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# OCR MEI Statistics 1 (S1)

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1. Discussion thread for this exam..

Ha I've been busy focussing on C1 and C2 I havent made enough time for S1...got four days to revise and do past papers...!!
2. Im gonna fail this paper , have you got the marking scheme for jan 2012?
3. I dont think s1 is too bad. The only tricky thing I think is the probability... sometimes its pretty hard to interpret the question. S1 is my A2 module though so maybe its a bit easier for me.

How are the grade boundaries for s1 usually ?
4. wow this thread is dead.... hows everyone finding s1 i thought it was ok untill i looked at june 06 paper daym TOO much PROBABILITY!!! ***scared now***

how are you guys finding s1

6 (ii) on this paper http://www.mei.org.uk/files/papers/s110ja_4766rev.pdf

and 7 (vi) on http://www.mei.org.uk/files/papers/s107ju_sw8f3.pdf

Thanks
6. also for 4 i) http://www.mei.org.uk/files/papers/s110ju_wtfd.pdf

how did they get their answer?
I thought it would be 1/5 x 1/5

7. Hey, unfortunately probability really isn't my strong point but I can try help with 7(vi)

You're working out the probability that the person is tested negative originally OR tests doubtful and is then cleared in the exam.

P(Negative Originally) = 0.91

That's easy. Now the hard(er) bit...

P(Doubtful THEN Cleared) you would just do as:
0.06 * 0.9

BUT with the new examination, if they HAVE the disease, it is still only 98% correct at identifying it. So, it's 2% or 0.02 likely to CLEAR the person incorrectly.

So as well as 0.06 * 0.9 you must include the outcome that they are incorrectly tested as clear, despite having the disease.

Which is 0.06 * (0.1 * 0.02)

So overall, it's:

0.91 + [(0.06 * 0.9) + (0.06 * 0.1 * 0.02)] = 0.96412

Hope this helps
8. (Original post by DrFantastic)
Can anyone explain to me how go about doing a hyposthesis test including the bits about CR's?

Help appreciated!
Do you have an example?
9. Well Im guessing you mean part (iv)...

H1: p < 0.15 as the new system should decrease number of no-shows.

Therefore, as the critical region is 8, values less than & equal to 8 lay in the critical region.

If a value is in the critical region then it is sufficient to reject the null hypothesis.

As 6 < 8 there is sufficient evidence to reject H0.
10. (Original post by Jamian)
Hey, unfortunately probability really isn't my strong point but I can try help with 7(vi)

You're working out the probability that the person is tested negative originally OR tests doubtful and is then cleared in the exam.

P(Negative Originally) = 0.91

That's easy. Now the hard(er) bit...

P(Doubtful THEN Cleared) you would just do as:
0.06 * 0.9

BUT with the new examination, if they HAVE the disease, it is still only 98% correct at identifying it. So, it's 2% or 0.02 likely to CLEAR the person incorrectly.

So as well as 0.06 * 0.9 you must include the outcome that they are incorrectly tested as clear, despite having the disease.

Which is 0.06 * (0.1 * 0.02)

So overall, it's:

0.91 + [(0.06 * 0.9) + (0.06 * 0.1 * 0.02)] = 0.96412

Hope this helps
Thanks! makes more sense

I just don't understand why you need to add P(doubtful then cleared) since the question just said "the probability that this person either tests negative originally or tests doubtful and is then cleared in the detailed medical examination..."
Why do you need to include doubtful and cleared without the med exam??
11. (Original post by helpme12345)
Thanks! makes more sense

I just don't understand why you need to add P(doubtful then cleared) since the question just said "the probability that this person either tests negative originally or tests doubtful and is then cleared in the detailed medical examination..."
Why do you need to include doubtful and cleared without the med exam??
There is no doubtful and cleared without the exam.

EVERYBODY who is doubtful is given the exam... 0.9 of whom are cleared with a 100% reliably. So 0.9 x 1.

