def __call__(self, a, mu=None, initscale=None, axis=0):
"""
Compute Huber's proposal 2 estimate of scale, using an optional
initial value of scale and an optional estimate of mu. If mu
is supplied, it is not reestimated.
Parameters
----------
a : array
1d array
mu : float or None, optional
If the location mu is supplied then it is not reestimated.
Default is None, which means that it is estimated.
initscale : float or None, optional
A first guess on scale. If initscale is None then the standardized
median absolute deviation of a is used.
Notes
-----
`Huber` minimizes the function
sum(psi((a[i]-mu)/scale)**2)
as a function of (mu, scale), where
psi(x) = np.clip(x, -self.c, self.c)
"""
a = np.asarray(a)
if mu is None:
n = a.shape[0] - 1
mu = np.median(a, axis=axis)
est_mu = True
else:
n = a.shape[0]
mu = mu
est_mu = False
if initscale is None:
scale = stand_mad(a, axis=axis)
else:
scale = initscale
scale = tools.unsqueeze(scale, axis, a.shape)
mu = tools.unsqueeze(mu, axis, a.shape)
return self._estimate_both(a, scale, mu, axis, est_mu, n)
def _estimate_both(self, a, scale, mu, axis, est_mu, n):
"""
Estimate scale and location simultaneously with the following
pseudo_loop:
while not_converged:
mu, scale = estimate_location(a, scale, mu), estimate_scale(a, scale, mu)
where estimate_location is an M-estimator and estimate_scale implements
the check used in Section 5.5 of Venables & Ripley
"""
for _ in range(self.maxiter):
# Estimate the mean along a given axis
if est_mu:
if self.norm is None:
# This is a one-step fixed-point estimator
# if self.norm == norms.HuberT
# It should be faster than using norms.HuberT
nmu = np.clip(a, mu-self.c*scale,
mu+self.c*scale).sum(axis) / a.shape[axis]
else:
nmu = norms.estimate_location(a, scale, self.norm, axis, mu,
self.maxiter, self.tol)
else:
# Effectively, do nothing
nmu = mu.squeeze()
nmu = tools.unsqueeze(nmu, axis, a.shape)
subset = np.less_equal(np.fabs((a - mu)/scale), self.c)
card = subset.sum(axis)
nscale = np.sqrt(np.sum(subset * (a - nmu)**2, axis) \
/ (n * self.gamma - (a.shape[axis] - card) * self.c**2))
nscale = tools.unsqueeze(nscale, axis, a.shape)
test1 = np.alltrue(np.less_equal(np.fabs(scale - nscale),
nscale * self.tol))
test2 = np.alltrue(np.less_equal(np.fabs(mu - nmu), nscale*self.tol))
if not (test1 and test2):
mu = nmu; scale = nscale
else:
return nmu.squeeze(), nscale.squeeze()
raise ValueError('joint estimation of location and scale failed to converge in %d iterations' % self.maxiter)
def stand_mad(a, c=Gaussian.ppf(3/4.), axis=0):
"""
The standardized Median Absolute Deviation along given axis of an array.
Parameters
----------
a : array-like
Input array.
c : float, optional
The normalization constant. Defined as scipy.stats.norm.ppf(3/4.),
which is approximately .6745.
axis : int, optional
The defaul is 0.
Returns
-------
mad : float
`mad` = median(abs(`a`-median(`a`))/`c`
"""
a = np.asarray(a)
d = np.median(a, axis = axis)
d = tools.unsqueeze(d, axis, a.shape)
return np.median(np.fabs(a - d)/c, axis = axis)
