Code:
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Group variable: state_id                         Number of obs      =      139
Time variable : time2                           Number of groups   =        76
Number of instruments = 51                      Obs per group: min =         1
F(30, 75)    =   2875.93                                      avg =      1.83
Prob > F     =     0.000                                      max =         2
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Arellano-Bond test for AR(1) in first differences: z =      . Pr > z =      .
Arellano-Bond test for AR(2) in first differences: z =      . Pr > z =      .
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AR tests don't show up because of the small number of obs per group. In this case, what can I do?