Dear SAS L'ers,

I have been asked to implement the "difference-in-difference"

approach -following up on an old post last September. I received some very

helpful suggestions- but still don't have a concrete way to implement this.

I developed a way to directly code the coefficients in a data step-but its

still very time consuming since I have multiple comparisons and multiple

outcomes.

To recap: In its simplest form the DD method provides an assessment of the

difference between the changes in the means for a treatment and comparison

group over a fixed time period. If the mean value M of some outcome measure

for eligible children is indicated by subscript i and the value for children

not income eligible is indicated by subscript j, then we have

DD = (M post implementation i M pre implementation i) (M post

implementation j M pre implementation j).

Since I have survey data- I would need to do this with the SURVEYREG

procedure- (this ones for you David :)- some of us pay attention at least

some of the time). I also need a regression procedure since I need to do

this controlling for other covariates.

The only procedure- that I've come across that can directly give me at least

the terms in parenthesis- is the LSMEANS/PDIFF in the proc mixed procedure.

If I have at least the results of the terms in parenthesis- I can export it

to excel- and do the final difference using sqrt of sums of squares errors

to generate the standard errors for the DD. But, I can't figure out how to

do this in surveyreg.

The key program parts in mixed:

CLASS nchscat sudyr ;

MODEL shealth = nchscat sudyr nchscat*sudyr;

LSMEANS nchscat*sudyr /PDIFF;

Differences of Least Squares Means

Standard

Effect nchscat sudyr _nchscat _sudyr Estimate

Error DF t Value >r > |t|

nchscat*sudyr 1<0-<200 2 1<0-<200 3 -0.00106

0.002630 79E3 -0.40 0.6877

nchscat*sudyr 300+ 2 300+ 3 0.003644

0.002177 79E3 1.67 0.0941

I was wondering if anyone could please help me figure out- how to implement

this approach in surveyreg- when I have 4 time periods and 4 income groups-

and need to control for other factors.

Outcome: Health

Time: T1, T2, T3, T4

Income: INC1, INC2, INC3, INC4

I want to be able to make multiple comparisons over time and across income

groups- so:

INC1 compared to INC2

INC1 compared to INC3

INC1 compared to INC4

for T1 compared to T2, T3 compared to T2, T4 compared to T3, and T4 compared

to T2

So the D&D for just one of these comparisons would be-comparing INC1 and

INC4 for T1 and T2

DD=(INC1_T2 -INC1_T1)-(INC4_T2 -INC4_T1)

Your time and expertise is very much appreciated.

Mah-J