# -*- coding: utf-8 -*-
"""
Created on Mon Jul 26 08:34:59 2010
Author: josef-pktd
changes:
added offset and zero-inflated version of Poisson
- kind of ok, need better test cases,
- a nan in ZIP bse, need to check hessian calculations
- found error in ZIP loglike
- all tests pass with
Issues
------
* If true model is not zero-inflated then numerical Hessian for ZIP has zeros
for the inflation probability and is not invertible.
-> hessian inverts and bse look ok if row and column are dropped, pinv also works
* GenericMLE: still get somewhere (where?)
"CacheWriteWarning: The attribute 'bse' cannot be overwritten"
* bfgs is too fragile, does not come back
* `nm` is slow but seems to work
* need good start_params and their use in genericmle needs to be checked for
consistency, set as attribute or method (called as attribute)
* numerical hessian needs better scaling
* check taking parts out of the loop, e.g. factorial(endog) could be precalculated
"""
import numpy as np
from scipy import stats
from scipy.special import factorial
from statsmodels.base.model import GenericLikelihoodModel
def maxabs(arr1, arr2):
return np.max(np.abs(arr1 - arr2))
def maxabsrel(arr1, arr2):
return np.max(np.abs(arr2 / arr1 - 1))
class NonlinearDeltaCov(object):
'''Asymptotic covariance by Deltamethod
the function is designed for 2d array, with rows equal to
the number of equations and columns equal to the number
of parameters. 1d params work by chance ?
fun: R^{m*k) -> R^{m} where m is number of equations and k is
the number of parameters.
equations follow Greene
'''
def __init__(self, fun, params, cov_params):
self.fun = fun
self.params = params
self.cov_params = cov_params
def grad(self, params=None, **kwds):
if params is None:
params = self.params
kwds.setdefault('epsilon', 1e-4)
from statsmodels.tools.numdiff import approx_fprime
return approx_fprime(params, self.fun, **kwds)
def cov(self):
g = self.grad()
covar = np.dot(np.dot(g, self.cov_params), g.T)
return covar
def expected(self):
# rename: misnomer, this is the MLE of the fun
return self.fun(self.params)
def wald(self, value):
m = self.expected()
v = self.cov()
df = np.size(m)
diff = m - value
lmstat = np.dot(np.dot(diff.T, np.linalg.inv(v)), diff)
return lmstat, stats.chi2.sf(lmstat, df)
[docs]class PoissonGMLE(GenericLikelihoodModel):
'''Maximum Likelihood Estimation of Poisson Model
This is an example for generic MLE which has the same
statistical model as discretemod.Poisson.
Except for defining the negative log-likelihood method, all
methods and results are generic. Gradients and Hessian
and all resulting statistics are based on numerical
differentiation.
'''
# copied from discretemod.Poisson
[docs] def nloglikeobs(self, params):
"""
Loglikelihood of Poisson model
Parameters
----------
params : array_like
The parameters of the model.
Returns
-------
The log likelihood of the model evaluated at `params`
Notes
-----
.. math:: \\ln L=\\sum_{i=1}^{n}\\left[-\\lambda_{i}+y_{i}x_{i}^{\\prime}\\beta-\\ln y_{i}!\\right]
"""
XB = np.dot(self.exog, params)
endog = self.endog
return np.exp(XB) - endog*XB + np.log(factorial(endog))
[docs] def predict_distribution(self, exog):
'''return frozen scipy.stats distribution with mu at estimated prediction
'''
if not hasattr(self, "result"):
# TODO: why would this be ValueError instead of AttributeError?
# TODO: Why even make this a Model attribute in the first place?
# It belongs on the Results class
raise ValueError
else:
result = self.result
params = result.params
mu = np.exp(np.dot(exog, params))
return stats.poisson(mu, loc=0)
[docs]class PoissonOffsetGMLE(GenericLikelihoodModel):
'''Maximum Likelihood Estimation of Poisson Model
This is an example for generic MLE which has the same
statistical model as discretemod.Poisson but adds offset
Except for defining the negative log-likelihood method, all
methods and results are generic. Gradients and Hessian
and all resulting statistics are based on numerical
differentiation.
'''
def __init__(self, endog, exog=None, offset=None, missing='none', **kwds):
# let them be none in case user wants to use inheritance
if offset is not None:
if offset.ndim == 1:
offset = offset[:,None] #need column
self.offset = offset.ravel()
else:
self.offset = 0.
super(PoissonOffsetGMLE, self).__init__(endog, exog, missing=missing,
**kwds)
#this was added temporarily for bug-hunting, but should not be needed
# def loglike(self, params):
# return -self.nloglikeobs(params).sum(0)
# original copied from discretemod.Poisson
[docs] def nloglikeobs(self, params):
"""
Loglikelihood of Poisson model
Parameters
----------
params : array_like
The parameters of the model.
Returns
-------
The log likelihood of the model evaluated at `params`
Notes
-----
.. math:: \\ln L=\\sum_{i=1}^{n}\\left[-\\lambda_{i}+y_{i}x_{i}^{\\prime}\\beta-\\ln y_{i}!\\right]
"""
XB = self.offset + np.dot(self.exog, params)
endog = self.endog
nloglik = np.exp(XB) - endog*XB + np.log(factorial(endog))
return nloglik
[docs]class PoissonZiGMLE(GenericLikelihoodModel):
'''Maximum Likelihood Estimation of Poisson Model
This is an example for generic MLE which has the same statistical model
as discretemod.Poisson but adds offset and zero-inflation.
Except for defining the negative log-likelihood method, all
methods and results are generic. Gradients and Hessian
and all resulting statistics are based on numerical
differentiation.
There are numerical problems if there is no zero-inflation.
'''
def __init__(self, endog, exog=None, offset=None, missing='none', **kwds):
# let them be none in case user wants to use inheritance
super(PoissonZiGMLE, self).__init__(endog, exog, missing=missing,
**kwds)
if offset is not None:
if offset.ndim == 1:
offset = offset[:,None] #need column
self.offset = offset.ravel() #which way?
else:
self.offset = 0.
#TODO: it's not standard pattern to use default exog
if exog is None:
self.exog = np.ones((self.nobs,1))
self.nparams = self.exog.shape[1]
#what's the shape in regression for exog if only constant
self.start_params = np.hstack((np.ones(self.nparams), 0))
self.cloneattr = ['start_params']
#needed for t_test and summary
self.exog_names.append('zi')
# original copied from discretemod.Poisson
[docs] def nloglikeobs(self, params):
"""
Loglikelihood of Poisson model
Parameters
----------
params : array_like
The parameters of the model.
Returns
-------
The log likelihood of the model evaluated at `params`
Notes
-----
.. math:: \\ln L=\\sum_{i=1}^{n}\\left[-\\lambda_{i}+y_{i}x_{i}^{\\prime}\\beta-\\ln y_{i}!\\right]
"""
beta = params[:-1]
gamm = 1 / (1 + np.exp(params[-1])) #check this
# replace with np.dot(self.exogZ, gamma)
#print(np.shape(self.offset), self.exog.shape, beta.shape
XB = self.offset + np.dot(self.exog, beta)
endog = self.endog
nloglik = -np.log(1-gamm) + np.exp(XB) - endog*XB + np.log(factorial(endog))
nloglik[endog==0] = - np.log(gamm + np.exp(-nloglik[endog==0]))
return nloglik