xtreg issue when running two different groups.

[Lama Hammad]

Hello,

I'm using the xtreg command.

. xtset newid

I have two groups. The first group is ( not_alternative)

 gen not_alternative=1 if date==2019 & civica_date==2019 & civica_treatment==0
 replace not_alternative=1 if date==2020 & civica_date==2020 & civica_treatment==0
replace not_alternative=1 if date==2020 & civica_date==2019 & civica_treatment==0
replace not_alternative=0 if date==2018 &  civica_treatment==1

#1

xtreg Tot_Clms i.not_alternative i.date, fe vce(cluster newid )

The other group is (alternative).

gen alternative=1 if date==2019 & civica_date==2019 & civica_treatment==1
replace alternative=1 if date==2020 & civica_date==2020 & civica_treatment==1
 replace alternative=1 if date==2020 & civica_date==2019 & civica_treatment==1

#2

xtreg Tot_Clms i.alternative i.date, fe vce(cluster newid )

Then I have the variable civica_treatment where the two previous groups combined.

xtreg Tot_Clms i.civica_treatment i.date, fe vce(cluster newid )

The result of the last command is not the sum of the two coefficients in commands #1 and #2. shouldn't be the sum, because it's combined?

* Example generated by -dataex-. For more info, type help dataex
clear
input str227 Brnd_Name str137 Gnrc_Name byte Tot_Mftr str16 Mftr_Name double Tot_Clms float(date civica_treatment) str27 civica_altrantive float(civica_date gvar newid not_alternative alternative not_alternative1)
"Alprazolam XR"       "Alprazolam"         1 "Overall"            14956 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam Intensol" "Alprazolam"         1 "Roxane/West-War"      263 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam Intensol" "Alprazolam"         1 "Overall"              263 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam ODT"      "Alprazolam"         1 "Par Pharm."          3447 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam ER"       "Alprazolam"         1 "Aurobindo Pharm"    35428 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam ODT"      "Alprazolam"         1 "Actavis/Teva"        4913 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam"          "Alprazolam"         1 "Mylan"             383139 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam ER"       "Alprazolam"         2 "Overall"            54929 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Xanax XR"            "Alprazolam"         1 "Pharmaci/Pfizer"      448 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam"          "Alprazolam"         1 "Actavis/Teva"     4531469 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam"          "Alprazolam"         1 "Major Pharmaceu"     2545 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam"          "Alprazolam"        10 "Overall"         13167478 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam"          "Alprazolam"         1 "Sandoz"           1618519 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam"          "Alprazolam"         1 "Mckesson Packag"     1212 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam"          "Alprazolam"         1 "Aurobindo Pharm"   665194 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Xanax"               "Alprazolam"         1 "Pharmaci/Pfizer"    31538 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam"          "Alprazolam"         1 "Dava Pharmaceut"   114837 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam"          "Alprazolam"         1 "Par Pharm."       4113397 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam ODT"      "Alprazolam"         2 "Overall"             8360 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam XR"       "Alprazolam"         1 "Greenstone LLC."    14956 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam ER"       "Alprazolam"         1 "Actavis/Teva"       19501 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Xanax XR"            "Alprazolam"         1 "Overall"              448 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam"          "Alprazolam"         1 "Breckenridge"      115998 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Xanax"               "Alprazolam"         1 "Overall"            31538 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam"          "Alprazolam"         1 "Greenstone LLC."  1621168 2018 1 "Benzodiazepines" 2019 2019 1 0 . .
"Alprazolam XR"       "Alprazolam"         1 "Overall"            18413 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Mylan"             271725 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Xanax XR"            "Alprazolam"         1 "Pharmaci/Pfizer"      519 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Greenstone LLC."  1525524 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Breckenridge"     1517812 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Sandoz"            458054 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Dava Pharmaceut"   110377 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam ER"       "Alprazolam"         2 "Overall"            50067 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam ODT"      "Alprazolam"         1 "Par Pharm."          2556 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Par Pharm."       2983939 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Actavis/Teva"     4625459 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"        10 "Overall"         12639059 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Major Pharmaceu"     2231 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Aurobindo Pharm"  1143482 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam ODT"      "Alprazolam"         2 "Overall"             7539 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam XR"       "Alprazolam"         1 "Greenstone LLC."    18413 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam ODT"      "Alprazolam"         1 "Actavis/Teva"        4983 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Mckesson Packag"      456 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam Intensol" "Alprazolam"         1 "Roxane/West-War"      367 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam Intensol" "Alprazolam"         1 "Overall"              367 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam ER"       "Alprazolam"         1 "Actavis/Teva"       11856 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Xanax"               "Alprazolam"         1 "Pharmaci/Pfizer"    30420 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Xanax XR"            "Alprazolam"         1 "Overall"              519 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Xanax"               "Alprazolam"         1 "Overall"            30420 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam ER"       "Alprazolam"         1 "Aurobindo Pharm"    38211 2019 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Actavis/Teva"     5426180 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Xanax XR"            "Alprazolam"         1 "Overall"              502 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam ODT"      "Alprazolam"         2 "Overall"             7094 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam XR"       "Alprazolam"         1 "Greenstone LLC."    24581 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Major Pharmaceu"     1906 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam Intensol" "Alprazolam"         1 "Roxane/West-War"      352 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam ER"       "Alprazolam"         2 "Overall"            41771 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Xanax"               "Alprazolam"         1 "Pharmaci/Pfizer"    29447 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam Intensol" "Alprazolam"         1 "Overall"              352 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam ODT"      "Alprazolam"         1 "Par Pharm."          