title: Fitting a model using formula and Negative binomial in statsmodels 0.6
body: <p>I`m trying to analyze a dataset with Negative Binomial in the statsmodels 0.6 module with python 3.4. </p>
  
  <pre><code>import pandas as pd
  import statsmodels.formula.api as smf
  import statsmodels.api as sm
  model2=smf.glm("an_darlingi ~ trat", data=dados,
               family=sm.families.NegativeBinomial()).fit()
  </code></pre>
  
  <p>which outputs:</p>
  
  <pre><code>                 Generalized Linear Model Regression Results                  
  ==============================================================================
  Dep. Variable:            an_darlingi   No. Observations:                  363
  Model:                            GLM   Df Residuals:                      352
  Model Family:        NegativeBinomial   Df Model:                           10
  Link Function:                    log   Scale:                   1.52662855632
  Method:                          IRLS   Log-Likelihood:                -1404.7
  Date:                Mon, 13 Oct 2014   Deviance:                       578.37
  Time:                        19:34:45   Pearson chi2:                     537.
  No. Iterations:                     7                                         
  =================================================================================
                      coef    std err          t      P&gt;|t|      [95.0% Conf. Int.]
  ---------------------------------------------------------------------------------
  Intercept         2.9988      0.220     13.607      0.000         2.567     3.431
  trat[T.CO2]      -0.3070      0.313     -0.981      0.327        -0.920     0.306
  trat[T.ML]       -0.1709      0.312     -0.547      0.584        -0.783     0.441
  trat[T.SFPF]     -0.3215      0.313     -1.027      0.304        -0.935     0.292
  trat[T.SFPM]     -0.4085      0.314     -1.303      0.193        -1.023     0.206
  trat[T.SFTF]     -0.0448      0.312     -0.144      0.886        -0.656     0.566
  trat[T.SFTM]     -0.1835      0.312     -0.587      0.557        -0.796     0.429
  trat[T.SIPF]     -0.3905      0.313     -1.246      0.213        -1.005     0.224
  trat[T.SIPM]     -0.1799      0.312     -0.576      0.565        -0.792     0.432
  trat[T.SIPTM]     0.0700      0.311      0.225      0.822        -0.540     0.680
  trat[T.SITF]      0.1968      0.311      0.633      0.527        -0.413     0.806
  =================================================================================
  </code></pre>
  
  <p>In the output I got values of t and p and I like to get values of z and P>(z) like in R. How can I do this using smf.glm instead sm.NegativeBinomial. Because if I try to use sm.NegativeBinomial I got error msg sayng that cant convert string SIPTM to a float.</p>