## Background

Backpropagation is a common method for training a neural network. There is no shortage of papers online that attempt to explain how backpropagation works, but few that include an example with actual numbers. This post is my attempt to explain how it works with a concrete example that folks can compare their own calculations to in order to ensure they understand backpropagation correctly.

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## Backpropagation in Python

You can play around with a Python script that I wrote that implements the backpropagation algorithm in this Github repo.

## Backpropagation Visualization

For an interactive visualization showing a neural network as it learns, check out my Neural Network visualization.

## Additional Resources

If you find this tutorial useful and want to continue learning about neural networks and their applications, I highly recommend checking out Adrian Rosebrock’s excellent tutorial on Getting Started with Deep Learning and Python.

## Overview

For this tutorial, we’re going to use a neural network with two inputs, two hidden neurons, two output neurons. Additionally, the hidden and output neurons will include a bias.

Here’s the basic structure:

In order to have some numbers to work with, here are the initial weights, the biases, and training inputs/outputs:

The goal of backpropagation is to optimize the weights so that the neural network can learn how to correctly map arbitrary inputs to outputs.

For the rest of this tutorial we’re going to work with a single training set: given inputs 0.05 and 0.10, we want the neural network to output 0.01 and 0.99.

## The Forward Pass

To begin, lets see what the neural network currently predicts given the weights and biases above and inputs of 0.05 and 0.10. To do this we’ll feed those inputs forward though the network.

We figure out the *total net input* to each hidden layer neuron, *squash* the total net input using an *activation function* (here we use the *logistic function*), then repeat the process with the output layer neurons.

*net input*by some sources.

Here’s how we calculate the total net input for :

We then squash it using the logistic function to get the output of :

Carrying out the same process for we get:

We repeat this process for the output layer neurons, using the output from the hidden layer neurons as inputs.

Here’s the output for :

And carrying out the same process for we get:

### Calculating the Total Error

We can now calculate the error for each output neuron using the squared error function and sum them to get the total error:

For example, the target output for is 0.01 but the neural network output 0.75136507, therefore its error is:

Repeating this process for (remembering that the target is 0.99) we get:

The total error for the neural network is the sum of these errors:

## The Backwards Pass

Our goal with backpropagation is to update each of the weights in the network so that they cause the actual output to be closer the target output, thereby minimizing the error for each output neuron and the network as a whole.

### Output Layer

Consider . We want to know how much a change in affects the total error, aka .

By applying the chain rule we know that:

Visually, here’s what we’re doing:

We need to figure out each piece in this equation.

First, how much does the total error change with respect to the output?

Next, how much does the output of change with respect to its total net input?

The partial derivative of the logistic function is the output multiplied by 1 minus the output:

Finally, how much does the total net input of change with respect to ?

Putting it all together:

You’ll often see this calculation combined in the form of the delta rule:

Alternatively, we have and which can be written as , aka (the Greek letter delta) aka the *node delta*. We can use this to rewrite the calculation above:

Therefore:

Some sources extract the negative sign from so it would be written as:

To decrease the error, we then subtract this value from the current weight (optionally multiplied by some learning rate, eta, which we’ll set to 0.5):

We can repeat this process to get the new weights , , and :

We perform the actual updates in the neural network *after* we have the new weights leading into the hidden layer neurons (ie, we use the original weights, not the updated weights, when we continue the backpropagation algorithm below).

### Hidden Layer

Next, we’ll continue the backwards pass by calculating new values for , , , and .

Big picture, here’s what we need to figure out:

Visually:

We’re going to use a similar process as we did for the output layer, but slightly different to account for the fact that the output of each hidden layer neuron contributes to the output (and therefore error) of multiple output neurons. We know that affects both and therefore the needs to take into consideration its effect on the both output neurons:

Starting with :

We can calculate using values we calculated earlier:

And is equal to :

Plugging them in:

Following the same process for , we get:

Therefore:

Now that we have , we need to figure out and then for each weight:

We calculate the partial derivative of the total net input to with respect to the same as we did for the output neuron:

Putting it all together:

You might also see this written as:

We can now update :

Repeating this for , , and

Finally, we’ve updated all of our weights! When we fed forward the 0.05 and 0.1 inputs originally, the error on the network was 0.298371109. After this first round of backpropagation, the total error is now down to 0.291027924. It might not seem like much, but after repeating this process 10,000 times, for example, the error plummets to 0.000035085. At this point, when we feed forward 0.05 and 0.1, the two outputs neurons generate 0.015912196 (vs 0.01 target) and 0.984065734 (vs 0.99 target).

If you’ve made it this far and found any errors in any of the above or can think of any ways to make it clearer for future readers, don’t hesitate to drop me a note. Thanks!

Quick question to anyone who can answer… I know that the Bias inputs remain 1 at all times. But do b1 and b2 change throughout the algorithm as w1-w8 do? Or do we keep them as the initial guess values?

Thank you in advance.

It stays the same throughout

Yes, in practice biases get updated as well. Moreover, each of the neurons should have its own bias. For example, this means that in practice there would not be just one b1 for both h1 and h2. Each of those 2 neurons would have a separate bias that also gets updated in the backward pass.

I had the same question and from available online literature that I read up on, it seems that the bias weights should also get adjusted. For example, what if one started with a bias weight b1 of 100 million – it might never converge since the net(h1) and net(h2) values would hardly be influenced by i1 and i2. Am not totally sure, but it seems only logical that bias weights should also be adjusted the same way w1-w8 are being currently done. If they shouldn’t get adjusted, then it would really be great if someone could explain why.

Ok, everything is clear! But at the moment when I choose for the output layer not a sigmoidal activation function, but linear, what changes in the algorithm?

The derivatives, for example \partial out_h1 / \partial new_h1 will be changed.

Is this same as gradient descent, or that is different from this?

I believe this is gradient descent.

Thankyou very much

Links to free machine learning courses & tutorials – Davidson Machine Learning

What is your suggestion if you have a huge number of input nodes and therefore the sum e.g. neth1 is huge, and so your activation function turns literally every neth1 into 1 because there is a (1 / e^-nethx) in the activation function?

It feels primitive to simply change the function by dividing for example

This is a very insightful question! There are alternative activation functions such as tanh, ReLU, Leaky ReLU etc.. Dead ReLU problem is usually referred as your question. One remedy is to use Leaky ReLU.

Hello, I’m university student in korea.

I study neural netwrok, so I like your writing because it is easy to understand.

Now i make program back propagation in c#.

I have a question one thing. AND gate, OR gate is understand, but

How do I handle actual data?? for example, weather data, fonding data..

Please teach me.

It’s too complicated question for such topic, in fact, this is what professionals use neural networks for. Main and the hardest task for making neural network is choosing parameters and form of input for neural network. You should read some articles about it, but basically you should represent input data in numbers that neural network can work with, choose certain number of hidden and output neurons and then experimentate. Experiments is what neural networks are all about.

Thank you. I wanted solution, but new thing leaned to your reply.

Outstanding explanation Matt ! Appreciate the excellent and detailed walk-through. This will help anybody trying to put together NN’s to get a complete understanding of the underlying math.

Machine Learning Engineer Interview Resources | liehendi

thanks!

After this line :

“`

Here’s how we calculate the total net input for h_1:

“`

instead of w2 there should be w3. means w1*i1 + w3*i2 + b1*1

You forgot to include how bias is updated.

Great!