I am running `ivprobit`

in Stata to look at the determinants of enrolment in health insurance (cbhi). I have several exogenous regressors and one endogenous regressor (consumption). I am using wealthindex as an intrumental variable for consumption. However, when I run the `ivprobit`

model all my exogenous regressors appear in the "instruments" list. Could someone please tell me how to prevent this from happening? I am copying the stata code and results below.

Thank you in advance for your help.

```
. # delimit;
delimiter now ;
. ivprobit cbhi age_hhead age2 edu_hh2 edu_hh3 edu_hh4 edu_hh5 mar_hh hhmem_cat2 hhmem_cat3
> hhhealth_poor hh_chronic hh_diffic risk pregnancy wra any_oldmem j_hh2 j_hh3 j_hh4
> hlthseek_mod campaign qual_percep urban door_to_door chief_mem
> pharm_in_vil drugsel_in_vil hlthcent_in_vil privclin_in_vil young edu_chief num_camp_vill home_collection
> qual2 qual3 size_ln name_hosp1 name_hosp3 name_hosp4 name_hosp5 name_hosp6 (cons_pcm=wealthindex) ;
Fitting exogenous probit model
Iteration 0: log likelihood = -1909.5425
Iteration 1: log likelihood = -1687.946
Iteration 2: log likelihood = -1681.6413
Iteration 3: log likelihood = -1681.6142
Iteration 4: log likelihood = -1681.6142
Fitting full model
Iteration 0: log likelihood = -42769.776
Iteration 1: log likelihood = -42766.676
Iteration 2: log likelihood = -42736.028
Iteration 3: log likelihood = -42734.629
Iteration 4: log likelihood = -42734.269
Iteration 5: log likelihood = -42734.266
Iteration 6: log likelihood = -42734.266
Probit model with endogenous regressors Number of obs = 3000
Wald chi2(42) = 1113.82
Log likelihood = -42734.266 Prob > chi2 = 0.0000
------------------------------------------------------------------------------
| Coef. Std. Err. z P>|z| [95% Conf. Interval]
-------------+----------------------------------------------------------------
cons_pcm | 3.59e-06 2.15e-07 16.68 0.000 3.17e-06 4.01e-06
age_hhead | .0320042 .0129973 2.46 0.014 .00653 .0574784
age2 | -.0001774 .0001228 -1.45 0.148 -.0004181 .0000632
edu_hh2 | .051426 .0879334 0.58 0.559 -.1209203 .2237723
edu_hh3 | -.0075595 .1023748 -0.07 0.941 -.2082106 .1930915
edu_hh4 | -.000683 .1285689 -0.01 0.996 -.2526735 .2513075
edu_hh5 | -.2986249 .1713749 -1.74 0.081 -.6345136 .0372638
mar_hh | -.0539035 .067056 -0.80 0.421 -.1853308 .0775237
hhmem_cat2 | .4222092 .0675468 6.25 0.000 .2898198 .5545985
hhmem_cat3 | .7099401 .0822271 8.63 0.000 .5487778 .8711023
hhhealth_p~r | .1550617 .0699122 2.22 0.027 .0180362 .2920872
hh_chronic | .159681 .0718297 2.22 0.026 .0188973 .3004647
hh_diffic | .2613713 .0741209 3.53 0.000 .116097 .4066456
risk | -.0482252 .0467953 -1.03 0.303 -.1399423 .0434919
pregnancy | .1200344 .1214774 0.99 0.323 -.1180569 .3581257
wra | .1033809 .0289192 3.57 0.000 .0467004 .1600614
any_oldmem | .0792428 .0680778 1.16 0.244 -.0541872 .2126729
j_hh2 | .0512404 .0815692 0.63 0.530 -.1086323 .211113
j_hh3 | -.349126 .0806268 -4.33 0.000 -.5071516 -.1911004
j_hh4 | .1928496 .0843208 2.29 0.022 .0275838 .3581154
hlthseek_mod | -.0359996 .0969011 -0.37 0.710 -.2259223 .153923
campaign | .5068671 .0781293 6.49 0.000 .3537365 .6599977
qual_percep | .1840314 .0514758 3.58 0.000 .0831408 .2849221
urban | -.0999875 .0590944 -1.69 0.091 -.2158103 .0158353
door_to_door | .0564977 .0719369 0.79 0.432 -.084496 .1974914
chief_mem | .0811248 .0599964 1.35 0.176 -.036466 .1987156
pharm_in_vil | .0269211 .0582651 0.46 0.644 -.0872763 .1411185
drugsel_in~l | .0693485 .0586169 1.18 0.237 -.0455385 .1842355
hlthcent_i~l | .0089279 .0823509 0.11 0.914 -.1524769 .1703327
privclin_i~l | -.0547559 .0857815 -0.64 0.523 -.2228846 .1133728
young | .0899074 .0715818 1.26 0.209 -.0503903 .2302051
edu_chief | -.0888131 .0528856 -1.68 0.093 -.1924671 .0148408
num_camp_v~l | .0655667 .0548026 1.20 0.232 -.0418444 .1729779
home_colle~n | .1944581 .0606159 3.21 0.001 .0756531 .3132631
qual2 | -.1778623 .0896638 -1.98 0.047 -.3536001 -.0021245
qual3 | -.2049839 .1081036 -1.90 0.058 -.416863 .0068952
size_ln | -.0059746 .042326 -0.14 0.888 -.0889321 .0769829
name_hosp1 | .1476783 .1168295 1.26 0.206 -.0813032 .3766598
name_hosp3 | .536909 .1350044 3.98 0.000 .2723053 .8015128
name_hosp4 | .4347936 .1069197 4.07 0.000 .2252348 .6443523
name_hosp5 | .228802 .1274495 1.80 0.073 -.0209945 .4785985
name_hosp6 | .9189969 .1301651 7.06 0.000 .6638779 1.174116
_cons | -4.82205 .4438201 -10.86 0.000 -5.691922 -3.952179
-------------+----------------------------------------------------------------
/athrho | -.8556093 .0998439 -8.57 0.000 -1.0513 -.6599188
/lnsigma | 12.27712 .01291 950.98 0.000 12.25181 12.30242
-------------+----------------------------------------------------------------
rho | -.6939885 .0517571 -.7823111 -.5783094
sigma | 214725.5 2772.097 209360.5 220228.1
------------------------------------------------------------------------------
Instrumented: cons_pcm
Instruments: age_hhead age2 edu_hh2 edu_hh3 edu_hh4 edu_hh5 mar_hh
hhmem_cat2 hhmem_cat3 hhhealth_poor hh_chronic hh_diffic risk
pregnancy wra any_oldmem j_hh2 j_hh3 j_hh4 hlthseek_mod
campaign qual_percep urban door_to_door chief_mem
pharm_in_vil drugsel_in_vil hlthcent_in_vil privclin_in_vil
young edu_chief num_camp_vill home_collection qual2 qual3
size_ln name_hosp1 name_hosp3 name_hosp4 name_hosp5
name_hosp6 wealthindex
------------------------------------------------------------------------------
Wald test of exogeneity (/athrho = 0): chi2(1) = 73.44 Prob > chi2 = 0.0000
```

## Best Answer

In general all exogenous variables are always included as instruments. Usually instruments are picked for variables which are endogenous, but we can think (it follows from the mathematical derivation of instrumental variable estimation) that we need to choose the instruments for all the variables. Instruments for exogenous variables then are naturally themselves.