The 'All Responses' Detail View gives you the 'big picture':
By accessing the 'All Responses' tab, you’ll be able to see how the viewers performed on the video as a whole. You'll be able to see the overall average score, how many questions were answered on average, and the total number of responses. You will also be able to see any questions that have not yet been graded at a glance!
Here's a quick walkthrough of that view: