How to train a Neural Network with Data
Introduction to Artificial Intelligence and Neural Networks
To learn from mistakes and take decisions on a new scenario by learning at past experiences is the trademark of human intelligence. The scientists have been long trying to implement this intelligence in machines as well. This study of making machines intelligent and self-learning is called Artificial Intelligence or Computational Intelligence.
Although Artificial Intelligence has been around since the 1950s, it became famous in the recent past. The main reason for this rise was the revival of deep learning or in other words deep neural networks. Neural networks have been around for more than 50 years now but, their use was not very common due to their computational cost. However, as the computer hardware started to improve neural networks started becoming a reality.
Everyone started taking the interest in using the neural networks for all sorts of tasks like speech recognition and image classification. Things finally come to a head in 2012 on the Large Scale Visual Recognition Challenge(LSVRC) 1. In 2010, a large database known as Imagenet containing millions of labeled images was created and published by Fei-Fei Li’s group at Stanford 2. This database was coupled with the annual LSVRC,in this competition the contests build their own models, make them predict on the test data and get ranked for their accuracy. So, along with all other discoveries like new non-linearity introducing functions, fast computing with help of GPUs, new kind of optimizers and improved architectures of neural networks, data also played a key role in fueling neural networks.
Neural Networks Basic Terminology and Learning
Now that we know what these neural networks are and how we make them learn to make them mimic human intelligence. Well, neural networks in general sense are made up of neurons. A neuron is a mathematical function that takes some number of inputs, for each input it assigns some weight and a same bias value for all inputs (these are the variables that we learn during the training), and applies some non-linearity (Rectified Linear Unit (ReLU)) in most cases and generates an output.
In a typical neural network, these neurons are stacked vertically to form a layer and then these layers are stacked horizontally to form a multilayer neural network. The first layer of a neural network is usually the input layer, the last layer is called the output layer and the middle layers are called hidden layers. The number of neurons in the first layer equals the number of input variables (features), the number of neurons in the middle layers are hyperparameters (that you decide for optimization) and the number of neurons in the last layer is equal to the number of desired outputs required from the network. We also use a loss function to calculate the deviation between the actual values and the output of our network, this helps in learning the weights of a neural network.
Once you are familiar with the basic terminology of neural networks mentioned above. It is not difficult to visualize the training process for a neural network. We start by building a neural network keeping the number of neurons in each layer according to the rules mentioned in the paragraph before this. The number of hidden layers in a neural network is also a hyperparameter just like the number of neurons in a hidden layer, we will discuss this later.
Once, we have built our neural network. Our next step is to initialize the weights and the biases, we try to keep this initialization random so that all our neuron learn something different. However, we do not need this randomness in case of biases and they can be initialized with zeros. These weights and biases are also called learnable parameters as well because we learn them over the cause of training. Now, to learn these parameters we first need to generate some output so that we can compare it with the actual values and start learning from them. To calculate the output of the network we need the output of each neuron which can be calculated by first multiplying each input coming to the neuron with the weights assigned for each input, summing the products, adding the bias term to the sum and then applying some non-linearity. This function can be represented by an equation like this f(x1,x2,…,xn) = RELU(w1*x1 + w2*x2 + … + wn*xn + b). Where RELU is the non-linear function, Ws are the weights, Xs are the inputs and b is a bias term. After calculating this result, we pass on the result to each neuron in the next layer.
This process as a whole is known as a forward pass. We use this to predict results for given inputs to a neural network. The forward pass is sufficient when, predicting results but for the training purposes, we need to go further and devise or use a loss function to calculate a deviation between our prediction and actual data. There are a variety of choices for loss functions that we can pick from or devise our own depending on the nature of the problem we are dealing with.
The actual learning starts right after the calculation of the loss, our goal is to minimize this loss and the approach we use for this is called backpropagation. What we do in backpropagation is that we take the partial derivatives of the loss with respect to each weight and each bias and try to update the weights and biases in the opposite direction of the derivatives because we want to decrease the loss. We usually use this formula to update the weights: Wnew = Wold – alpha*(partial derivative of loss with respect to Wold). All other variables are self-explanatory except for alpha which is known as the learning rate. This learning rate basically defines the magnitude of the step that we are going to take to update the weights, we do not want this alpha to be big so that we do not keep oscillating and miss the values of Ws that give the best result. This does not mean that we should always keep alpha very very small because doing this can really slow down our learning as the step becomes very small to produce any significant improvement.
