In machine learning, there are a set of analytical techniques know as black box methods. What is meant by black box methods is that the actual models developed are derived from complex mathematical processes that are difficult to understand and interpret. This difficulty in understanding them is what makes them mysterious.
One black method is artificial neural network (ANN). This method tries to imitate mathematically the behavior of neurons in the nervous system of humans. This post will attempt to explain ANN in simplistic terms.
The Human Mind and the Artificial One
We will begin by looking at how real neurons work before looking at ANN. In simple terms, as this is not a biology blog, neurons send and receive signals. They receive signals through their dendrites, process information in the soma, and the send signals through their axon terminal. Below is a picture of a neuron.
An ANN works in a highly similar manner. The x variables are the dendrites that are providing information to the cell body when they are summed. Different dendrites or x variables can have different weights (w). Next, the summation of the x variables is passed to an activation function before moving to the output or dependent variable y. Below is a picture of this process.
￼If you compare the two pictures they are similar yet different. ANN mimics the mind in a way that has fascinated people for over 50 years.
Activation Function (f)
The activation function purpose is to determine if there should be an activation. In the human body, activation takes place when the nerve cell sends the message to the next cell. This indicates that the message was strong enough to have it move forward.
The same concept applies in ANN. A signal will not be passed on unless it meets a minimum threshold. This threshold can vary depending on how the ANN is model.
The makeup of an ANNs can vary greatly. Some models have more than one output variable as shown below.
Other models have what are called hidden layers. These are variables that are both input and output variables. They could be seen as mediating variables. Below is a visual example.
How many layers to developed is left to the researcher. When models become really complex with several hidden layers and or outcome variables it is referred to as deep learning in the machine learning community.
Another complexity of ANN is the direction of information. Just as in the human body information can move forward and backward in an ANN. This provides for opportunities to model highly complex data and relationships.
ANN can be used for classifying virtually anything. They are a highly accurate model as well that is not bogged down by many assumptions. However, ANN’s are so hard to understand that it makes it difficult to use them despite their advantages. As such, this form of analysis can be beneficial if the user is able to explain the results.