Hello,

I'd like to do a differences-in-differences-in-differences analysis (DDD) using Stata 15.


I'm measuring the impact of removing financial incentives on quality indicator performance, in ~450 GP practices from 2016 to 2017. Financial incentives remained for two quality indicators during that period (my two controls).

I'd like to estimate the precise effect of removing the incentives on quality achievement, whilst adjusting for time-invariant factors pertaining to individual practices and quality indicators.

So far, I've done a basic DiD (with dummy variables for the pre- and post- incentive removal, and for treatment control).

My Stata command is:

reg Achievement Time Treated Time##Treated i.indicator i.practice



My output is:


Achievement | Coef. Std. Err. t P>|t| [95% Conf. Interval]
-------------+----------------------------------------------------------------
Time | -1.063959 .5073119 -2.10 0.036 -2.058322 -.0695965
Treated | 20.38859 .5700967 35.76 0.000 19.27117 21.50601
1.Time | 0 (omitted)
1.Treated | 0 (omitted)
|
Time#Treated |
1 1 | -4.78109 .5263227 -9.08 0.000 -5.812714 -3.749465
|
indicator |
2 | -10.22745 .5072466 -20.16 0.000 -11.22169 -9.233218
3 | -19.64932 .5071011 -38.75 0.000 -20.64327 -18.65537
4 | -8.730226 .5071011 -17.22 0.000 -9.724175 -7.736276
5 | -6.353807 .5075383 -12.52 0.000 -7.348613 -5.359001
6 | -9.002199 .5073981 -17.74 0.000 -9.99673 -8.007668
7 | -8.68239 .5071011 -17.12 0.000 -9.676339 -7.68844
8 | -1.745006 .5073923 -3.44 0.001 -2.739526 -.7504858
9 | -15.92869 .5071011 -31.41 0.000 -16.92264 -14.93474
10 | -18.55426 .5069612 -36.60 0.000 -19.54793 -17.56058
11 | -5.963069 .5071011 -11.76 0.000 -6.957018 -4.969119
12 | -21.0394 .5071011 -41.49 0.000 -22.03335 -20.04545
13 | -26.1869 .5071011 -51.64 0.000 -27.18085 -25.19295
14 | -8.664169 .5072464 -17.08 0.000 -9.658403 -7.669934
15 | -8.35708 .5073923 -16.47 0.000 -9.3516 -7.36256
16 | -7.64188 .5076896 -15.05 0.000 -8.636983 -6.646777
17 | 1.926681 .5071011 3.80 0.000 .9327313 2.92063
18 | 0 (omitted)
19 | -16.04299 .5072464 -31.63 0.000 -17.03723 -15.04876
20 | -13.34192 .5071011 -26.31 0.000 -14.33587 -12.34797
21 | -7.2508 .5071011 -14.30 0.000 -8.244749 -6.25685
22 | -1.456801 .5105569 -2.85 0.004 -2.457524 -.4560785

..... (and so on...)


So the effect of removing financial incentives was a drop of -4.8 % points in performance.


I think this approach needs refining, but I'd appreciate some help/advice on this:

1) I'm using (unbalanced) panel data, so does this need a different regression command?

2) Within the 'treated' group of indicators, there are two types of indicator (ones that measure health outcomes, and ones that measure clinical processes); so I'd like to do a DDD analysis to examine whether there was a significant difference in outcome between the two types. Please can you advise how to do this? (Is it a case of using '0', for the control group dummy, '1' for one type of indicator, and '2' for the second type)

3) One of the quality indicators gets omitted due to collinearity. Why would this be? Does it matter?

4) Is it OK that I'm combining two quality indicators as a control? (they are both '0' in the Treatment dummy variable)

5) I'd also like to assess whether there was any difference in relative variance between the treatment and control group. i.e. were quality scores less consistent between practices, once the financial incentive was removed (relative to the control group). How can I calculate this on Stata, with confidence intervals?

Many thanks for your help.