4 Jack's, not John's
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Yes, we do need to know all of Jack's ratings to calculate their mean.

And we need John's rating for object 3 to calculate correlation between John and Jack's ratings.

However, if Jack's rating for object 3 were equal to the mean of all of other John'sJack's ratings (which it is not), then we could calculate Pearson's correlation without knowing John's rating for object 3. You can look at the formula for covariance to see the reason.

Yes, we do need to know all of Jack's ratings to calculate their mean.

And we need John's rating for object 3 to calculate correlation between John and Jack's ratings.

However, if Jack's rating for object 3 were equal to the mean of all of other John's ratings (which it is not), then we could calculate Pearson's correlation without knowing John's rating for object 3. You can look at the formula for covariance to see the reason.

Yes, we do need to know all of Jack's ratings to calculate their mean.

And we need John's rating for object 3 to calculate correlation between John and Jack's ratings.

However, if Jack's rating for object 3 were equal to the mean of all of other Jack's ratings (which it is not), then we could calculate Pearson's correlation without knowing John's rating for object 3. You can look at the formula for covariance to see the reason.

3 grammar
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Yes, we do need to know all of Jack's ratings to calculate their mean.

And we need John's rating for object 3 to calculate correlation between John and Jack's ratings.

However, if Jack's rating for object 3 were equal to the mean of all of other John's ratings (itwhich it is not), thethen we could calculate Pearson's correlation without knowing John's rating for object 3. You can look at the formula for covariance to see the reason.

Yes, we do need to know all of Jack's ratings to calculate their mean.

And we need John's rating for object 3 to calculate correlation between John and Jack's ratings.

However, if Jack's rating for object 3 were equal to the mean of all of John's ratings (it is not), the we could calculate Pearson's correlation without knowing John's rating for object 3. You can look at the formula for covariance to see the reason.

Yes, we do need to know all of Jack's ratings to calculate their mean.

And we need John's rating for object 3 to calculate correlation between John and Jack's ratings.

However, if Jack's rating for object 3 were equal to the mean of all of other John's ratings (which it is not), then we could calculate Pearson's correlation without knowing John's rating for object 3. You can look at the formula for covariance to see the reason.

2 added 12 characters in body
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Yes, we do need to know all of Jack's ratings to calculate their mean.

And we need John's rating for object 3 to calculate correlation between John and Jack's ratings.

However, if Jack's rating for object 3 were equal to the mean of all of John's ratings (it is not), the we could calculate Pearson's correlation without knowing John's rating for object 3. You can look at the formula for covariance to see the reason.

Yes, we do need to know all of Jack's ratings to calculate their mean.

And we need John's rating for object 3 to calculate correlation between John and Jack's ratings.

However, if Jack's rating for object 3 were equal to the mean of all of John's ratings, the we could calculate Pearson's correlation without knowing John's rating for object 3. You can look at the formula for covariance to see the reason.

Yes, we do need to know all of Jack's ratings to calculate their mean.

And we need John's rating for object 3 to calculate correlation between John and Jack's ratings.

However, if Jack's rating for object 3 were equal to the mean of all of John's ratings (it is not), the we could calculate Pearson's correlation without knowing John's rating for object 3. You can look at the formula for covariance to see the reason.

1
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