# Source code for quantecon.inequality

"""
Implements inequality and segregation measures such as Gini, Lorenz Curve

"""

import numpy as np
from numba import njit, prange

[docs]@njit
def lorenz_curve(y):
"""
Calculates the Lorenz Curve, a graphical representation of the distribution of income
or wealth.

It returns the cumulative share of people (x-axis) and the cumulative share of income earned

Parameters
----------
y : array_like(float or int, ndim=1)
Array of income/wealth for each individual. Unordered or ordered is fine.

Returns
-------
cum_people : array_like(float, ndim=1)
Cumulative share of people for each person index (i/n)
cum_income : array_like(float, ndim=1)
Cumulative share of income for each person index

References
----------
.. [1] https://en.wikipedia.org/wiki/Lorenz_curve

Examples
--------
>>> a_val, n = 3, 10_000
>>> y = np.random.pareto(a_val, size=n)
>>> f_vals, l_vals = lorenz(y)

"""

n = len(y)
y = np.sort(y)
s = np.zeros(n + 1)
s[1:] = np.cumsum(y)
cum_people = np.zeros(n + 1)
cum_income = np.zeros(n + 1)
for i in range(1, n + 1):
cum_people[i] = i / n
cum_income[i] = s[i] / s[n]
return cum_people, cum_income

[docs]@njit(parallel=True)
def gini_coefficient(y):
r"""
Implements the Gini inequality index

Parameters
-----------
y : array_like(float)
Array of income/wealth for each individual. Ordered or unordered is fine

Returns
-------
Gini index: float
The gini index describing the inequality of the array of income/wealth

References
----------

https://en.wikipedia.org/wiki/Gini_coefficient
"""
n = len(y)
i_sum = np.zeros(n)
for i in prange(n):
for j in range(n):
i_sum[i] += abs(y[i] - y[j])
return np.sum(i_sum) / (2 * n * np.sum(y))

[docs]def shorrocks_index(A):
r"""
Implements Shorrocks mobility index

Parameters
-----------
A : array_like(float)
Square matrix with transition probabilities (mobility matrix) of
dimension m

Returns
--------
Shorrocks index: float
The Shorrocks mobility index calculated as

.. math::

s(A) = \frac{m - \sum_j a_{jj} }{m - 1} \in (0, 1)

An index equal to 0 indicates complete immobility.

References
-----------
.. [1] Wealth distribution and social mobility in the US: A quantitative approach
(Benhabib, Bisin, Luo, 2017).
https://www.econ.nyu.edu/user/bisina/RevisionAugust.pdf
"""

A = np.asarray(A)  # Convert to array if not already
m, n = A.shape

if m != n:
raise ValueError('A must be a square matrix')

diag_sum = np.diag(A).sum()

return (m - diag_sum) / (m - 1)