问题
I have a quick question regarding backpropagation. I am looking at the following:
http://www4.rgu.ac.uk/files/chapter3%20-%20bp.pdf
In this paper, it says to calculate the error of the neuron as
Error = Output(i) * (1 - Output(i)) * (Target(i) - Output(i))
I have put the part of the equation that I don't understand in bold. In the paper, it says that the Output(i) * (1 - Output(i)) term is needed because of the sigmoid function - but I still don't understand why this would be nessecary.
What would be wrong with using
Error = abs(Output(i) - Target(i))
?
Is the error function regardless of the neuron activation/transfer function?
回答1:
The reason you need this is that you are calculating the derivative of the error function with respect to the neuron's inputs.
When you take the derivative via the chain rule, you need to multiply by the derivative of the neuron's activation function (which happens to be a sigmoid)
Here's the important math.
Calculate the derivative of the error on the neuron's inputs via the chain rule:
E = -(target - output)^2
dE/dinput = dE/doutput * doutput/dinput
Work out doutput/dinput:
output = sigmoid (input)
doutput/dinput = output * (1 - output) (derivative of sigmoid function)
therefore:
dE/dinput = 2 * (target - output) * output * (1 - output)
回答2:
The choice of the sigmoid function is by no means arbitrary. Basically you are trying to estimate the conditional probability of a class label given some sample. If you take the absolute value, you are doing something different, and you will get different results.
For a practical introduction in the topic I would recommend you to check out the online Machine Learning course by Prof. Andrew Ng
https://www.coursera.org/course/ml
and the book by Prof. Christopher Bishop for an in depth study on the topic
http://www.amazon.com/Neural-Networks-Pattern-Recognition-Christopher/dp/0198538642/ref=sr_1_1?ie=UTF8&qid=1343123246&sr=8-1&keywords=christopher+bishop+neural+networks
来源：https://stackoverflow.com/questions/11627773/calculate-the-error-using-a-sigmoid-function-in-backpropagation