OCR (non mei) S2 Wednesday 15th June 2016

Scroll to see replies

Reply 20

16characterlimit

Original post by duncanjgraham

there's misunderstanding in this thread

fix it then

Reply 21

duncanjgraham

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%20S2%20Revision%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

(edited 7 years ago)

Reply 22

16characterlimit

Original post by duncanjgraham

Yeah I like doing that also, though it works out the same regardless of what you choose to do.

Okay, thank you!

When is S^2 used? Like in June 2012, why is S^2 used in question 5 but not question 2?

Reply 25

hallo.C

Original post by MrZebraCookie

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

Reply 26

MrZebraCookie

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

Reply 27

hallo.C

Original post by MrZebraCookie

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

Reply 28

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.

Reply 29

Parallex

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.

Reply 30

16characterlimit

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.

Reply 31

Parallex

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.

Reply 32

16characterlimit

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

Reply 33

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

Reply 34

hallo.C

Original post by curious_M

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

Reply 35

curious_M

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