DLS Course 4 Week 4 Quiz


One of the questions in the C4 W4 Quiz asks about the number of output units of the landmarks.


When building a neural network that inputs a picture of a person’s face and outputs N landmarks on the face (assume that the input image contains exactly one face), we need two coordinates for each landmark, thus we need 2N output units. True/False?"

The answer is marked as True. However, doesn’t the output need one more unit telling the probability of whether there is a face or not? At least that’s how I understood Andrew’s explanation (at 2:37 of the recording). Thus, 2N+1. His note says 129 units output for 64 landmarks. Can someone explain why the Quiz says 2N is correct?


The quiz says 2N because the scope of the question is the number of units. It does not include the detection output.

I have similar question in a different way.

I think the answer to shape of y estimate is (1, 2N), but got wrong.
Then I’m confused what the shape of y should be? Is it in the options?

(2N,1) or (1,2N) is determined by how you design the NN. Either one could be true.

But the last statement is always true.

The thing is after I tried (1,2N), it said wrong, giving the reason " y hat is column vector, not row vector". Then I tried the last option, but still wrong. That’s why I am confused.

Thank you for the confirmation. At lease I have an answer.