Stats standard deviation question - HELP! Watch

figure-it-out
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I'm getting really confused over when to use normal standard deviation calculations and when to use an unbiased estimate (though I don't seem to be understanding this either too much) If anyone could give me a rule which says when to use an estimate, I'd love them.

Eg. is it just for small samples etc

An example question would be this, from June 2007 S2, where they use n-1 to divide by, rather than just n. Why is this?? I'm rather confused! Thanks for anyone who can help in any way!!

David is the professional coach at the golf club where Becki is a member. He claims that,
after having a series of lessons with him, the mean number of putts that Becki takes per
round of golf will reduce from her present mean of 36.
After having the series of lessons with David, Becki decides to investigate his claim.
She therefore records, for each of a random sample of 50 rounds of golf, the number of
putts, x, that she takes to complete the round. Her results are summarised below, where x
denotes the sample mean.
Ex= 1730 and E(x-xbar)^2= 784
Using a z-test and the 1% level of significance, investigate David’s claim.
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Maths Buster
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(Original post by figure-it-out)
I'm getting really confused over when to use normal standard deviation calculations and when to use an unbiased estimate (though I don't seem to be understanding this either too much) If anyone could give me a rule which says when to use an estimate, I'd love them.

Eg. is it just for small samples etc

An example question would be this, from June 2007 S2, where they use n-1 to divide by, rather than just n. Why is this?? I'm rather confused! Thanks for anyone who can help in any way!!

David is the professional coach at the golf club where Becki is a member. He claims that,
after having a series of lessons with him, the mean number of putts that Becki takes per
round of golf will reduce from her present mean of 36.
After having the series of lessons with David, Becki decides to investigate his claim.
She therefore records, for each of a random sample of 50 rounds of golf, the number of
putts, x, that she takes to complete the round. Her results are summarised below, where x
denotes the sample mean.
Ex= 1730 and E(x-xbar)^2= 784
Using a z-test and the 1% level of significance, investigate David’s claim.
If you are finding the standard deviation of a sample, then you use n underneath. If you are using a sample to estimate the standard deviation of the population, then you use n - 1 underneath.
Does that help?
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