Maximum Likelihood Estimation (Generic models)

This tutorial explains how to quickly implement new maximum likelihood models in statsmodels. We give two examples:

  1. Probit model for binary dependent variables
  2. Negative binomial model for count data

The GenericLikelihoodModel class eases the process by providing tools such as automatic numeric differentiation and a unified interface to scipy optimization functions. Using statsmodels, users can fit new MLE models simply by "plugging-in" a log-likelihood function.

Example 1: Probit model

In [1]:
from __future__ import print_function
import numpy as np
from scipy import stats
import statsmodels.api as sm
from statsmodels.base.model import GenericLikelihoodModel
/build/statsmodels-GE7Zhw/statsmodels-0.8.0/.pybuild/cpython3_3.6_statsmodels/build/statsmodels/compat/pandas.py:56: FutureWarning: The pandas.core.datetools module is deprecated and will be removed in a future version. Please use the pandas.tseries module instead.
  from pandas.core import datetools

The Spector dataset is distributed with statsmodels. You can access a vector of values for the dependent variable (endog) and a matrix of regressors (exog) like this:

In [2]:
data = sm.datasets.spector.load_pandas()
exog = data.exog
endog = data.endog
print(sm.datasets.spector.NOTE)
print(data.exog.head())
::

    Number of Observations - 32

    Number of Variables - 4

    Variable name definitions::

        Grade - binary variable indicating whether or not a student's grade
                improved.  1 indicates an improvement.
        TUCE  - Test score on economics test
        PSI   - participation in program
        GPA   - Student's grade point average

    GPA  TUCE  PSI
0  2.66  20.0  0.0
1  2.89  22.0  0.0
2  3.28  24.0  0.0
3  2.92  12.0  0.0
4  4.00  21.0  0.0

Them, we add a constant to the matrix of regressors:

In [3]:
exog = sm.add_constant(exog, prepend=True)

To create your own Likelihood Model, you simply need to overwrite the loglike method.

In [4]:
class MyProbit(GenericLikelihoodModel):
    def loglike(self, params):
        exog = self.exog
        endog = self.endog
        q = 2 * endog - 1
        return stats.norm.logcdf(q*np.dot(exog, params)).sum()

Estimate the model and print a summary:

In [5]:
sm_probit_manual = MyProbit(endog, exog).fit()
print(sm_probit_manual.summary())
Optimization terminated successfully.
         Current function value: 0.400588
         Iterations: 292
         Function evaluations: 494
                               MyProbit Results                               
==============================================================================
Dep. Variable:                  GRADE   Log-Likelihood:                -12.819
Model:                       MyProbit   AIC:                             33.64
Method:            Maximum Likelihood   BIC:                             39.50
Date:                Wed, 27 Jun 2018                                         
Time:                        23:24:37                                         
No. Observations:                  32                                         
Df Residuals:                      28                                         
Df Model:                           3                                         
==============================================================================
                 coef    std err          z      P>|z|      [0.025      0.975]
------------------------------------------------------------------------------
const         -7.4523      2.542     -2.931      0.003     -12.435      -2.469
GPA            1.6258      0.694      2.343      0.019       0.266       2.986
TUCE           0.0517      0.084      0.617      0.537      -0.113       0.216
PSI            1.4263      0.595      2.397      0.017       0.260       2.593
==============================================================================

Compare your Probit implementation to statsmodels' "canned" implementation:

In [6]:
sm_probit_canned = sm.Probit(endog, exog).fit()
Optimization terminated successfully.
         Current function value: 0.400588
         Iterations 6
In [7]:
print(sm_probit_canned.params)
print(sm_probit_manual.params)
const   -7.452320
GPA      1.625810
TUCE     0.051729
PSI      1.426332
dtype: float64
[-7.45233176  1.62580888  0.05172971  1.42631954]
In [8]:
print(sm_probit_canned.cov_params())
print(sm_probit_manual.cov_params())
          const       GPA      TUCE       PSI
const  6.464166 -1.169668 -0.101173 -0.594792
GPA   -1.169668  0.481473 -0.018914  0.105439
TUCE  -0.101173 -0.018914  0.007038  0.002472
PSI   -0.594792  0.105439  0.002472  0.354070
[[ 6.46416770e+00 -1.16966617e+00 -1.01173180e-01 -5.94788993e-01]
 [-1.16966617e+00  4.81472116e-01 -1.89134586e-02  1.05438227e-01]
 [-1.01173180e-01 -1.89134586e-02  7.03758392e-03  2.47189191e-03]
 [-5.94788993e-01  1.05438227e-01  2.47189191e-03  3.54069512e-01]]

Notice that the GenericMaximumLikelihood class provides automatic differentiation, so we didn't have to provide Hessian or Score functions in order to calculate the covariance estimates.

