I'm using the difference-in-difference technique to investigate the impact of a law reform (effective as of October 2012) intended to lower entry barriers for setting up a new business.
The treatment group consists of Dutch firms, whereas the control group are Belgian firms. Both the pre-treatment and post-period are precisely 12 months. My data is repeated cross-sections as it consists of the daily number of new firms. I use the nbreg command because my data is heavily dispersed count data.
My current analysis appears to show that the law change has had a positive effect on the number of new firms and now I am doing robustness checks.
Code:
drop pick gen PostreformXTreated = Postreform*Treated gen month_date = month(Date) nbreg Nfirms Postreform Treated PostreformXTreated IndustryOutput i.month_date, irr
Now, similar to Branstetter et al. (2014), I wish to estimate the monthly values of the coefficient of interest (PostXTreated) in the pre-treatment period to show that there hasn't been an existing trend leading up to the introduction of the law change. Please find below the graph in question, note that this graph includes both leads and lags.
[ATTACH=CONFIG]temp_14692_1559480203026_270[/ATTACH]
My question: How do I code this in Stata?
Do I use monthly time dummies for each month in the pre-treatment period and interact those with the dummy Treated (which indicates the treated group)? If so, shouldn't I omit one month dummy which would serve as a reference and to avoid perfect collinearity?
Thank you for your time.
Leon
A sample of my data is attached here:
Code:
* Example generated by -dataex-. To install: ssc install dataex clear input int(Date Nfirms) double IndustryOutput byte(Treated Postreform pick) 18904 26 97.9 1 0 1 18911 18 97.9 1 0 1 18914 23 97.9 1 0 1 18915 6 97.9 1 0 1 18917 13 97.9 1 0 1 18921 26 97.9 1 0 1 18925 31 97.9 1 0 1 18932 70 98.1 1 0 1 18935 21 98.1 1 0 1 18938 27 98.1 1 0 1 18946 28 98.1 1 0 1 18949 34 98.1 1 0 1 18952 22 98.1 1 0 1 18954 32 98.1 1 0 1 18956 31 98.1 1 0 1 18973 18 100.2 1 0 1 18974 22 100.2 1 0 1 18976 24 100.2 1 0 1 18980 25 100.2 1 0 1 18981 24 100.2 1 0 1 18996 56 99.9 1 0 1 18997 59 99.9 1 0 1 19001 71 99.9 1 0 1 19010 68 99.9 1 0 1 19013 1 99.9 1 0 1 19015 63 99.9 1 0 1 19019 61 99.9 1 0 1 19022 71 99.9 1 0 1 19023 63 99.9 1 0 1 19024 132 97.7 1 0 1 19027 1 97.7 1 0 1 19028 1 97.7 1 0 1 19031 51 97.7 1 0 1 19035 1 