2019-01-09 Logistic Regression with a Neural Network mindset v5
What you need to remember:
Common steps for pre-processing a new dataset are:
Figure out the dimensions and shapes of the problem (m_train, m_test, num_px, ...)
Reshape the datasets such that each example is now a vector of size (num_px * num_px * 3, 1)
"Standardize" the data
What to remember: You've implemented several functions that:
Initialize (w,b)
Optimize the loss iteratively to learn parameters (w,b):
computing the cost and its gradient
updating the parameters using gradient descent
Use the learned (w,b) to predict the labels for a given set of examples
What to remember from this assignment:
Preprocessing the dataset is important.
You implemented each function separately: initialize(), propagate(), optimize(). Then you built a model().
Tuning the learning rate (which is an example of a "hyperparameter") can make a big difference to the algorithm. You will see more examples of this later in this course!
2019-01-11
Planar data classification with one hidden layer v5
Reminder: The general methodology to build a Neural Network is to:
- Define the neural network structure ( # of input units, # of hidden units, etc).
- Initialize the model's parameters
- Loop:
- Implement forward propagation
- Compute loss
- Implement backward propagation to get the gradients
- Update parameters (gradient descent)
You often build helper functions to compute steps 1-3 and then merge them into one function we call nn_model(). Once you've built nn_model() and learnt the right parameters, you can make predictions on new data.
2019-01-16
3.3 - General methodology
As usual you will follow the Deep Learning methodology to build the model:
1. Initialize parameters / Define hyperparameters
2. Loop for num_iterations:
a. Forward propagation
b. Compute cost function
c. Backward propagation
d. Update parameters (using parameters, and grads from backprop)
4. Use trained parameters to predict labels
2019-01-20
What you should remember:
The weights W[l]W[l] should be initialized randomly to break symmetry.
It is however okay to initialize the biases b[l]b[l] to zeros. Symmetry is still broken so long as W[l]W[l] is initialized randomly.
In summary:
Initializing weights to very large random values does not work well.
Hopefully intializing with small random values does better. The important question is: how small should be these random values be? Lets find out in the next part!
What you should remember from this notebook:
Different initializations lead to different results
Random initialization is used to break symmetry and make sure different hidden units can learn different things
Don't intialize to values that are too large
He initialization works well for networks with ReLU activations.
Regularization
Welcome to the second assignment of this week. Deep Learning models have so much flexibility and capacity that overfitting can be a serious problem, if the training dataset is not big enough. Sure it does well on the training set, but the learned network doesn't generalize to new examples that it has never seen!
What is L2-regularization actually doing?:
L2-regularization relies on the assumption that a model with small weights is simpler than a model with large weights. Thus, by penalizing the square values of the weights in the cost function you drive all the weights to smaller values. It becomes too costly for the cost to have large weights! This leads to a smoother model in which the output changes more slowly as the input changes.
What you should remember -- the implications of L2-regularization on:
The cost computation:
A regularization term is added to the cost
The backpropagation function:
There are extra terms in the gradients with respect to weight matrices
Weights end up smaller ("weight decay"):
Weights are pushed to smaller values.
What you should remember about dropout:
Dropout is a regularization technique.
You only use dropout during training. Don't use dropout (randomly eliminate nodes) during test time.
Apply dropout both during forward and backward propagation.
During training time, divide each dropout layer by keep_prob to keep the same expected value for the activations. For example, if keep_prob is 0.5, then we will on average shut down half the nodes, so the output will be scaled by 0.5 since only the remaining half are contributing to the solution. Dividing by 0.5 is equivalent to multiplying by 2. Hence, the output now has the same expected value. You can check that this works even when keep_prob is other values than 0.5.
What we want you to remember from this notebook:
Regularization will help you reduce overfitting.
Regularization will drive your weights to lower values.
L2 regularization and Dropout are two very effective regularization techniques.
2019-01-22
Optimization methods
What you should remember:
The difference between gradient descent, mini-batch gradient descent and stochastic gradient descent is the number of examples you use to perform one update step.
You have to tune a learning rate hyperparameter αα.
With a well-turned mini-batch size, usually it outperforms either gradient descent or stochastic gradient descent (particularly when the training set is large).
What you should remember:
Shuffling and Partitioning are the two steps required to build mini-batches
Powers of two are often chosen to be the mini-batch size, e.g., 16, 32, 64, 128.
What you should remember:
Momentum takes past gradients into account to smooth out the steps of gradient descent. It can be applied with batch gradient descent, mini-batch gradient descent or stochastic gradient descent.
You have to tune a momentum hyperparameter ββ and a learning rate αα.
Adam on the other hand, clearly outperforms mini-batch gradient descent and Momentum. If you run the model for more epochs on this simple dataset, all three methods will lead to very good results. However, you've seen that Adam converges a lot faster.
Some advantages of Adam include:
Relatively low memory requirements (though higher than gradient descent and gradient descent with momentum)
Usually works well even with little tuning of hyperparameters (except αα)