Example 2: Negative Binomial Regression for Count Data

Consider a negative binomial regression model for count data with log-likelihood (type NB-2) function expressed as:

$$ \mathcal{L}(\beta_j; y, \alpha) = \sum_{i=1}^n y_i ln \left ( \frac{\alpha exp(X_i'\beta)}{1+\alpha exp(X_i'\beta)} \right ) - \frac{1}{\alpha} ln(1+\alpha exp(X_i'\beta)) + ln \Gamma (y_i + 1/\alpha) - ln \Gamma (y_i+1) - ln \Gamma (1/\alpha) $$

with a matrix of regressors $X$, a vector of coefficients $\beta$, and the negative binomial heterogeneity parameter $\alpha$.

Using the nbinom distribution from scipy, we can write this likelihood simply as:

In [9]:
import numpy as np
from scipy.stats import nbinom
In [10]:
def _ll_nb2(y, X, beta, alph):
    mu = np.exp(np.dot(X, beta))
    size = 1/alph
    prob = size/(size+mu)
    ll = nbinom.logpmf(y, size, prob)
    return ll

New Model Class

We create a new model class which inherits from GenericLikelihoodModel:

In [11]:
from statsmodels.base.model import GenericLikelihoodModel
In [12]:
class NBin(GenericLikelihoodModel):
    def __init__(self, endog, exog, **kwds):
        super(NBin, self).__init__(endog, exog, **kwds)
        
    def nloglikeobs(self, params):
        alph = params[-1]
        beta = params[:-1]
        ll = _ll_nb2(self.endog, self.exog, beta, alph)
        return -ll 
    
    def fit(self, start_params=None, maxiter=10000, maxfun=5000, **kwds):
        # we have one additional parameter and we need to add it for summary
        self.exog_names.append('alpha')
        if start_params == None:
            # Reasonable starting values
            start_params = np.append(np.zeros(self.exog.shape[1]), .5)
            # intercept
            start_params[-2] = np.log(self.endog.mean())
        return super(NBin, self).fit(start_params=start_params, 
                                     maxiter=maxiter, maxfun=maxfun, 
                                     **kwds) 

Two important things to notice:

  • nloglikeobs: This function should return one evaluation of the negative log-likelihood function per observation in your dataset (i.e. rows of the endog/X matrix).
  • start_params: A one-dimensional array of starting values needs to be provided. The size of this array determines the number of parameters that will be used in optimization.

That's it! You're done!

Usage Example

The Medpar dataset is hosted in CSV format at the Rdatasets repository. We use the read_csv function from the Pandas library to load the data in memory. We then print the first few columns:

In [13]:
import statsmodels.api as sm
In [14]:
medpar = sm.datasets.get_rdataset("medpar", "COUNT", cache=True).data

medpar.head()
---------------------------------------------------------------------------
gaierror                                  Traceback (most recent call last)
/usr/lib/python3.6/urllib/request.py in do_open(self, http_class, req, **http_conn_args)
   1317                 h.request(req.get_method(), req.selector, req.data, headers,
-> 1318                           encode_chunked=req.has_header('Transfer-encoding'))
   1319             except OSError as err: # timeout error

/usr/lib/python3.6/http/client.py in request(self, method, url, body, headers, encode_chunked)
   1238         """Send a complete request to the server."""
-> 1239         self._send_request(method, url, body, headers, encode_chunked)
   1240 

/usr/lib/python3.6/http/client.py in _send_request(self, method, url, body, headers, encode_chunked)
   1284             body = _encode(body, 'body')
-> 1285         self.endheaders(body, encode_chunked=encode_chunked)
   1286 

