Empirical Likelihood with Moment Constraints

empirical_likelihood.py

from statsmodels.model import LikelihoodModel
import numpy as np
from numpy import asarray
from scipy import optimize

def log_star(z, eps):
    """
    Owens' log* function. Provides curvature at eps.
    """
    log_idx = z > eps
    loggedarr = np.zeros_like(z)
    z_ok = z[log_idx]
    z_star = z[~log_idx]
    loggedarr[log_idx] = np.log(z_ok)
    loggedarr[~log_idx] = np.log(eps) - 1.5 + 2*z_star/eps - \
            z_star**2/(2*eps**2)
    return loggedarr

class EstEqs(LikelihoodModel):
    """
    Class for Estimating Equations

    esteqs is an iterable of functions (func1, func2, ...) and args is an
    iterable of arguments to be passed to the functions ((args1,),
    (args2,)).
    The estimating equations are expected to be called, for instance, as
    func1(params, *args1)
    func2(params, *args2)
    """
    def __init__(self, endog, exog=None, esteqs=(), args=()):
        if esteqs is None:
            raise ValueError("Specify estimating equations")
#        if len(esteqs) != len(args):
#            raise ValueError("The length of esteq and args are not the same.")
#        self.J = len(esteqs) # number of estimating equations
        self.esteqs = esteqs
        self._args = args
        super(EstEqs, self).__init__(endog, exog)

    def _get_esteqs(self, params):
#        esteqs = self.esteqs
#        arr = np.empty((self.nobs,self.J))
#        args = self._args
#        for i,eq in enumerate(esteqs):
#            arr[:,i,None] = eq(params, *args[i])
        return self.esteqs(params, *self._args)
        #unnecessary, really with this syntax

    def initialize(self):
        pass


class EL(EstEqs):
    """
    Empirical Likelihood

    Notes
    -----
    The API is experimental
    """
    def __init__(self, endog, exog=None, esteqs=(), args=()):
        super(EL, self).__init__(endog, exog, esteqs, args)

    def pmf(self, params):
        nobs = self.nobs
        esteqs = self._get_esteqs(params)
        constraints = self.constraints
        return (nobs * (1+(constraints*esteqs).sum(1)))**-1 # should use dot

    def _get_constraints(self, constraints, params):
        esteqs = self._get_esteqs(params)
        nobs = self.nobs
        return 1./nobs*np.sum(1./(1-(constraints*esteqs).sum(1))[:,None]\
                    *esteqs, axis=0)

    def fit(self, start_params, params_bounds=None):
#        if self.exog is None: # no way to guess params
                              # you're going to *have* to give start_params
#            start_params = np.array([0.1])

#TODO: if it's a linear model, then do this.
#            exog = self.exog
#            k = exog.shape[1]
#            start_params = np.array([0.1]*k)

        start_params = np.array(start_params)
        interm_func = self._get_constraints
#        self.constraints = optimize.fsolve(interm_func, [0,0],
#                                            args=(start_params,))
        func = self.emploglike
        ret = optimize.fmin_tnc(func, start_params, approx_grad=True, eta=1e-4,
                                pgtol=1e-8, xtol=1e-5, ftol=1e-24, messages=0)
#        constraints = self._get_constraints(constraints, ret[0])
#        self.constraints = optimize.fsolve(interm_func, [0,0], args=ret[0])
        self.params = ret[0]
        self.results = ret

    def emploglike(self, params):
        """
        Dual (scaled) empirical likelihood. log(L_{EL}(params;endog))
        """
        interm_func = self._get_constraints
        print params, "params"
#        constraints = optimize.fsolve(interm_func, [0,0], args=(params,),
#            factor=10, epsfcn=1e-5, xtol=1e-6)

        nobs = self.nobs
        eps = 1./nobs
#Try R docs way
        def P(constraints, params):
            esteq = self._get_esteqs(params)
            return -1./nobs * np.sum(log_star(1-(constraints*esteq).sum(1),
                            eps))

        constraints = optimize.fmin_tnc(P, [0,0], args=(params,),
                        approx_grad=True, messages=0)[0]
        print constraints, "constraints"
        self.constraints = constraints
        esteqs = self._get_esteqs(params)
        return 1./nobs*np.sum(log_star(1-(constraints*esteqs).sum(1),
                                    eps))

#TODO: looks close, but we run sometimes into negative values in the
#params, check the constraint that can't be violated
if __name__ == "__main__":
    from scipy import stats
    theta = 1.0
#    np.random.seed(1234)
#    Y = stats.norm.rvs(theta, theta**2 + 1,size=(40,1))
    Y = np.loadtxt('./Y_EL.txt')
    Y = Y[:,None]

    def esteqs(params, Y):
        params = asarray([params])
        return np.column_stack((Y - params,Y**2 - 2*params**2 -1))

    def esteqs_grad(params, Y):
        return np.column_stack((-np.ones((len(Y),1)),
                    np.repeat(-4*params,(40,1))))

    ELmod = EL(Y, esteqs=esteqs, args=(Y,))

    # (Y,) TODO: set it so that it accepts (Y) as well
    ELmod.fit(np.array([Y.mean()]), params_bounds=[(Y.min(),Y.max())])