97.7 1 0 1 19038 55 97.7 1 0 1 19043 50 97.7 1 0 1 19045 69 97.7 1 0 1 19046 53 97.7 1 0 1 19048 1 97.7 1 0 1 19051 58 97.7 1 0 1 19052 62 97.7 1 0 1 19061 46 102 1 0 1 19067 56 102 1 0 1 19076 0 102 1 0 1 19093 43 98.3 1 0 1 19102 53 98.3 1 0 1 19108 54 98.3 1 0 1 19110 71 98.3 1 0 1 19113 0 98.3 1 0 1 19117 36 98.1 1 0 1 19120 44 98.1 1 0 1 19121 42 98.1 1 0 1 19128 68 98.1 1 0 1 19129 72 98.1 1 0 1 19134 58 98.1 1 0 1 19135 60 98.1 1 0 1 19137 66 98.1 1 0 1 19139 4 98.1 1 0 1 19149 39 98.7 1 0 1 19157 45 98.7 1 0 1 19159 63 98.7 1 0 1 19164 32 98.7 1 0 1 19167 2 98.7 1 0 1 19168 1 98.7 1 0 1 19170 52 98.7 1 0 1 19179 56 97 1 0 1 19181 4 97 1 0 1 19185 44 97 1 0 1 19186 46 97 1 0 1 19193 45 97 1 0 1 19194 46 97 1 0 1 19197 22 97 1 0 1 19198 43 97 1 0 1 19199 22 97 1 0 1 19202 1 97 1 0 1 19205 44 97 1 0 1 19208 26 98.8 1 0 1 19214 38 98.8 1 0 1 19218 23 98.8 1 0 1 19219 55 98.8 1 0 1 19220 31 98.8 1 0 1 19222 30 98.8 1 0 1 19225 29 98.8 1 0 1 19229 32 98.8 1 0 1 19230 1 98.8 1 0 1 19232 36 98.8 1 0 1 19237 50 97.6 1 0 1 19240 29 97.6 1 0 1 19243 27 97.6 1 0 1 19244 2 97.6 1 0 1 19245 1 97.6 1 0 1 19249 28 97.6 1 0 1 19251 11 97.6 1 0 1 19253 28 97.6 1 0 1 19261 32 97.6 1 0 1 19270 62 96.4 1 1 1 19272 1 96.4 1 1 1 19276 52 96.4 1 1 1 19281 47 96.4 1 1 1 19283 55 96.4 1 1 1 19285 59 96.4 1 1 1 19289 36 96.4 1 1 1 19297 38 96.4 1 1 1 19299 54 98.4 1 1 1 19302 55 98.4 1 1 1 19305 56 98.4 1 1 1 19307 3 98.4 1 1 1 19314 5 98.4 1 1 1 19317 42 98.4 1 1 1 19320 43 98.4 1 1 1 19321 2 98.4 1 1 1 19325 48 98.4 1 1 1 19326 64 98.4 1 1 1 19328 14 99.4 1 1 1 19332 36 99.4 1 1 1 19340 34 99.4 1 1 1 19351 15 99.4 1 1 1 19356 15 99.4 1 1 1 19360 137 96.6 1 1 1 19362 96 96.6 1 1 1 19367 87 96.6 1 1 1 19371 1 96.6 1 1 1 19373 60 96.6 1 1 1 19379 65 96.6 1 1 1 19381 82 96.6 1 1 1 19382 80 96.6 1 1 1 19388 75 96.6 1 1 1 19389 79 96.6 1 1 1 19393 58 96.1 1 1 1 19401 64 96.1 1 1 1 19408 73 96.1 1 1 1 19409 60 96.1 1 1 1 19410 76 96.1 1 1 1 19411 79 96.1 1 1 1 19412 1 96.1 1 1 1 19418 131 96.9 1 1 1 19419 0 96.9 1 1 1 19420 1 96.9 1 1 1 19421 83 96.9 1 1 1 19422 66 96.9 1 1 1 19423 60 96.9 1 1 1 19424 66 96.9 1 1 1 19428 64 96.9 1 1 1 19430 62 96.9 1 1 1 19432 86 96.9 1 1 1 19438 81 96.9 1 1 1 19456 51 97 1 1 1 19459 74 97 1 1 1 19460 226 97 1 1 1 19466 71 97 1 1 1 19467 83 97 1 1 1 19470 64 97 1 1 1 19471 62 97 1 1 1 19472 60 97 1 1 1 19474 102 97 1 1 1 19478 1 97 1 1 1 19481 66 96.2 1 