Description
About The Data Our goal for this lab is construct a model that can take a certain set of housing features and give us back a price estimate. Since price is a continuous variable, linear regression may be a good place to start from. The dataset that we’ll be using for this task comes from kaggle.com and contains the following attributes: ‘Avg. Area Income’: Avg. income of residents of the city house is located in. ‘Avg. Area House Age’: Avg age of houses in same city ‘Avg. Area Number of Rooms’: Avg number of rooms for houses in same city ‘Avg. Area Number of Bedrooms’: Avg number of bedrooms for houses in same city ‘Area Population’: Population of city house is located in ‘Price’: Price that the house sold at (target) ‘Address’: Address for the house Exploratory Data Analysis Let’s begin by importing some necessary libraries that we’ll be using to explore the data. Our first step is to load the data into a pandas DataFrame Avg. Area Income Avg. Area House Age Avg. Area Number of Rooms Avg. Area Number of Bedrooms Area Population Price Address 0 79545.458574 5.682861 7.009188 4.09 23086.800503 1.059034e+06 208 Michael Ferry Apt. 674\nLaurabury, NE 3701… 1 79248.642455 6.002900 6.730821 3.09 40173.072174 1.505891e+06 188 Johnson Views Suite 079\nLake Kathleen, CA… 2 61287.067179 5.865890 8.512727 5.13 36882.159400 1.058988e+06 9127 Elizabeth Stravenue\nDanieltown, WI 06482… 3 63345.240046 7.188236 5.586729 3.26 34310.242831 1.260617e+06 USS Barnett\nFPO AP 44820 4 59982.197226 5.040555 7.839388 4.23 26354.109472 6.309435e+05 USNS Raymond\nFPO AE 09386 From here, it’s always a good step to use describe() and info() to get a better sense of the data and see if we have any missing values. Avg. Area Income Avg. Area House Age Avg. Area Number of Rooms Avg. Area Number of Bedrooms Area Population Price count 5000.000000 5000.000000 5000.000000 5000.000000 5000.000000 5.000000e+03 mean 68583.108984 5.977222 6.987792 3.981330 36163.516039 1.232073e+06 std 10657.991214 0.991456 1.005833 1.234137 9925.650114 3.531176e+05 min 17796.631190 2.644304 3.236194 2.000000 172.610686 1.593866e+04 25% 61480.562388 5.322283 6.299250 3.140000 29403.928702 9.975771e+05 50% 68804.286404 5.970429 7.002902 4.050000 36199.406689 1.232669e+06 75% 75783.338666 6.650808 7.665871 4.490000 42861.290769 1.471210e+06 max 107701.748378 9.519088 10.759588 6.500000 69621.713378 2.469066e+06 The info below lets us know that we have 5,000 entries and 5,000 non‑null values in each feature/column. Therefore, there are no missing values in this dataset. <class ’pandas.core.frame.DataFrame’> RangeIndex: 5000 entries, 0 to 4999 Data columns (total 7 columns): # Column NonNull Count Dtype 0 Avg. Area Income 5000 nonnull float64 1 Avg. Area House Age 5000 nonnull float64 2 Avg. Area Number of Rooms 5000 nonnull float64 3 Avg. Area Number of Bedrooms 5000 nonnull float64 4 Area Population 5000 nonnull float64 5 Price 5000 nonnull float64 6 Address 5000 nonnull object dtypes: float64(6), object(1) memory usage: 273.6+ KB A quick pairplot lets us get an idea of the distributions and relationships in our dataset. From here, we could choose any interesting features that we’d like to later explore in greater depth. Warning: The more features in our dataset, the harder our pairplot will be to interpret. Taking a closer look at price, we see that it’s normally distributed with a peak around 1.232073e+06, and 75% of houses sold were at a price of 1.471210e+06 or lower. count 5.000000e+03 mean 1.232073e+06 std 3.531176e+05 min 1.593866e+04 25% 9.975771e+05 50% 1.232669e+06 75% 1.471210e+06 max 2.469066e+06 Name: Price, dtype: float64 A scatterplot of Price vs. Avg. Area Income shows a strong positive linear relationship between the two. Creating a boxplot of Avg. Area Number of Bedrooms lets us see that the median average area number of bedrooms is around 4, with a minimum of 2 and max of around 6.5. We can also so that there are no outliers present. Try plotting some of the other features for yourself to see if you can discover some interesting findings. Refer back to the matplotlib lab if you’re having trouble creating any graphs. Another important thing to look for while we’re exploring our data is multicollinearity. Multicollinearity means that several variables are essentially measuring the same thing. Not only is there no point to having more than one measure of the same thing in a model, but doing so can actually cause our model results to fluctuate. Luckily, checking for multicollinearity can be done easily with the help of a heatmap. Note: Depending on the situation, it may not be a problem for your model if only slight or moderate collinearity issue occur. However, it is strongly advised to solve the issue if severe collinearity issue exists(e.g. correlation >0.8 between 2 variables) This dataset is quite clean, and so there’s no severe collinearity issues. We’ll later dive into some messier datasets which will require some type of feature engineering or PCA to resolve. Creating Our Linear Model We’re now ready to begin creating and training our model. We first need to split our data into training and testing sets. This can be done using sklearn’s train_test_split(X, y, test_size) function. This function takes in your features (X), the target variable (y), and the test_size you’d like (Generally a test size of around 0.3 is good enough). It will then return a tuple of X_train, X_test, y_train, y_test sets for us. We will train our model on the training set and then use the test set to evaluate the model. We’ll now import sklearn’s LinearRegression model and begin training it using the fit(train_data, train_data_labels) method. In a nutshell, fitting is equal to training. Then, after it is trained, the model can be used to make predictions, usually with a predict(test_data) method call. You can think of fit as the step that finds the coefficients for the equation. LinearRegression() Model Evaluation Now that we’ve finished training, we can make predictions off of the test data and evaluate our model’s performance using the corresponding test data labels (y_test). To get a rough idea of how well the model is predicting, we can make a scatterplot with the true test labels (y_test) on the x‑axis, and our predictions on the y‑axis. Ideally, we’d like a 45 degree line. The straighter the line, the better our predictions are. Something that you may recall from MATH 3339 is that we’d like to see the residuals be normally distributed in regression analysis. We can exam this as follows: Here are the most common evaluation metrics for regression problems: Mean Absolute Error (MAE) is the mean of the absolute value of the errors: Mean Squared Error (MSE) is the mean of the squared errors: Root Mean Squared Error (RMSE) is the square root of the mean of the squared errors: Comparing these metrics: MAE is the easiest to understand, because it’s the average error. MSE is more popular than MAE, because MSE “punishes” larger errors, which tends to be useful in the real world. RMSE is even more popular than MSE, because RMSE is interpretable in the “y” units. All of these are loss functions, because we want to minimize them. Luckily, sklearn can calculate all of these metrics for us. All we need to do is pass the true labels (y_test) and our predictions to the functions below. What’s more important is that we understand what each of these means. Root Mean Square Error (RMSE) is what we’ll most commonly use, which is the standard deviation of the residuals (prediction errors). Residuals are a measure of how far from the regression line data points are; RMSE is a measure of how spread out these residuals are. In other words, it tells us how concentrated the data is around the line of best fit. Determining a good RMSE depends on your data. You can find a great example here, or refer back to the power points. MAE: 83410.59496702514 MSE: 10608579825.136667 RMSE: 102997.96029600133 Something we also like to look at is the coefficient of determination ( ), which is the percentage of variation in y explained by all the x variables together. Usually an of .70 is considered good. R2 Score: 0.9150208174786678 Finally, let’s see how we can interpret our model’s coefficients. We can access the coefficients by calling coef_ on our linear model (lm in this case). We’ll use this and put it in a nice pandas DataFrame for visual purposes. Note: You can also call intercept_ if you’d like to get the intercept. Coefficient Avg. Area Income 21.564645 Avg. Area House Age 166102.423648 Avg. Area Number of Rooms 122398.915857 Avg. Area Number of Bedrooms 887.665746 Area Population 15.309706 What these coefficients mean: Holding all other features fixed, a 1 unit increase in Avg. Area Income is associated with an increase of $21.564645 . Holding all other features fixed, a 1 unit increase in Avg. Area House Age is associated with an increase of $166102.423648 . Holding all other features fixed, a 1 unit increase in Avg. Area Number of Rooms is associated with an increase of $122398.915857 . Holding all other features fixed, a 1 unit increase in Avg. Area Number of Bedrooms is associated with an increase of $887.665746 . Holding all other features fixed, a 1 unit increase in Area Population is associated with an increase of $15.309706 . Congratulations! You now know how to create and evaluate linear models using sklearn. As extra practice, I’d recommend now trying to find a used car or similar housing dataset on kaggle.com and use this notebook as a guide. In [1]: import numpy as np import pandas as pd import matplotlib.pyplot as plt import seaborn as sns In [2]: from matplotlib import rcParams rcParams[‘figure.figsize’] = 15, 5 sns.set_style(‘darkgrid’) In [3]: housing_data = pd.read_csv(‘USA_Housing.csv’) housing_data.head() Out[3]: In [4]: housing_data.describe() Out[4]: In [5]: housing_data.info() In [6]: sns.pairplot(housing_data) plt.show() In [7]: sns.histplot(housing_data[‘Price’]) plt.show() print(housing_data[‘Price’].describe()) In [8]: sns.scatterplot(x=’Price’, y=’Avg. Area Income’, data=housing_data) plt.show() In [9]: sns.boxplot(x=’Avg. Area Number of Bedrooms’, data=housing_data) plt.show() In [10]: sns.heatmap(housing_data.corr(), annot=True) plt.show() In [11]: from sklearn.model_selection import train_test_split X = housing_data[[‘Avg. Area Income’, ‘Avg. Area House Age’, ‘Avg. Area Number of Room ’Avg. Area Number of Bedrooms’, ‘Area Population’]] y = housing_data[‘Price’] X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.3) In [12]: from sklearn.linear_model import LinearRegression lm = LinearRegression() lm.fit(X_train,y_train) Out[12]: In [13]: predictions = lm.predict(X_test) plt.scatter(y_test,predictions) plt.show() In [14]: residuals = y_test predictions sns.histplot(residuals) plt.show() n∑ i=1 |yi − y^i | 1 n n∑ i=1 (yi − y^i) 2 1 n ⎷ n∑ i=1 (yi − y^i) 2 1 n In [15]: from sklearn import metrics print(‘MAE:’, metrics.mean_absolute_error(y_test, predictions)) print(‘MSE:’, metrics.mean_squared_error(y_test, predictions)) print(‘RMSE:’, np.sqrt(metrics.mean_squared_error(y_test, predictions))) R2 R2 In [16]: from sklearn.metrics import r2_score print(‘R2 Score: ’, r2_score(y_test, predictions)) In [17]: coeff_df = pd.DataFrame(lm.coef_,X.columns,columns=[‘Coefficient’]) coeff_df Out[17]:
