I have attempted Week 3 Quiz and can’t understand the correct answer for Q5 (unanswered screenshot below). Is the correct answer referring to the wrong m? Thanks.

Hey, while I can’t give you the right answer, you just have to use the formula for perplexity provided in the lecture and reading material.

Thank you for the pointer. My main confusion is that m = 5 in this case right? (Which is not what the correct answer says…)

I don’t think it’s explained, but the slide with the cross-eyed penguin says we count and do not count .

Vincent’s notation for the start and end sequences in his answer below didn’t get through (probably interpreted as HTML tags), so for others wondering this same question as I was…

Perplexity includes the end-of-sentence tokens but not the start-of-sentence tokens.

I think this is because the problem isn’t symmetric: we are predicting likely next words… so every sentence has to start with a start token – there’s no predictive value there. Versus some words are statistically more likely than others to end a sentence (thus have predictive value of the next word (end token)).