5023 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Breckenridge"     2712954 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"        10 "Overall"         12464336 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Par Pharm."        134088 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Mylan"             165567 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Xanax XR"            "Alprazolam"         1 "Pharmaci/Pfizer"      502 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam ODT"      "Alprazolam"         1 "Actavis/Teva"        2071 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Aurobindo Pharm"  1837249 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Mckesson Packag"      189 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam XR"       "Alprazolam"         1 "Overall"            24581 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam ER"       "Alprazolam"         1 "Actavis/Teva"        5290 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Greenstone LLC."  1583820 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Sandoz"            597354 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam ER"       "Alprazolam"         1 "Aurobindo Pharm"    36481 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Xanax"               "Alprazolam"         1 "Overall"            29447 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Alprazolam"          "Alprazolam"         1 "Dava Pharmaceut"     5029 2020 1 "Benzodiazepines" 2019 2019 1 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                     703 2018 1 "Lidocaine"       2019 2019 2 0 . .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                     339 2018 1 "Lidocaine"       2019 2019 2 0 . .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                  376243 2018 1 "Lidocaine"       2019 2019 2 0 . .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                    1602 2018 1 "Lidocaine"       2019 2019 2 0 . .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                  131679 2018 1 "Lidocaine"       2019 2019 2 0 . .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                    1471 2018 1 "Lidocaine"       2019 2019 2 0 . .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                 2659253 2018 1 "Lidocaine"       2019 2019 2 0 . .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                  373077 2018 1 "Lidocaine"       2019 2019 2 0 . .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                 1486360 2018 1 "Lidocaine"       2019 2019 2 0 . .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                      32 2018 1 "Lidocaine"       2019 2019 2 0 . .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                  287747 2018 1 "Lidocaine"       2019 2019 2 0 . .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                  192739 2019 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                  315145 2019 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                    1770 2019 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                  878205 2019 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                 1046398 2019 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                   37389 2019 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                   17033 2019 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                     301 2019 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                     645 2019 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                 2657176 2019 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                  167551 2019 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                 2555914 2020 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                   16170 2020 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                  313130 2020 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                  273866 2020 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                 1378888 2020 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                  180901 2020 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                     835 2020 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                   28855 2020 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                     166 2020 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                  362874 2020 1 "Lidocaine"       2019 2019 2 . 1 .
"Amiodarone HCl*"     "Amiodarone HCl"     . ""                     229 2020 1 "Lidocaine"       2019 2019 2 . 1 .
""                    "Amitriptyline HCl"  . ""                      32 2018 0 "Benzodiazepines" 2019 2019 3 . 0 0
""                    "Amitriptyline HCl"  . ""                  663026 2018 0 "Benzodiazepines" 2019 2019 3 . 0 0
""                    "Amitriptyline HCl"  . ""                    6031 2018 0 "Benzodiazepines" 2019 2019 3 . 0 0
""                    "Amitriptyline HCl"  . ""                  460081 2018 0 "Benzodiazepines" 2019 2019 3 . 0 0
""                    "Amitriptyline HCl"  . ""                 3800000 2018 0 "Benzodiazepines" 2019 2019 3 . 0 0
""                    "Amitriptyline HCl"  . ""                    2645 2018 0 "Benzodiazepines" 2019 2019 3 . 0 0
""                    "Amitriptyline HCl"  . ""                  112917 2018 0 "Benzodiazepines" 2019 2019 3 . 0 0
""                    "Amitriptyline HCl"  . ""                  316224 2018 0 "Benzodiazepines" 2019 2019 3 . 0 0
""                    "Amitriptyline HCl"  . ""                  424760 2018 0 "Benzodiazepines" 2019 2019 3 . 0 0
""                    "Amitriptyline HCl"  . ""                 1800000 2018 0 "Benzodiazepines" 2019 2019 3 . 0 0
""                    "Amitriptyline HCl"  . ""                     965 2018 0 "Benzodiazepines" 2019 2019 3 . 0 0
""                    "Amitriptyline HCl"  . ""                  194110 2019 0 "Benzodiazepines" 2019 2019 3 1 . 1
""                    "Amitriptyline HCl"  . ""                  391922 2019 0 "Benzodiazepines" 2019 2019 3 1 . 1
""                    "Amitriptyline HCl"  . ""                 1900000 2019 0 "Benzodiazepines" 2019 2019 3 1 . 1
""                    "Amitriptyline HCl"  . ""                 3700000 2019 0 "Benzodiazepines" 2019 2019 3 1 . 1
""                    "Amitriptyline HCl"  . ""                  157876 2019 0 "Benzodiazepines" 2019 2019 3 1 . 1
""                    "Amitriptyline HCl"  . ""                  163167 2019 0 "Benzodiazepines" 2019 2019 3 1 . 1
""                    "Amitriptyline HCl"  . ""                      24 2019 0 "Benzodiazepines" 2019 2019 3 1 . 1
""                    "Amitriptyline HCl"  . ""                  810930 2019 0 "Benzodiazepines" 2019 2019 3 1 . 1
""                    "Amitriptyline HCl"  . ""                      25 2019 0 "Benzodiazepines" 2019 2019 3 1 . 1
""                    "Amitriptyline HCl"  . ""                     747 2019 0 "Benzodiazepines" 2019 2019 3 1 . 1
""                    "Amitriptyline HCl"  . ""                    2880 2019 0 "Benzodiazepines" 2019 2019 3 1 . 1
end