Advanced practices for training Neural Networks
So far, the process is just the bare minimum required to train a neural network. But to train a neural network for good results requires some certain procedures to be followed. Besides, following these steps, you also need to have sound knowledge of the subject to make certain decisions. These decisions can include increasing or decreasing the value of learning-rate looking at the behavior of the learning curve, deciding the architecture of the network and decide whether the networking in overfitting or underfitting during the training.
Splitting of Dataset for Validation and Testing
To start with it is a very good idea to split our dataset into three splits a training set, a validation set, and a test set. These splits help us in keeping track of our model’s learning, a good model is one that performs relatively well on all three splits. The training set is used to make the model learn by providing expected outputs, validation split is used to monitor the learning of model on this split compared to the training set, this helps in fine-tuning the hyperparameters and the test split is used to test the accuracy or any other criteria for the model to see its performance on unseen data.
Underfitting and Overfitting
Once we start the training it is helpful to plot a graph for training loss and validation loss after each iteration. We can decide from looking at these curves whether the model in underfitting, overfitting or is a good fit for the data. If the training loss is not decreasing after two to three iterations or is increasing and all other calculations like partial derivatives are correct then it is underfitting the training data. This means that the network is not complex enough to map this relationship and probably needs more neurons and layers to be included. Another problem that a neural network can suffer from is overfitting this is the case when the validation loss is higher than the training loss. This case usually occurs when we make our neural network overly complex such that it becomes specific to the training data and does not generalize the overall data. This model will perform poorly in real life settings.
Remedies for overfitting
There are many ways to get rid of this overfitting problem. These include but are not limited to regularization, dropout, reducing the complexity of the network and augmenting data. Regularization is the process of penalizing the weights for being high in magnitude, it is carried out by adding a term in the loss function which is directly proportional to the magnitude of the weights. As we train our neural network we get a higher loss for higher values of the weights and in turn, the values for these weights are penalized and are kept small in magnitude by the model to avoid the higher losses. Dropout is a technique which is used during the training of a neural network to avoid dependency on certain neurons in the network. It is done by choosing a random probability value and that number of random neurons in left out during training in each iteration. Different neurons are left out during each iteration of the network. Reducing the complexity of the network simply means reducing the weights of the networks by removing neurons or layers in the network. Data augmentation means increasing the data in our hand so that our training data is not too small for the network to overfit. Data augmentation can be performed in various ways depending on the type of data for example in the case of images it can be done by blurring some images, flipping some images and performing some other operations. If done properly data augmentation can be a very good step in the betterment of the performance of the network.
Initialization of Weights
Another important step during the training of a neural network is to make sure the proper initialization of the weights. It is a very bad idea to initialize weights with zeros, the neural net does not perform symmetry-breaking. If we set all the weights to be zero, then all the neurons in all the layers perform the same calculations and learn same things and the neural network fails to generalize the overall aspects of data which does not help in improving the performance of the network. Whereas the random initialization helps in breaking the symmetry and each neuron learns something different from the training. It is also important to keep the values of these weights near to zero to keep the values of weights compact and without high standard deviation. Another approach to initialize the weights keeping in mind the size of the previous layer which helps in attaining a global minimum of the loss function faster and more efficiently. The weights are still random but differ in range depending on the size of the previous layer of neurons and help in faster convergence
However, all these steps are just the tip of the iceberg, a lot more facts are kept in mind while training these neural networks. More modern methods include batch normalization, using neural networks other than conventional artificial neural networks which include convolutional neural networks, recurrent neural networks 3 and capsule networks, training ensemble of the networks and using the different types of optimizers. Training also requires careful fine-tuning of hyperparameters like looking at the loss plots, deciding the number of iterations to run and adjusting the learning rata in between the iterations.
Neural networks are very powerful learning models and are widely used these days in fields object detection, language translation, speech recognition, cancer detection, autonomous driving and many more. Another approach to use neural networks is reinforcement learning which does not have great deal of data as in deep learning and is gaining real popularity among the researchers.
Useful Links for Neural Networks: