#!/usr/bin/env python

# DO NOT EDIT
# Autogenerated from the notebook ols.ipynb.
# Edit the notebook and then sync the output with this file.
#
# flake8: noqa
# DO NOT EDIT

# # Ordinary Least Squares

import matplotlib.pyplot as plt
import numpy as np
import pandas as pd
import statsmodels.api as sm

np.random.seed(9876789)

# ## OLS estimation
#
# Artificial data:

nsample = 100
x = np.linspace(0, 10, 100)
X = np.column_stack((x, x**2))
beta = np.array([1, 0.1, 10])
e = np.random.normal(size=nsample)

# Our model needs an intercept so we add a column of 1s:

X = sm.add_constant(X)
y = np.dot(X, beta) + e

# Fit and summary:

model = sm.OLS(y, X)
results = model.fit()
print(results.summary())

# Quantities of interest can be extracted directly from the fitted model.
# Type ``dir(results)`` for a full list. Here are some examples:

print("Parameters: ", results.params)
print("R2: ", results.rsquared)

# ## OLS non-linear curve but linear in parameters
#
# We simulate artificial data with a non-linear relationship between x and
# y:

nsample = 50
sig = 0.5
x = np.linspace(0, 20, nsample)
X = np.column_stack((x, np.sin(x), (x - 5)**2, np.ones(nsample)))
beta = [0.5, 0.5, -0.02, 5.0]

y_true = np.dot(X, beta)
y = y_true + sig * np.random.normal(size=nsample)

# Fit and summary:

res = sm.OLS(y, X).fit()
print(res.summary())

# Extract other quantities of interest:

print("Parameters: ", res.params)
print("Standard errors: ", res.bse)
print("Predicted values: ", res.predict())

# Draw a plot to compare the true relationship to OLS predictions.
# Confidence intervals around the predictions are built using the
# ``wls_prediction_std`` command.

pred_ols = res.get_prediction()
iv_l = pred_ols.summary_frame()["obs_ci_lower"]
iv_u = pred_ols.summary_frame()["obs_ci_upper"]

fig, ax = plt.subplots(figsize=(8, 6))

ax.plot(x, y, "o", label="data")
ax.plot(x, y_true, "b-", label="True")
ax.plot(x, res.fittedvalues, "r--.", label="OLS")
ax.plot(x, iv_u, "r--")
ax.plot(x, iv_l, "r--")
ax.legend(loc="best")

# ## OLS with dummy variables
#
# We generate some artificial data. There are 3 groups which will be
# modelled using dummy variables. Group 0 is the omitted/benchmark category.

nsample = 50
groups = np.zeros(nsample, int)
groups[20:40] = 1
groups[40:] = 2
# dummy = (groups[:,None] == np.unique(groups)).astype(float)

dummy = pd.get_dummies(groups).values
x = np.linspace(0, 20, nsample)
# drop reference category
X = np.column_stack((x, dummy[:, 1:]))
X = sm.add_constant(X, prepend=False)

beta = [1.0, 3, -3, 10]
y_true = np.dot(X, beta)
e = np.random.normal(size=nsample)
y = y_true + e

# Inspect the data:

print(X[:5, :])
print(y[:5])
print(groups)
print(dummy[:5, :])

# Fit and summary:

res2 = sm.OLS(y, X).fit()
print(res2.summary())

# Draw a plot to compare the true relationship to OLS predictions:

pred_ols2 = res2.get_prediction()
iv_l = pred_ols2.summary_frame()["obs_ci_lower"]
iv_u = pred_ols2.summary_frame()["obs_ci_upper"]

fig, ax = plt.subplots(figsize=(8, 6))

ax.plot(x, y, "o", label="Data")
ax.plot(x, y_true, "b-", label="True")
ax.plot(x, res2.fittedvalues, "r--.", label="Predicted")
ax.plot(x, iv_u, "r--")
ax.plot(x, iv_l, "r--")
legend = ax.legend(loc="best")

# ## Joint hypothesis test
#
# ### F test
#
# We want to test the hypothesis that both coefficients on the dummy
# variables are equal to zero, that is, $R \times \beta = 0$. An F test
# leads us to strongly reject the null hypothesis of identical constant in
# the 3 groups:

R = [[0, 1, 0, 0], [0, 0, 1, 0]]
print(np.array(R))
print(res2.f_test(R))

# You can also use formula-like syntax to test hypotheses

print(res2.f_test("x2 = x3 = 0"))

# ### Small group effects
#
# If we generate artificial data with smaller group effects, the T test
# can no longer reject the Null hypothesis:

beta = [1.0, 0.3, -0.0, 10]
y_true = np.dot(X, beta)
y = y_true + np.random.normal(size=nsample)

res3 = sm.OLS(y, X).fit()

print(res3.f_test(R))

print(res3.f_test("x2 = x3 = 0"))

# ### Multicollinearity
#
# The Longley dataset is well known to have high multicollinearity. That
# is, the exogenous predictors are highly correlated. This is problematic
# because it can affect the stability of our coefficient estimates as we
# make minor changes to model specification.

from statsmodels.datasets.longley import load_pandas

y = load_pandas().endog
X = load_pandas().exog
X = sm.add_constant(X)

# Fit and summary:

ols_model = sm.OLS(y, X)
ols_results = ols_model.fit()
print(ols_results.summary())

# #### Condition number
#
# One way to assess multicollinearity is to compute the condition number.
# Values over 20 are worrisome (see Greene 4.9). The first step is to
# normalize the independent variables to have unit length:

norm_x = X.values
for i, name in enumerate(X):
    if name == "const":
        continue
    norm_x[:, i] = X[name] / np.linalg.norm(X[name])
norm_xtx = np.dot(norm_x.T, norm_x)

# Then, we take the square root of the ratio of the biggest to the
# smallest eigen values.

eigs = np.linalg.eigvals(norm_xtx)
condition_number = np.sqrt(eigs.max() / eigs.min())
print(condition_number)

# #### Dropping an observation
#
# Greene also points out that dropping a single observation can have a
# dramatic effect on the coefficient estimates:

ols_results2 = sm.OLS(y.iloc[:14], X.iloc[:14]).fit()
print("Percentage change %4.2f%%\n" * 7 % tuple([
    i for i in (ols_results2.params - ols_results.params) /
    ols_results.params * 100
]))

# We can also look at formal statistics for this such as the DFBETAS -- a
# standardized measure of how much each coefficient changes when that
# observation is left out.

infl = ols_results.get_influence()

# In general we may consider DBETAS in absolute value greater than
# $2/\sqrt{N}$ to be influential observations

2.0 / len(X)**0.5

print(infl.summary_frame().filter(regex="dfb"))
