Hi Everyone,

I am doing a Poisson regression (ppmlhdfe) with a specification using the Zipcode-specific-trend. The code pasted below is used for a difference-in-difference design. With MW is my outcome variable, POLICY is the treatment intervention and lineartrend is the dummy variable for generated by the Zipcode-specific-trend.

Code:
xi: ppmlhdfe MW POLICY i.year  lineartrend_*  heat_elect1 house_size1 graduates  median_age , absorb(zipcode) vce(cluster zipcode);
I got the results but the standard errors are not listed actually. An error message also appears which means I have something wrong in the variables that have lots of zeros: "Warning: variance matrix is nonsymmetric or highly singular"

my data example (I have lineartrend_1 to lineartrend_1345, but couldnt add them all here in the example):


Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input str5 zipcode float(year POLICY) double heat_elect1 int house_size1 double(graduates median_age) float(MW lineartrend_1 lineartrend_2 lineartrend_3)
"90001" 2011 0 18.7 11285                  1               26.6           0 2011    0    0
"90001" 2012 0 19.4 11168                 .7               26.5      .00306 2012    0    0
"90001" 2013 0 22.2 11119                 .6                 27     .075075 2013    0    0
"90001" 2014 0 23.4 11284                 .7               27.3  .004897959 2014    0    0
"90001" 2015 0 23.1 11417                 .5                 28   .09254032 2015    0    0
"90001" 2016 0 21.4 11381                 .6                 28    .0998847 2016    0    0
"90001" 2017 1 21.3 11345                 .6               28.7   .05581061 2017    0    0
"90001" 2018 1 21.3 11345                 .6  29.40000000000009   .05288979 2018    0    0
"90001" 2019 1 21.3 11345                 .6 30.100000000000136   .01230204 2019    0    0
"90002" 2011 0 13.4  8902                1.2               25.2   .03794612    0 2011    0
"90002" 2012 0 14.2  9329                 .9               25.4  .013152653    0 2012    0
"90002" 2013 0 15.2  9437                 .9               25.8           0    0 2013    0
"90002" 2014 0 16.5  9741                1.1               26.2 .0035714286    0 2014    0
"90002" 2015 0 18.7  9898                  1               26.4   .05865959    0 2015    0
"90002" 2016 0 19.9 10173                  1               27.5   .11238775    0 2016    0
"90002" 2017 1 20.5 10294                  1               27.7   .05637326    0 2017    0
"90002" 2018 1 20.5 10294                  1 27.899999999999977   .04104388    0 2018    0
"90002" 2019 1 20.5 10294                  1 28.100000000000023  .008405102    0 2019    0
"90022" 2011 0   22 13779                  1               29.5  .026052613    0    0 2011
"90022" 2012 0   24 13954                1.1               29.2   .06128286    0    0 2012
"90022" 2013 0 25.6 14113                1.2               29.7   .01880041    0    0 2013
"90022" 2014 0   29 14105                1.2               30.5  .026266836    0    0 2014
"90022" 2015 0 35.2 13998                1.2               31.6    .0320151    0    0 2015
"90022" 2016 0 42.1 14118                1.1               31.9   .13632184    0    0 2016
"90022" 2017 1 44.6 14222                1.3               32.1   .13176908    0    0 2017
"90022" 2018 1 44.6 14222                1.5 32.299999999999955  .065070406    0    0 2018
"90022" 2019 1 44.6 14222 1.6999999999999886               32.5  .036545917    0    0 2019
"90040" 2011 0 23.2  2681                1.5               30.8  .003469388    0    0    0
"90040" 2012 0 23.3  2595                1.5               32.1           0    0    0    0
"90040" 2013 0 26.7  2670                2.1               33.5  .012209714    0    0    0
"90040" 2014 0 25.8  2716                2.9               35.2  .013571428    0    0    0
"90040" 2015 0 22.6  2658                3.1               33.9  .018959183    0    0    0
"90040" 2016 0 26.4  2662                2.4                 34  .069083676    0    0    0
"90040" 2017 1 27.5  2750                2.5               34.6   .02416735    0    0    0
"90040" 2018 1 27.5  2750 2.5999999999999943 35.200000000000045  .004793878    0    0    0
"90040" 2019 1 27.5  2750 2.6999999999999886 35.799999999999955  .015468367    0    0    0
"90044" 2011 0 20.9 18490                1.9                 28           0    0    0    0
"90044" 2012 0 26.4 17847                2.3               28.9      .00552    0    0    0
"90044" 2013 0 28.3 17931                2.2               29.8           0    0    0    0
"90044" 2014 0 28.1 18119                1.8               30.2   .02049796    0    0    0
"90044" 2015 0 28.7 18303                1.7               30.5    .0346051    0    0    0
"90044" 2016 0 26.6 18631                  2               30.7   .07026633    0    0    0
"90044" 2017 1 20.5 19628                  2               30.8   .06878775    0    0    0
"90044" 2018 1 20.5 19628                  2 30.899999999999977   .04111633    0    0    0
"90044" 2019 1 20.5 19628                  2                 31   .02434388    0    0    0
