Week 4's Quiz. Is the grader right?

Last one for today:

In Week 4’s Quiz, the grader insists that the first dZ to compute, dZ^{[L]}, is

dZ^{[L]} = A^{[L]}-Z.

This does not seem to be true. It is only true if:

  • The network used \sigma() as activation function at layer L, and;
  • The loss function is the two-class log-likelihood.

i.e. only for a very particular architecture, which, it is true, has been used heretofore.

Otherwise you will have to compute the dZ^{[L]} (here, dZ^{[2]}) the usual way:

Or am I mistaken?

Also, shouldn’t Week 4 come before the exercise of Week 3, to give the student additional perspective and answer questions that might arise in Week 3? But then the programming assignment of Week 4 would have to come directly after the programming assignment of Week 3.

Picture unrelated!

Hello, @dtonhofer,

Here comes perspective again. Yes - you are right, if we go general to just any loss function, we don’t know the exact form for dZ^{[L]}. However, if we look at just the binary classification problem which is covered by the first course and even the regression problem (linear activation + squared loss), we will have the same form for dZ^{[L]}.

I think, when you were considering this question, your scope, perhaps, might have already been larger than what the course had covered. IMHO, I think it is good for you because you have a big picture in mind and I think you will be cautious at what to use when you make your own network. However, it is just like a context of a conversation, when we are discussing shopping apples, then when I say $5, then it should imply that is the price of apples and not something else. I mean, here there are two parallel contexts - the course’s and yours (or mine as a learner).

Is there anything being tested/asked in week 3 that’s covered only in week 4? Because, you know, we are building up knowledge and I don’t see any problem for week 4 to ask anything taught in week 1 through week 3. In fact, in course 2-5, we might use things in course 1 from time to time.

Cheers,
Raymond

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Btw, I thought you were asking about Question 7 of the Quiz. You might also state the question number next time.

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But aren’t the question shuffled? They definitely change between trials.

Also, concerning

Is there anything being tested/asked in week 3 that’s covered only in week 4?

Not directly, but the “box structure” of the backward propagation is explained in the week 4 lecture, and it would be useful to know about it when doing the programming assignment of week 3. Just my opinion.

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Yes, they are shuffled, but trust me, the question number is still helpful to us. :wink: :wink:

I see. I see how it can be helpful. At least I can show learners this slide/lecture when it is appropriate, then I can see how others think.

Cheers,
Raymond

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