[Clyde Schechter]

The result of the last command is not the sum of the two coefficients in commands #1 and #2. shouldn't be the sum, because it's combined?

No. When you partition the data into two subsets and regress separately in each of them, the regression coefficient for a variable in the combined data is not necessarily the sum of (nor the average of, nor in the range of, nor even have the same sign as) the separate regression coefficients. See #6 in https://www.statalist.org/forums/for...treatment-post for a demonstration.

That said, your premise that civica_alternative is partitioned into two classes by alternative and not_alternative is not correct, as can be seen in your example data. You do not even have it right that not_alternative and alternative are opposites of each other when civica_treatment is non-missing because you always have at least one of alternative or not_alternative missing. So you are not handling missing values correctly in your construction of these variables. So if you want the set of observations with non-missing civica_alternative to be the union of the subsets with alternative = 1 and not_alternative = 1, you need to revise your code to make that happen. As I don't understand what exactly the variable alternative is supposed to represent, I'm not going to try to offer you revised code for that.

I should also add that if you want it to be the case the alternative = 0 when not_alternative = 1 and vice versa, then you don't need both variables. You can just create alternative. But perhaps that is really not what you want, in which case I would choose a less confusing name than not_alternative to avoid confusion.

[Lama Hammad]

Thank you Clyde,
I didn't know how to handle the missing values in both variables alternative and not_alteranative.
the alternative is the drugs produced by a company called Civica that can be used as a substitute for Gnrc_name when it's in shortage and the not_alternative is the opposite of that.
2018 is the pre Civica's entry and 2019 & 2020 post Civica's entry.