/usr/lib/python3.6/http/client.py in endheaders(self, message_body, encode_chunked)
   1233             raise CannotSendHeader()
-> 1234         self._send_output(message_body, encode_chunked=encode_chunked)
   1235 

/usr/lib/python3.6/http/client.py in _send_output(self, message_body, encode_chunked)
   1025         del self._buffer[:]
-> 1026         self.send(msg)
   1027 

/usr/lib/python3.6/http/client.py in send(self, data)
    963             if self.auto_open:
--> 964                 self.connect()
    965             else:

/usr/lib/python3.6/http/client.py in connect(self)
   1391 
-> 1392             super().connect()
   1393 

/usr/lib/python3.6/http/client.py in connect(self)
    935         self.sock = self._create_connection(
--> 936             (self.host,self.port), self.timeout, self.source_address)
    937         self.sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)

/usr/lib/python3.6/socket.py in create_connection(address, timeout, source_address)
    703     err = None
--> 704     for res in getaddrinfo(host, port, 0, SOCK_STREAM):
    705         af, socktype, proto, canonname, sa = res

/usr/lib/python3.6/socket.py in getaddrinfo(host, port, family, type, proto, flags)
    744     addrlist = []
--> 745     for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
    746         af, socktype, proto, canonname, sa = res

gaierror: [Errno -2] Name or service not known

During handling of the above exception, another exception occurred:

URLError                                  Traceback (most recent call last)
<ipython-input-14-0e9026c9d126> in <module>()
----> 1 medpar = sm.datasets.get_rdataset("medpar", "COUNT", cache=True).data
      2 
      3 medpar.head()

/build/statsmodels-GE7Zhw/statsmodels-0.8.0/.pybuild/cpython3_3.6_statsmodels/build/statsmodels/datasets/utils.py in get_rdataset(dataname, package, cache)
    288                      "master/doc/"+package+"/rst/")
    289     cache = _get_cache(cache)
--> 290     data, from_cache = _get_data(data_base_url, dataname, cache)
    291     data = read_csv(data, index_col=0)
    292     data = _maybe_reset_index(data)

/build/statsmodels-GE7Zhw/statsmodels-0.8.0/.pybuild/cpython3_3.6_statsmodels/build/statsmodels/datasets/utils.py in _get_data(base_url, dataname, cache, extension)
    219     url = base_url + (dataname + ".%s") % extension
    220     try:
--> 221         data, from_cache = _urlopen_cached(url, cache)
    222     except HTTPError as err:
    223         if '404' in str(err):

/build/statsmodels-GE7Zhw/statsmodels-0.8.0/.pybuild/cpython3_3.6_statsmodels/build/statsmodels/datasets/utils.py in _urlopen_cached(url, cache)
    210     # not using the cache or didn't find it in cache
    211     if not from_cache:
--> 212         data = urlopen(url).read()
    213         if cache is not None:  # then put it in the cache
    214             _cache_it(data, cache_path)

/usr/lib/python3.6/urllib/request.py in urlopen(url, data, timeout, cafile, capath, cadefault, context)
    221     else:
    222         opener = _opener
--> 223     return opener.open(url, data, timeout)
    224 
    225 def install_opener(opener):

/usr/lib/python3.6/urllib/request.py in open(self, fullurl, data, timeout)
    524             req = meth(req)
    525 
--> 526         response = self._open(req, data)
    527 
    528         # post-process response

/usr/lib/python3.6/urllib/request.py in _open(self, req, data)
    542         protocol = req.type
    543         result = self._call_chain(self.handle_open, protocol, protocol +
--> 544                                   '_open', req)
    545         if result:
    546             return result

/usr/lib/python3.6/urllib/request.py in _call_chain(self, chain, kind, meth_name, *args)
    502         for handler in handlers:
    503             func = getattr(handler, meth_name)
--> 504             result = func(*args)
    505             if result is not None:
    506                 return result

/usr/lib/python3.6/urllib/request.py in https_open(self, req)
   1359         def https_open(self, req):
   1360             return self.do_open(http.client.HTTPSConnection, req,
-> 1361                 context=self._context, check_hostname=self._check_hostname)
   1362 
   1363         https_request = AbstractHTTPHandler.do_request_

/usr/lib/python3.6/urllib/request.py in do_open(self, http_class, req, **http_conn_args)
   1318                           encode_chunked=req.has_header('Transfer-encoding'))
   1319             except OSError as err: # timeout error
-> 1320                 raise URLError(err)
   1321             r = h.getresponse()
   1322         except:

URLError: <urlopen error [Errno -2] Name or service not known>

The model we are interested in has a vector of non-negative integers as dependent variable (los), and 5 regressors: Intercept, type2, type3, hmo, white.

