How to calculate the confidence intervals for prediction in Regression? and also how to plot it in python

enter image description here

Fig 7.1, An Introduction To Statistical Learning

I am currently studying a book named Introduction to Statistical Learning with applications in R, and also converting the solutions to python language.
I am not able to get how to get the confidence intervals and plot them as shown in the above image(dashed lines). I have plotted the line. Here's my code for that - (I am using polynomial regression with predictiors - 'age' and response - 'wage',degree is 4)

poly = PolynomialFeatures(4)
X = poly.fit_transform(data['age'].to_frame())
y = data['wage']
# X.shape

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

# So, what we want here is not only the final line, but also the standart error related to the line
# TO find that we need to calcualte the predictions for some values of age
test_ages = np.linspace(data['age'].min(),data['age'].max(),100)

X_test = poly.transform(test_ages.reshape(-1,1))
pred = model.predict(X_test)

plt.figure(figsize = (12,8))
plt.scatter(data['age'],data['wage'],facecolors='none', edgecolors='darkgray')

Here data is WAGE data which is available in R. This is the resulting graph i get -

This is what i was able to plot


  • I have used bootstraping to calculate the confidence intervals, for this i have used a self customed module -

    import numpy as np
    import pandas as pd
    from tqdm import tqdm
    class Bootstrap_ci:
        def boot(self,X_data,y_data,R,test_data,model):
            predictions = []
            for i in tqdm(range(R)):
            return np.percentile(predictions,2.5,axis = 0),np.percentile(predictions,97.5,axis = 0)
        def alpha(self,X_data,y_data,index,test_data,model):
            X = X_data.loc[index]
            y = y_data.loc[index]
            lr = model
            return lr.predict(pd.DataFrame(test_data))
        def get_indices(self,data,num_samples):
            return  np.random.choice(data.index, num_samples, replace=True)

    The above module can be used as -

    poly = PolynomialFeatures(4)
    X = poly.fit_transform(data['age'].to_frame())
    y = data['wage']
    X_test = np.linspace(min(data['age']),max(data['age']),100)
    X_test_poly = poly.transform(X_test.reshape(-1,1))
    from bootstrap import Bootstrap_ci
    bootstrap = Bootstrap_ci()
    li,ui = bootstrap.boot(pd.DataFrame(X),y,1000,X_test_poly,LinearRegression())

    This will give us the lower confidence interval, and upper confidence interval. To plot the graph -

    plt.scatter(data['age'],data['wage'],facecolors='none', edgecolors='darkgray')
    plt.plot(X_test,pred,label = 'Fitted Line')
    plt.plot(X_test,ui,linestyle = 'dashed',color = 'r',label = 'Confidence Intervals')
    plt.plot(X_test,li,linestyle = 'dashed',color = 'r')

    The resultant graph is

    enter image description here