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# [OCR not MEI] Statistics 2: Deciding on when to used an unbiased estimator

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1. I have found throughout S2 that I have confusion with when and when not to use the unbiased estimator for the variance.

I understand if you have a sample from a population of UNKNOWN variance, you must use it then. However, there seems to be discontinuity in some questions. Sometimes I have noticed that with this question: (taken from June 2012 S2 q2 i):

For the continuous random variable V, it is known that E(V) = 72.0. The mean of a random sample of 40 observations of V is denoted V bar. Given that P(V bar < 71.2) = 0.35, estimate the value of Var(V).

Now I managed to get Var(V) before applying the unbiased estimator. I then applied the unbiased estimating method (multiplying by the sample size/ sample size -1) and that was not the answer.

However, when you are given data for the sample of a population (given the summation information) you have to use the unbiased estimator.

I am not entirely sure why, but can someone please explain this to me.

Thank you,

2. Sorry you've not had any responses about this. Are you sure you've posted in the right place? Here's a link to our subject forum which should help get you more responses if you post there.

You can also find the Exam Thread list for A-levels here and GCSE here.

Just quoting in Puddles the Monkey so she can move the thread if needed
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(Original post by Puddles the Monkey)
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