For estimation, we need to create two variables to hold our regressors and the outcome variable. These can be ndarrays or pandas objects.

In [15]:
y = medpar.los
X = medpar[["type2", "type3", "hmo", "white"]].copy()
X["constant"] = 1
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-15-69a1d8d48bb6> in <module>()
----> 1 y = medpar.los
      2 X = medpar[["type2", "type3", "hmo", "white"]].copy()
      3 X["constant"] = 1

NameError: name 'medpar' is not defined

Then, we fit the model and extract some information:

In [16]:
mod = NBin(y, X)
res = mod.fit()
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-16-a894549a4046> in <module>()
----> 1 mod = NBin(y, X)
      2 res = mod.fit()

NameError: name 'y' is not defined

Extract parameter estimates, standard errors, p-values, AIC, etc.:

In [17]:
print('Parameters: ', res.params)
print('Standard errors: ', res.bse)
print('P-values: ', res.pvalues)
print('AIC: ', res.aic)
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-17-0c9c0635ee9b> in <module>()
----> 1 print('Parameters: ', res.params)
      2 print('Standard errors: ', res.bse)
      3 print('P-values: ', res.pvalues)
      4 print('AIC: ', res.aic)

NameError: name 'res' is not defined

As usual, you can obtain a full list of available information by typing dir(res). We can also look at the summary of the estimation results.

In [18]:
print(res.summary())
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-18-ba064a039ab1> in <module>()
----> 1 print(res.summary())

NameError: name 'res' is not defined

Testing

We can check the results by using the statsmodels implementation of the Negative Binomial model, which uses the analytic score function and Hessian.

In [19]:
res_nbin = sm.NegativeBinomial(y, X).fit(disp=0)
print(res_nbin.summary())
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-19-ed0df43e1ba9> in <module>()
----> 1 res_nbin = sm.NegativeBinomial(y, X).fit(disp=0)
      2 print(res_nbin.summary())

NameError: name 'y' is not defined
In [20]:
print(res_nbin.params)
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-20-8bd64ee14ecb> in <module>()
----> 1 print(res_nbin.params)

NameError: name 'res_nbin' is not defined
In [21]:
print(res_nbin.bse)
---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-21-4c2dcece9ae4> in <module>()
----> 1 print(res_nbin.bse)

NameError: name 'res_nbin' is not defined

Or we could compare them to results obtained using the MASS implementation for R:

url = 'http://vincentarelbundock.github.com/Rdatasets/csv/COUNT/medpar.csv'
medpar = read.csv(url)
f = los~factor(type)+hmo+white

library(MASS)
mod = glm.nb(f, medpar)
coef(summary(mod))
                 Estimate Std. Error   z value      Pr(>|z|)
(Intercept)    2.31027893 0.06744676 34.253370 3.885556e-257
factor(type)2  0.22124898 0.05045746  4.384861  1.160597e-05
factor(type)3  0.70615882 0.07599849  9.291748  1.517751e-20
hmo           -0.06795522 0.05321375 -1.277024  2.015939e-01
white         -0.12906544 0.06836272 -1.887951  5.903257e-02

Numerical precision

The statsmodels generic MLE and R parameter estimates agree up to the fourth decimal. The standard errors, however, agree only up to the second decimal. This discrepancy is the result of imprecision in our Hessian numerical estimates. In the current context, the difference between MASS and statsmodels standard error estimates is substantively irrelevant, but it highlights the fact that users who need very precise estimates may not always want to rely on default settings when using numerical derivatives. In such cases, it is better to use analytical derivatives with the LikelihoodModel class.