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OCR (non mei) S2 Wednesday 15th June 2016 Watch

1. (Original post by duncanjgraham)
fix it then
2. (Original post by PotAuFeu)
Are you sure that the continuity correction for P(X<5) is P(X<5+0.5)?
The way I remember it is that you want it <5 so take away 0.5 so you don't include 5 and if you want >5 then add 0.5 so you don't include 5. Likewise, if you want <=5 then add 0.5 to include 5 and if you want >=5 then you take away 0.5 to include 5.
Hope that makes sense, I haven't seen particularly many 1/2n corrections but when I have, I just do my continuity corrections as I said so above and the answer comes out fine.

For when to use 1/2n, I found http://www.mathshelper.co.uk/OCR%20S...on%20Sheet.pdf to be helpful. Basically, do it when you are approximating a discrete distribution using the CLT.
Hi hi, my bad, I'd never be continuity correcting P(X<5) or P(X>5) because I'd make the probability asked for into the form <= or => before considering the approximate normal distribution . It then follows what you say. I meant it in a more general case (I should have been more mathematically accurate and so am going to delete the comment from the thread in case it confuses people)

I'm a fan of 1/2n continuity corrections
3. (Original post by duncanjgraham)
Hi hi, my bad, I'd never be continuity correcting P(X<5) or P(X>5) because I'd make the probability asked for into the form <= or => before considering the approximate normal distribution . It then follows what you say. I meant it in a more general case (I should have been more mathematically accurate and so am going to delete the comment from the thread in case it confuses people)

I'm a fan of 1/2n continuity corrections
Yeah I like doing that also, though it works out the same regardless of what you choose to do.
4. Okay, thank you!
5. When is S^2 used? Like in June 2012, why is S^2 used in question 5 but not question 2?
When is S^2 used? Like in June 2012, why is S^2 used in question 5 but not question 2?
In question 2 you are trying to find the population variance, in question 5 you are using the sample variance to estimate the population variance
7. (Original post by hallo.C)
In question 2 you are trying to find the population variance, in question 5 you are using the sample variance to estimate the population variance
Thank you. In general, am I only gonna need S^2 when the question involves working out the mean and variance from sigma x and sgma x^2
Thank you. In general, am I only gonna need S^2 when the question involves working out the mean and variance from sigma x and sgma x^2
I'm pretty sure that's the only way you can use it, I've never seen it in any other context on any of the papers
9. For June 2012 Q8 why do you divide the variance by n? There's no mention of sample means or mean of observations, nor can you use the CLT as n is too small.
10. (Original post by 16characterlimit)
For June 2012 Q8 why do you divide the variance by n? There's no mention of sample means or mean of observations, nor can you use the CLT as n is too small.
You always divide the variance by n when it gives you a sample size.
11. (Original post by Parallex)
You always divide the variance by n when it gives you a sample size.
What about Jan 2013 2ii? I got that right but that's a sample.
12. (Original post by 16characterlimit)
What about Jan 2013 2ii? I got that right but that's a sample.
Sorry my bad. In Jan 2013, you're calculating P(C>40) rather than P(C bar>40), i.e. not the distribution of the means.
13. (Original post by Parallex)
Sorry my bad. In Jan 2013, you're calculating P(C>40) rather than P(C bar>40), i.e. not the distribution of the means.
np I think I get it now
14. Hi !! It is bit urgent !!
My calculator has functions to calculate the probability distributions; I was wondering if I could use my calculator instead of the tables that are provided; What do you guys think? If it would be okay
15. (Original post by curious_M)
Hi !! It is bit urgent !!
My calculator has functions to calculate the probability distributions; I was wondering if I could use my calculator instead of the tables that are provided; What do you guys think? If it would be okay
If you are going to do that, I would then check on the tables that you get the same value. Use your calculator as the checking tool rather than solely relying on it
16. (Original post by hallo.C)
If you are going to do that, I would then check on the tables that you get the same value. Use your calculator as the checking tool rather than solely relying on it
Thanks very much
17. (Original post by curious_M)
Hi !! It is bit urgent !!
My calculator has functions to calculate the probability distributions; I was wondering if I could use my calculator instead of the tables that are provided; What do you guys think? If it would be okay
Personally I always use the calculator for normal distributions.
18. I can share my answers if anyone wants? pretty decent paper tbh
19. So how did everyone find it?
20. (Original post by andygeor)
I can share my answers if anyone wants? pretty decent paper tbh
Would be great if you could!! I can also post my answers

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