Backpropagation of Error

• The MLP is cool, but how do you set the weights?
• No one sets the weights manually.
• With a single perceptron, you can use the perceptron learning rule.
• What's really cool is that an MLP can be learned by the backpropagation of error.
• You train the system in a supervised manner.
• That is, you have instances that you know the output of.
• You put those instances through the system, and record the error.
• You then move the weights in a direction that will reduce the error.
• You backpropagate the error.
• So if the desired value is higher than the actual value, and the inputs are positive, increase the weights.
• The actual maths behind this is complex, and there are a few variants of the rule.
• You then go through another training cycle, and repeat.
• Eventually, if you have enough neurons, this will converge on a weight matrix that closely approximates the input output pairs.
• It's a universal learner.