Single feature linear regression is really simple. All you need is to find function that fits training data best. It is also easy to plot data and learned curve. But in reality regression analysis is based on multiple features. So in most cases we cannot imagine the multidimensional space where data could be plotted. We need to rely on methods we use. You have to feel comfortable with linear algebra where matrices and vectors are used. If previously we had one feature (temperature) now we need to introduce more of them. So we need to expand hypothesis to accept more features. From now and later on instead of output y we are gonna use h(x) notation:
As you can see with more variables (features) we also end up with more parameters θ that has to be learned. Before we move lets find suitable data that we could use for building machine learning algorithm.
The data set
Again we are going to use data set college cengage. This time we select health data set with several variables.
The data (X1, X2, X3, X4, X5) are by city.
X1 = death rate per 1000 residents
X2 = doctor availability per 100,000 residents
X3 = hospital availability per 100,000 residents
X4 = annual per capita income in thousands of dollars
X5 = population density people per square mile
Reference: Life In America’s Small Cities, by G.S. Thomas
Here we take variables X2, X3, X4, X5 as input and X1 as output. And then we build a linear regression model where we are going to predict death rate per 1000 residents.
Death rate per 1000 residents (y) 
Doctor availability per 100,000 residents (x1) 
Hospital availability per 100,000 (x2) 
Annual per capita income in thousands of dollars (x3) 
Population density people per square mile (x4) 
8 
78 
284 
9.100000381 
109 
9.300000191 
68 
433 
8.699999809 
144 
7.5 
70 
739 
7.199999809 
113 
… 
… 
… 
… 
… 
There are n = 53 sets of features in this data set.
Linear regression hypothesis
We have four features in each training example and one output so we will need to find hyphothesis:
When working with linear algebra we must follow the few notation rules in order to make it easier to follow. So we annotate i^{th} training example for instance our first training example vector looks like:
And further we can annotate j^{th} feature in i^{th} training example . For instance from our training set we can see that 2 feature of 2 training example is .
Since we are using linear algebra, it is convenient to write hypothesis by including x_{0} variable which is always equal to 1. So we can write hypothesis h(x) in very compact form:
where
and
So this is pure linear algebra and we won’t go deeper in to this.
Feature normalization
When building linear regression model with multiple features we face another problem. The values of features may differ by orders of magnitude. For instance Annual per capita income in thousands of dollars (x3) value is about 10 while Hospital availability per 100,000 (x2) may reach several hundreds. This may influence the gradient descent calculation effectiveness. So we need to scale features to make algorithm converge much faster. The procedure is really simple:

Subtract the mean value of each feature from the training set.

Then additionally scale (divide) the feature values by their respective standard deviations.
To do this we can write pretty simple python algorithm:
def normalization(x): mean_x = []; std_x = []; X_normalized = x; temp = x.shape[1] for i in range(temp): m = np.mean(x[:, i]) s = np.std(x[:, i]) mean_x.append(m) std_x.append(s) X_normalized[:, i] = (X_normalized[:, i]  m) / s return X_normalized, mean_x, std_x
Since we are dealing with multidimensional data, we also need to write vectorized cost and gradient descent functions:
Vectorized Cost function
def cost(x,y,theta): m = y.size #number of training examples predicted = np.dot(x,theta) sqErr = (predicted  y) J = (1.0) / (2 * m) * np.dot(sqErr.T, sqErr) return J
Vectorized gradient algorithm
Run until converge:
def gradient_descent(x, y, theta, alpha, iterations): #gradient descent algorithm to find optimal theta values m = y.size #theta size theta_n = theta.size #cost history J_theta_log = np.zeros(shape=(iterations+1, 1)) #store initial values in to log J_theta_log[0, 0] = cost(x, y, theta) for i in range(iterations): #split equation in to several parts predicted = x.dot(theta) for thetas in range(theta_n): tmp = x[:,thetas] tmp.shape = (m,1) err = (predicted  y) * tmp theta[thetas][0] = theta[thetas][0]  alpha * (1.0 / m) * err.sum() J_theta_log[i+1, 0] = cost(x, y, theta) return theta, J_theta_log
How to know what number of iterations and learning rate alpha to choose? Normally you should keep an eye on Cost function change during each iteration. We are keeping cost function values in J_theta_log that can be put on chart. If everything is selected correctly you should see normal convergence function towards zero:
If you see cost increasing or wavy, then try decreasing learning rate parameter alpha (try 0.01, 0.001 and so on). In other hand if cost function doesn’t reach flat shape, try increasing number of iterations. If you see that convergence doesn’t decrease by less than 0.001 you can stop learning and use learned parameters in your regression formula.
Gradient descent algorithm can be improved by implementing automated check on minimal error, this way it could stop earlier than performing all iterations.
With this record we are going to finish on linear regression. It is fairly simple and fast machine learning method which is used in many areas. It is ideal solution to b implemented in to microcontroller applications where processing power and memory is limited. But in reality, not all data can be fitted with linear models, so there are more advanced methods used like Logistic regression, support vector machines, neural networks, classifiers. We will take a look at them gradually.
You can download Anaconda project files here (linear_regression_multiple.zip) if you would like to try running linear regression algorithm by yourself.