1 1 19484 50 96.2 1 1 1 19485 41 96.2 1 1 1 19486 56 96.2 1 1 1 19487 1 96.2 1 1 1 19495 70 96.2 1 1 1 19498 1 96.2 1 1 1 19506 77 96.2 1 1 1 19507 75 96.2 1 1 1 19510 32 97.4 1 1 1 19513 67 97.4 1 1 1 19515 73 97.4 1 1 1 19516 64 97.4 1 1 1 19517 3 97.4 1 1 1 19519 53 97.4 1 1 1 19522 71 97.4 1 1 1 19530 74 97.4 1 1 1 19533 77 97.4 1 1 1 19535 112 97.4 1 1 1 19544 62 96.6 1 1 1 19548 57 96.6 1 1 1 19555 50 96.6 1 1 1 19557 73 96.6 1 1 1 19558 60 96.6 1 1 1 19565 48 96.6 1 1 1 19566 3 96.6 1 1 1 19568 29 96.6 1 1 1 19569 34 96.6 1 1 1 19572 41 97.2 1 1 1 19576 35 97.2 1 1 1 19578 32 97.2 1 1 1 19584 38 97.2 1 1 1 19592 49 97.2 1 1 1 19598 43 97.2 1 1 1 19600 61 97.2 1 1 1 19602 13 97.5 1 1 1 19603 43 97.5 1 1 1 19611 41 97.5 1 1 1 19614 39 97.5 1 1 1 19616 1 97.5 1 1 1 19618 49 97.5 1 1 1 19620 53 97.5 1 1 1 19621 49 97.5 1 1 1 18903 2 96.7 0 0 1 18904 0 96.7 0 0 1 18905 2 96.7 0 0 1 18906 1 96.7 0 0 1 18908 0 96.7 0 0 1 18913 0 96.7 0 0 1 18914 1 96.7 0 0 1 18917 0 96.7 0 0 1 18928 1 96.7 0 0 1 18931 0 96.7 0 0 1 18935 0 96.7 0 0 1 18940 1 96.7 0 0 1 18941 1 96.7 0 0 1 18947 0 96.7 0 0 1 18948 0 96.7 0 0 1 18952 1 96.7 0 0 1 18956 2 96.7 0 0 1 18959 1 96.7 0 0 1 18966 0 96.8 0 0 1 18977 0 96.8 0 0 1 18982 0 96.8 0 0 1 18984 0 96.8 0 0 1 18991 0 96.8 0 0 1 18992 0 96.8 0 0 1 18993 1 97.6 0 0 1 18996 41 97.6 0 0 1 18997 47 97.6 0 0 1 19002 49 97.6 0 0 1 19006 0 97.6 0 0 1 19012 38 97.6 0 0 1 19017 42 97.6 0 0 1 19018 50 97.6 0 0 1 19023 35 97.6 0 0 1 19024 48 99.8 0 0 1 19025 41 99.8 0 0 1 19027 1 99.8 0 0 1 19032 18 99.8 0 0 1 19033 39 99.8 0 0 1 19036 31 99.8 0 0 1 19038 41 99.8 0 0 1 19039 39 99.8 0 0 1 19040 45 99.8 0 0 1 19047 24 99.8 0 0 1 19051 39 99.8 0 0 1 19054 30 102.4 0 0 1 19055 2 102.4 0 0 1 19058 31 102.4 0 0 1 19059 32 102.4 0 0 1 19060 25 102.4 0 0 1 19066 39 102.4 0 0 1 19067 29 102.4 0 0 1 19069 1 102.4 0 0 1 19072 30 102.4 0 0 1 19074 26 102.4 0 0 1 19078 12 102.4 0 0 1 19080 41 102.4 0 0 1 19086 29 95.7 0 0 1 19095 28 95.7 0 0 1 19100 18 95.7 0 0 1 19102 19 95.7 0 0 1 19103 16 95.7 0 0 1 19105 0 95.7 0 0 1 19106 15 95.7 0 0 1 19107 12 95.7 0 0 1 19108 18 95.7 0 0 1 19109 15 95.7 0 0 1 19117 12 97.9 0 0 1 19118 1 97.9 0 0 1 19121 23 97.9 0 0 1 19138 14 97.9 0 0 1 19139 0 97.9 0 0 1 19141 0 97.9 0 0 1 19150 8 98.2 0 0 1 19153 1 98.2 0 0 1 19155 8 98.2 0 0 1 19156 9 98.2 0 0 1 19157 6 98.2 0 0 1 19162 17 98.2 0 0 1 19163 8 98.2 0 0 1 19165 5 98.2 0 0 1 19172 9 98.2 0 0 1 19175 0 97.8 0 0 1 19179 6 97.8 0 0 1 19180 2 97.8 0 0 1 19181 0 97.8 0 0 1 19183 1 97.8 0 0 1 19192 4 97.8 0 0 1 19193 6 97.8 0 0 1 19201 3 97.8 0 0 1 19205 3 97.8 0 0 1 19207 3 99.7 0 0 1 19212 3 99.7 0 0 1 19227 1 99.7 0 0 1 19230 0 99.7 0 0 1 19232 1 99.7 0 0 1 19233 3 99.7 0 0 1 19235 2 99.7 0 0 1 19242 2 99.7 0 0 1 19245 0 99.7 0 0 1 19246 1 99.7 0 0 1 19247 3 99.7 0 0 1 19249 0 99.7 0 0 1 19253 4 99.7 0 0 1 19260 1 99.7 0 0 1 19261 2 99.7 0 0 1 19263 1 99.7 0 0 1 19264 0 99.7 0 0 1 19269 0 96.3 0 1 1 19270 1 96.3 0 1 1 19271 0 96.3 0 1 1 19277 1 96.3 0 1 1 19284 0 96.3 0 1 1 19289 1 96.3 0 1 1 19290 1 96.3 0 1 1 19292 2 96.3 0 1 1 19295 0 96.3 0 1 1 19297 2 96.3 0 1 1 19298 0 95.1 0 1 1 19303 0 95.1 0 1 1 19304 0 95.1 0 1 1 19309 0 95.1 0 1 1 19310 0 95.1 0 1 1 19316 0 95.1 0 1 1 19317 0 95.1 0 1 1 19321 0 95.1 0 1 1 19323 0 95.1 0 1 1 19332 0 96.3 0 1 1 19335 0 96.3 0 1 1 19338 1 96.3 0 1 1 19339 0 96.3 0 1 1 19345 0 96.3 0 1 1 19351 0 96.3 0 1 1 19357 0 96.3 0 1 1 19360 32 95.1 0 1 1 19361 50 95.1 0 1 1 19362 37 95.1 0 1 1 19366 35 95.1 0 1 1 19368 50 95.1 0 1 1 19370 2 95.1 0 1 1 19371 1 95.1 0 1 1 19374 48 95.1 0 1 1 19382 44 95.1 0 1 1 19393 36 93.9 0 1 1 19395 48 93.9 0 1 1 19400 20 93.9 0 1 1 19404 12 93.9 0 1 1 19407 32 93.9 0 1 1 19410 46 93.9 0 1 1 19411 32 93.9 0 1 1 19416 38 93.9 0 1 1 19419 1 96.5 0 1 1 19421 22 96.5 0 1 1 19425 35 96.5 0 1 1 19426 0 96.5 0 1 1 19433 1 96.5 0 1 1 19438 21 96.5 0 1 1 19441 0 96.5 0 1 1 19445 38 96.5 0 1 1 19452 26 97.4 0 1 1 19459 15 97.4 0 1 1 19467 20 97.4 0 1 1 19468 1 97.4 0 1 1 19472 14 97.4 0 1 1 19477 22 97.4 0 1 1 19482 0 99.3 0 1 1 19491 13 99.3 0 1 1 19492 10 99.3 0 1 1 19500 10 99.3 0 1 1 19501 15 99.3 0 1 1 19509 8 99.3 0 1 1 19513 3 98.3 0 1 1 19523 4 98.3 0 1 1 19529 1 98.3 0 1 1 19532 0 98.3 0 1 1 19534 8 98.3 0 1 1 19541 4 99.4 0 1 1 19545 0 99.4 0 1 1 19548 6 99.4 0 1 1 19549 2 99.4 0 1 1 19550 2 99.4 0 1 1 19551 7 99.4 0 1 1 19553 1 99.4 0 1 1 19555 5 99.4 0 1 1 19556 3 99.4 0 1 1 19558 1 99.4 0 1 1 19565 2 99.4 0 1 1 19566 0 99.4 0 1 1 19572 0 97.5 0 1 1 19575 4 97.5 0 1 1 19576 4 97.5 0 1 1 19580 0 97.5 0 1 1 19582 1 97.5 0 1 1 19586 0 97.5 0 1 1 19591 1 97.5 0 1 1 19593 1 97.5 0 1 1 19602 0 96.4 0 1 1 19606 1 96.4 0 1 1 19611 5 96.4 0 1 1 19613 3 96.4 0 1 1 19614 0 96.4 0 1 1 19618 0 96.4 0 1 1 end format %td Date
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