# Theil-Sen Regression#

Computes a Theil-Sen Regression on a synthetic dataset.

Compared to the OLS (ordinary least squares) estimator, the Theil-Sen estimator is robust against outliers. It has a breakdown point of about 29.3% in case of a simple linear regression which means that it can tolerate arbitrary corrupted data (outliers) of up to 29.3% in the two-dimensional case.

The estimation of the model is done by calculating the slopes and intercepts of a subpopulation of all possible combinations of p subsample points. If an intercept is fitted, p must be greater than or equal to n_features + 1. The final slope and intercept is then defined as the spatial median of these slopes and intercepts.

In certain cases Theil-Sen performs better than RANSAC which is also a robust method. This is illustrated in the second example below where outliers with respect to the x-axis perturb RANSAC. Tuning the `residual_threshold` parameter of RANSAC remedies this but in general a priori knowledge about the data and the nature of the outliers is needed. Due to the computational complexity of Theil-Sen it is recommended to use it only for small problems in terms of number of samples and features. For larger problems the `max_subpopulation` parameter restricts the magnitude of all possible combinations of p subsample points to a randomly chosen subset and therefore also limits the runtime. Therefore, Theil-Sen is applicable to larger problems with the drawback of losing some of its mathematical properties since it then works on a random subset.

```# Authors: The scikit-learn developers

import time

import matplotlib.pyplot as plt
import numpy as np

from sklearn.linear_model import LinearRegression, RANSACRegressor, TheilSenRegressor

estimators = [
("OLS", LinearRegression()),
("Theil-Sen", TheilSenRegressor(random_state=42)),
("RANSAC", RANSACRegressor(random_state=42)),
]
colors = {"OLS": "turquoise", "Theil-Sen": "gold", "RANSAC": "lightgreen"}
lw = 2
```

## Outliers only in the y direction#

```np.random.seed(0)
n_samples = 200
# Linear model y = 3*x + N(2, 0.1**2)
x = np.random.randn(n_samples)
w = 3.0
c = 2.0
noise = 0.1 * np.random.randn(n_samples)
y = w * x + c + noise
# 10% outliers
y[-20:] += -20 * x[-20:]
X = x[:, np.newaxis]

plt.scatter(x, y, color="indigo", marker="x", s=40)
line_x = np.array([-3, 3])
for name, estimator in estimators:
t0 = time.time()
estimator.fit(X, y)
elapsed_time = time.time() - t0
y_pred = estimator.predict(line_x.reshape(2, 1))
plt.plot(
line_x,
y_pred,
color=colors[name],
linewidth=lw,
label="%s (fit time: %.2fs)" % (name, elapsed_time),
)

plt.axis("tight")
plt.legend(loc="upper left")
_ = plt.title("Corrupt y")
```

## Outliers in the X direction#

```np.random.seed(0)
# Linear model y = 3*x + N(2, 0.1**2)
x = np.random.randn(n_samples)
noise = 0.1 * np.random.randn(n_samples)
y = 3 * x + 2 + noise
# 10% outliers
x[-20:] = 9.9
y[-20:] += 22
X = x[:, np.newaxis]

plt.figure()
plt.scatter(x, y, color="indigo", marker="x", s=40)

line_x = np.array([-3, 10])
for name, estimator in estimators:
t0 = time.time()
estimator.fit(X, y)
elapsed_time = time.time() - t0
y_pred = estimator.predict(line_x.reshape(2, 1))
plt.plot(
line_x,
y_pred,
color=colors[name],
linewidth=lw,
label="%s (fit time: %.2fs)" % (name, elapsed_time),
)

plt.axis("tight")
plt.legend(loc="upper left")
plt.title("Corrupt x")
plt.show()
```

Total running time of the script: (0 minutes 0.524 seconds)

Related examples

Robust linear model estimation using RANSAC

Robust linear model estimation using RANSAC

Robust linear estimator fitting

Robust linear estimator fitting

Compare the effect of different scalers on data with outliers

Compare the effect of different scalers on data with outliers

Logistic function

Logistic function

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