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  • Omitted constant in multilevel model

    Dear all,

    I am tryin to run a multilevel model (individuals nested in countries), while controlling for year-effects (2012-2018). See my code below. The result omits the constant, and I don't understand why nor what this means. I tried including explicit dummies for each year instead, but the results remain the same. Why is this the constant omitted? I do not have missing values for these variables. I also tried changing the base category, without success.

    Code:
    mixed demosatisfaction equalityvalue gini gindifnat gindifeu i.year || country:
    
    Mixed-effects ML regression                     Number of obs     =    171,631
    Group variable: country                         Number of groups  =         28
                                                    Obs per group:
                                                                  min =      2,775
                                                                  avg =    6,129.7
                                                                  max =     10,099
                                                    Wald chi2(10)     =    5146.44
    Log likelihood = -192192.93                     Prob > chi2       =     0.0000
    
    ----------------------------------------------------------------------------------
    demosatisfaction | Coefficient  Std. err.      z    P>|z|     [95% conf. interval]
    -----------------+----------------------------------------------------------------
       equalityvalue |   .0100899   .0045895     2.20   0.028     .0010946    .0190852
                gini      |   .0806512   .0012274    65.71   0.000     .0782454    .0830569
           gindifnat |  -.0127411   .0101866    -1.25   0.211    -.0327064    .0072242
            gindifeu |  -.0779339   .0099304    -7.85   0.000    -.0973972   -.0584707
                     |
                year |
               2013  |  -.0215762   .0067843    -3.18   0.001    -.0348731   -.0082792
               2014  |  -.0485003   .0085952    -5.64   0.000    -.0653466    -.031654
               2015  |  -.0392094   .0085768    -4.57   0.000    -.0560195   -.0223993
               2016  |   .0025607   .0074815     0.34   0.732    -.0121028    .0172243
               2017  |   .0538484   .0068795     7.83   0.000     .0403648     .067332
               2018  |   .0823129   .0074179    11.10   0.000     .0677741    .0968518
                     |
               _cons |          0  (omitted)
    ----------------------------------------------------------------------------------
    
    ------------------------------------------------------------------------------
      Random-effects parameters  |   Estimate   Std. err.     [95% conf. interval]
    -----------------------------+------------------------------------------------
    country: Identity            |
                      var(_cons) |   .0380295   .0101915       .022491    .0643032
    -----------------------------+------------------------------------------------
                   var(Residual) |   .5492214    .001875      .5455587    .5529086
    ------------------------------------------------------------------------------
    LR test vs. linear model: chibar2(01) = 10590.29      Prob >= chibar2 = 0.0000
    Best,
    Harry
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