*From the video series: Introduction to machine learning with scikit-learn*

- Train the model on the
**entire dataset**. - Test the model on the
**same dataset**, and evaluate how well we did by comparing the**predicted**response values with the**true**response values.

```
In [2]:
```# read in the iris data
from sklearn.datasets import load_iris
iris = load_iris()
# create X (features) and y (response)
X = iris.data
y = iris.target

```
In [3]:
```# import the class
from sklearn.linear_model import LogisticRegression
# instantiate the model (using the default parameters)
logreg = LogisticRegression()
# fit the model with data
logreg.fit(X, y)
# predict the response values for the observations in X
logreg.predict(X)

```
Out[3]:
```

```
In [4]:
```# store the predicted response values
y_pred = logreg.predict(X)
# check how many predictions were generated
len(y_pred)

```
Out[4]:
```

Classification accuracy:

**Proportion**of correct predictions- Common
**evaluation metric**for classification problems

```
In [5]:
```# compute classification accuracy for the logistic regression model
from sklearn import metrics
print metrics.accuracy_score(y, y_pred)

```
```

- Known as
**training accuracy**when you train and test the model on the same data

```
In [6]:
```from sklearn.neighbors import KNeighborsClassifier
knn = KNeighborsClassifier(n_neighbors=5)
knn.fit(X, y)
y_pred = knn.predict(X)
print metrics.accuracy_score(y, y_pred)

```
```

```
In [7]:
```knn = KNeighborsClassifier(n_neighbors=1)
knn.fit(X, y)
y_pred = knn.predict(X)
print metrics.accuracy_score(y, y_pred)

```
```

*Image Credit: Overfitting by Chabacano. Licensed under GFDL via Wikimedia Commons.*

- Split the dataset into two pieces: a
**training set**and a**testing set**. - Train the model on the
**training set**. - Test the model on the
**testing set**, and evaluate how well we did.

```
In [8]:
```# print the shapes of X and y
print X.shape
print y.shape

```
```

```
In [9]:
```# STEP 1: split X and y into training and testing sets
from sklearn.cross_validation import train_test_split
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.4, random_state=4)

What did this accomplish?

- Model can be trained and tested on
**different data** - Response values are known for the testing set, and thus
**predictions can be evaluated** **Testing accuracy**is a better estimate than training accuracy of out-of-sample performance

```
In [10]:
```# print the shapes of the new X objects
print X_train.shape
print X_test.shape

```
```

```
In [11]:
```# print the shapes of the new y objects
print y_train.shape
print y_test.shape

```
```

```
In [12]:
```# STEP 2: train the model on the training set
logreg = LogisticRegression()
logreg.fit(X_train, y_train)

```
Out[12]:
```

```
In [13]:
```# STEP 3: make predictions on the testing set
y_pred = logreg.predict(X_test)
# compare actual response values (y_test) with predicted response values (y_pred)
print metrics.accuracy_score(y_test, y_pred)

```
```

Repeat for KNN with K=5:

```
In [14]:
```knn = KNeighborsClassifier(n_neighbors=5)
knn.fit(X_train, y_train)
y_pred = knn.predict(X_test)
print metrics.accuracy_score(y_test, y_pred)

```
```

Repeat for KNN with K=1:

```
In [15]:
```knn = KNeighborsClassifier(n_neighbors=1)
knn.fit(X_train, y_train)
y_pred = knn.predict(X_test)
print metrics.accuracy_score(y_test, y_pred)

```
```

Can we locate an even better value for K?

```
In [16]:
```# try K=1 through K=25 and record testing accuracy
k_range = range(1, 26)
scores = []
for k in k_range:
knn = KNeighborsClassifier(n_neighbors=k)
knn.fit(X_train, y_train)
y_pred = knn.predict(X_test)
scores.append(metrics.accuracy_score(y_test, y_pred))

```
In [17]:
```# import Matplotlib (scientific plotting library)
import matplotlib.pyplot as plt
# allow plots to appear within the notebook
%matplotlib inline
# plot the relationship between K and testing accuracy
plt.plot(k_range, scores)
plt.xlabel('Value of K for KNN')
plt.ylabel('Testing Accuracy')

```
Out[17]:
```

**Training accuracy**rises as model complexity increases**Testing accuracy**penalizes models that are too complex or not complex enough- For KNN models, complexity is determined by the
**value of K**(lower value = more complex)

```
In [18]:
```# instantiate the model with the best known parameters
knn = KNeighborsClassifier(n_neighbors=11)
# train the model with X and y (not X_train and y_train)
knn.fit(X, y)
# make a prediction for an out-of-sample observation
knn.predict([3, 5, 4, 2])

```
Out[18]:
```

- Provides a
**high-variance estimate**of out-of-sample accuracy **K-fold cross-validation**overcomes this limitation- But, train/test split is still useful because of its
**flexibility and speed**

- Quora: What is an intuitive explanation of overfitting?
- Video: Estimating prediction error (12 minutes, starting at 2:34) by Hastie and Tibshirani
- Understanding the Bias-Variance Tradeoff
- Guiding questions when reading this article

- Video: Visualizing bias and variance (15 minutes) by Abu-Mostafa

- Email: kevin@dataschool.io
- Website: http://dataschool.io
- Twitter: @justmarkham

```
In [1]:
```from IPython.core.display import HTML
def css_styling():
styles = open("styles/custom.css", "r").read()
return HTML(styles)
css_styling()

```
Out[1]:
```