# of nodes in a layer

Course Q&A Deep Learning Specialization Neural Networks and Deep Learning
1

Hi,

Can someone help me understand, how the number of nodes in a layer is determined?

For example, in the Week3 videos, there is an example of a NN that has 3 features in the input layer, 4 nodes in the hidden layer and 1 node in the output layer. Why 4 nodes in the hidden layer? What decides this? Does the output layer always have 1 node?

Thank you.

2

Hi @lzednik ,

The number of nodes, and even the hidden layers, are determined by the architect of the NN: by you, by me, by the person or group that is creating the NN.

There are several options:

  1. You can get started with a known model which has been created by some researchers, and this model proposes already a certain amount of layers, and nodes inside of each layer.
  2. You can start from scratch.
    2.1 Assuming you are a very experienced ML designer, may be you’ll get started with a configuration that, from experience, it is the best layout for the task at hand.
    2.2 Assuming you are a novice, you may start with your best guess. For instance, 2 hidden layers, each with 4 nodes.

In any case, you’ll design your NN, train it, and watch the results. If the results are not meeting your objectives, then you start ‘fine-tuning’ your architecture (although fine-tuning the data may be more important, but that’s another topic). At the end you will hopefully reach a design (number of layers, number of units inside each layer, etc) that meets your objective.

The other question is:

“Does the output layer always have 1 node?”

No, not always. You’ll soon find out that the 1-node output is for the very specific case of a binary-classification. That is, when you want to determine if something is of a given class or not. Like for instance: Cat / No-Cat.

You’ll learn about many other types of NN that provide many other types of outputs.

I hope this sheds light on your questions. If you still have any doubt, please don’t hesitate to share it.

Thanks!

Juan

4 Likes
3

@Juan_Olano , thank you for the quick and detailed answer. This makes sense.

4

Excellent! I am glad this makes sense!

Good luck with the rest of the lessons!

Juan