Fit Statistics

-2 Res Log Likelihood 1018.5

AIC (Smaller is Better) 1020.5

AICC (Smaller is Better) 1020.5

BIC (Smaller is Better) 1023.3

Solution for Fixed Effects

Effect Estimate Standard Error DF t Value Pr > |t| Alpha Lower Upper

Intercept -59.6842 25.3976 115 -2.35 0.0205 0.05 -109.99 -9.3765

SEX -7.7764 3.2996 115 -2.36 0.0201 0.05 -14.3122 -1.2406

pre_CSBP_BB -0.8725 0.1466 115 -5.95 <.0001 0.05 -1.1628 -0.5821

pre_CSBP_TD 1.3641 0.1476 115 9.24 <.0001 0.05 1.0717 1.6566

Type 3 Tests of Fixed Effects

Effect Num DF Den DF F Value Pr > F

SEX 1 115 5.55 0.0201

pre_CSBP_BB 1 115 35.42 <.0001

pre_CSBP_TD 1 115 85.38 <.0001

So, how can I know the difference between the 2 drugs adjusted for the covariates in the model, like the case of unpaired data (I used the "Drug" parameter estimate from the output).

Thanks in advance