A novel on the Amazon.com web site has the following ratings, in number of stars

| August 31, 2017

Question
A novel on the Amazon.com web site has the following ratings, in number of stars, from reviewers:

Number of Stars Frequency
1 7
2 4
3 20
4 9
5 10
1. What is the standard deviation of this distribution?

2. What is the probability that a randomly selected review is more than 3 stars?

Which formula do you use for each?

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