Hello,

I am conducting a multilevel logistic regression to identify determinants of the availability of diagnostic tests.
I am using a multilevel approach because I want to examine the effects of medical surgeries’ characteristics (level 1) and the effects of country (level 2) on the availability of the test in the surgery.
I am using 5 surgery explanatory variables (three are binary variables and two are continuous variables) and two country explanatory variables (both categorical). I have 847 observations (i.e. 847 surgeries) and 19 countries. The continuous variables are centered to the mean.

I am using the nlcom command to obtain the country variance after the melogit command.
My problem is that I obtain a negative lower boundary for the 95 %CI of the variance, but I understand the variance cannot be negative.


Does anyone have a suggestion about why could this happen?
Is there another command in stata instead of nlcom that I could use to avoid having negative boundaries for variance?

Thank you !
Code:
melogit dipstick_available_for_use ib2.public_private2 ib2.group_solo2 centered_distance_lab99 centered_pc_turna_time3 i.bloods_taker3 ib3.expend_capita_quart i.source_financing2 [pweight=scaled_pweight] || country:
Code:
Mixed-effects logistic regression               Number of obs     =        847
Group variable:         country                 Number of groups  =         19

                                                Obs per group:
                                                              min =          3
                                                              avg =       44.6
                                                              max =        162

Integration method: mvaghermite                 Integration pts.  =          7

                                                Wald chi2(10)     =      62.55
Log pseudolikelihood = -223.54672               Prob > chi2       =     0.0000
                                                    (Std. Err. adjusted for 19 clusters in country)
---------------------------------------------------------------------------------------------------
                                  |               Robust
       dipstick_available_for_use |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
----------------------------------+----------------------------------------------------------------
                  public_private2 |
                         Private  |   .6230582   1.365527     0.46   0.648    -2.053326    3.299443
                                  |
                      group_solo2 |
Solo practice. I am the only h..  |   .4136781   .5851945     0.71   0.480    -.7332821    1.560638
                                  |
          centered_distance_lab99 |   -.036124    .031088    -1.16   0.245    -.0970553    .0248072
          centered_pc_turna_time3 |   .1929808   .1399966     1.38   0.168    -.0814074     .467369
                  3.bloods_taker3 |  -.8310664   .6179098    -1.34   0.179    -2.042147    .3800147
                                  |
              expend_capita_quart |
                              0-  |  -3.333408   1.727199    -1.93   0.054    -6.718656    .0518407
                           2400-  |  -1.755793   1.432835    -1.23   0.220    -4.564098    1.052511
                           2900-  |    -2.2454   1.490375    -1.51   0.132     -5.16648     .675681
                                  |
                source_financing2 |
                             gov  |   1.308304   .7677665     1.70   0.088     -.196491    2.813098
                         vhi_oop  |   1.075064   1.279517     0.84   0.401    -1.432742    3.582871
                                  |
                            _cons |   4.341836   1.405842     3.09   0.002     1.586436    7.097237
----------------------------------+----------------------------------------------------------------
country                           |
                        var(_cons)|    1.22608   1.076046                      .2195218    6.847939
---------------------------------------------------------------------------------------------------
Code:
nlcom _b[/var(_cons[country])], cformat(%9.3f)
Code:
       _nl_1:  _b[/var(_cons[country])]

------------------------------------------------------------------------------
dipsti~r_use |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
-------------+----------------------------------------------------------------
       _nl_1 |      1.226      1.076     1.14   0.255       -0.883       3.335
------------------------------------------------------------------------------