"90059" 2011 0 26.6  8043                2.1                 25   .04813306    0    0    0
"90059" 2012 0 20.8  7941                  2               25.3   .04108569    0    0    0
"90059" 2013 0 19.8  7981                  2               25.8   .07947745    0    0    0
"90059" 2014 0   20  7850                2.3                 26  .028121226    0    0    0
"90059" 2015 0 21.6  8128                2.1               26.1   .05320306    0    0    0
"90059" 2016 0 21.8  8340                1.8               26.3   .12532449    0    0    0
"90059" 2017 1 22.1  8431                1.6               26.3   .05046939    0    0    0
"90059" 2018 1 22.1  8431  1.400000000000034               26.3   .04064082    0    0    0
"90059" 2019 1 22.1  8431 1.2000000000000455               26.3   .01812245    0    0    0
"90061" 2011 0   17  5753                1.6               28.2           0    0    0    0
"90061" 2012 0 16.2  5716                1.6               28.5 .0044387756    0    0    0
"90061" 2013 0 18.1  5727                1.6               27.7      .13793    0    0    0
"90061" 2014 0 18.2  5748                1.4               27.6  .020522857    0    0    0
"90061" 2015 0 21.7  5646                1.6                 28   .06546225    0    0    0
"90061" 2016 0 19.8  5564                1.7               29.1   .08671428    0    0    0
"90061" 2017 1 20.4  5591                1.2               29.9   .05559286    0    0    0
"90061" 2018 1 20.4  5591  .7000000000000455 30.699999999999818   .03022347    0    0    0
"90061" 2019 1 20.4  5591 .20000000000004547               31.5  .017564286    0    0    0
"90063" 2011 0 26.5 10275                1.1                 30  .024380613    0    0    0
"90063" 2012 0 32.4 10293                  1               30.1  .013806735    0    0    0
"90063" 2013 0 36.1 10434                1.1               29.7  .003265306    0    0    0
"90063" 2014 0 39.2 10593                1.2               30.9  .008803469    0    0    0
"90063" 2015 0 44.1 10769                1.2               30.8   .06187041    0    0    0
"90063" 2016 0 46.3 11010                1.3               31.2    .1611194    0    0    0
"90063" 2017 1 49.8 10956                1.8               31.8   .11878153    0    0    0
"90063" 2018 1 49.8 10956 2.2999999999999545  32.40000000000009   .07928061    0    0    0
"90063" 2019 1 49.8 10956 2.7999999999999545                 33  .005481633    0    0    0
"90201" 2011 0 20.7 21177                1.2               27.3      .00525    0    0    0
"90201" 2012 0 22.4 20960                1.1               27.3  .006835306    0    0    0
"90201" 2013 0 23.4 20768                1.1               27.9   .13054307    0    0    0
"90201" 2014 0 24.5 21064                1.1               28.2   .03972784    0    0    0
"90201" 2015 0 22.6 21031                1.2               28.2   .14367245    0    0    0
"90201" 2016 0 21.9 21018                1.1               28.5   .22825103    0    0    0
"90201" 2017 1 22.5 21121                1.1               28.9   .19656327    0    0    0
"90201" 2018 1 22.5 21121                1.1 29.299999999999955  .069312245    0    0    0
"90201" 2019 1 22.5 21121                1.1 29.700000000000045  .033293877    0    0    0
"90220" 2011 0 18.6 10181                2.6               27.7  .017239796    0    0    0
"90220" 2012 0   20 10094                2.1               27.6  .014676122    0    0    0
"90220" 2013 0 18.8  9764                1.9               29.6  .013577143    0    0    0
"90220" 2014 0 17.3 10162                1.7               30.3   .07018449    0    0    0
"90220" 2015 0 17.7 10397                2.3               31.2   .16516836    0    0    0
"90220" 2016 0 17.8 10589                  3               32.1    .3033388    0    0    0
"90220" 2017 1 18.9 10637                3.2               32.4   .12942041    0    0    0
"90220" 2018 1 18.9 10637 3.3999999999999773 32.700000000000045    .1204694    0    0    0
"90220" 2019 1 18.9 10637  3.599999999999966                 33   .03507143    0    0    0
"90221" 2011 0 20.8  9702                1.6               26.5 .0012244898    0    0    0
"90221" 2012 0 20.9  9780                1.6                 27  .010136327    0    0    0
"90221" 2013 0   20  9972                1.5               27.2  .031064695    0    0    0
"90221" 2014 0 20.1 10068                1.5               27.6   .04099306    0    0    0
"90221" 2015 0   20 10215                1.6                 28   .24060205    0    0    0
"90221" 2016 0 21.8 10451                1.3               28.8    .4446643    0    0    0
"90221" 2017 1 25.8 10526                  1               29.6   .25495246    0    0    0
"90221" 2018 1 25.8 10526  .6999999999999318 30.399999999999864    .1569949    0    0    0
"90221" 2019 1 25.8 10526 .39999999999997726 31.199999999999818    .0405949    0    0    0
"90222" 2011 0 22.4  6390                  2               27.9      .02081    0    0    0
end

Please, would anyone be able to suggest any solution?

Best regards
Ali