Then out of those that are tested that HAVE the disease, only 98% are successfully identified, the other 2% have the disease but are cleared. In this event it is 0.06 * (0.1 * 0.02) the 0.1 * 0.02 being the probability they have the disease but get cleared anyway.

Hope this makes it clearer
12. Can someone please help me with what you label histogram axis with?? I was fine till I looked at markschemes...

like http://www.mei.org.uk/files/papers/2011_Jan_s1.pdf Q7
they said Accept any suitable unit for fd, eg freq per £10K, BUT
NOT FD per £1000

why can't it be FD per £1000 ???
13. (Original post by Jamian)
There is no doubtful and cleared without the exam.

EVERYBODY who is doubtful is given the exam... 0.9 of whom are cleared with a 100% reliably. So 0.9 x 1.

Then out of those that are tested that HAVE the disease, only 98% are successfully identified, the other 2% have the disease but are cleared. In this event it is 0.06 * (0.1 * 0.02) the 0.1 * 0.02 being the probability they have the disease but get cleared anyway.

Hope this makes it clearer
Thanks makes sense completely now
14. (Original post by helpme12345)
Can someone please help me with what you label histogram axis with?? I was fine till I looked at markschemes...

like http://www.mei.org.uk/files/papers/2011_Jan_s1.pdf Q7
they said Accept any suitable unit for fd, eg freq per £10K, BUT
NOT FD per £1000

why can't it be FD per £1000 ???
Because frequency density IS frequency/£1000. So it's Frequency per £1000.

Saying frequency density is frequency density per £1000 is like saying: frequency density = frequency density/1000
15. (Original post by Jamian)
Because frequency density IS frequency/£1000. So it's Frequency per £1000.

Saying frequency density is frequency density per £1000 is like saying: frequency density = frequency density/1000
thanks again

for the rest of that question, do we not need to multiply mean and sd by 1000? since income is given in 1000s?
for this markscheme they don't do it, but they did in another one :/
16. also for 4 i) http://www.mei.org.uk/files/papers/s110ju_wtfd.pdf

how did they get their answer?
I thought it would be 1/5 x 1/5
17. On this paper http://www.mei.org.uk/files/papers/s110ju_wtfd.pdf
in the examiners report they say Candidates should be reminded that total accuracy in intermediate working is important when dealing with probabilities eg 0.857375 should not be rounded to 0.86 so do they mean we should give the whole thing?? I know they mean in the working we shouldnt round off

but for the answer to Q7 ii C they gave their answer to 6dp... and it said CAO (correct answer only) so i don't really know what we should do

also, would it be ok to leave answers as fractions??
18. (Original post by helpme12345)
thanks again

for the rest of that question, do we not need to multiply mean and sd by 1000? since income is given in 1000s?
for this markscheme they don't do it, but they did in another one :/
It says on the extra points "Also accept £40760, £40800 etc"

I honestly can't imagine them severely penalizing you in any case like this, you've done the maths correctly... the only case I remember seeing things like this mattering are when you're dealing with £s and pennies.
19. (Original post by helpme12345)
On this paper http://www.mei.org.uk/files/papers/s110ju_wtfd.pdf
in the examiners report they say Candidates should be reminded that total accuracy in intermediate working is important when dealing with probabilities eg 0.857375 should not be rounded to 0.86 so do they mean we should give the whole thing?? I know they mean in the working we shouldnt round off

but for the answer to Q7 ii C they gave their answer to 6dp... and it said CAO (correct answer only) so i don't really know what we should do

also, would it be ok to leave answers as fractions??
NEVER round during calculations... it will give an incorrect answer. I personally store long decimal placed answers as letters on the calculator.

Normally you can leave answers as fractions or decimals. If the answer is recurring like 0.33333333333 I normally put 1/3 as it's more accurate than rounding to 0.333 :P

Honestly, who knows what is going through the examiner's head other than them though. Just do what you think applies best at the time and go with it... but in most cases an answer can be fraction OR to 3sf. (Unless binomial then it's 4sf).
20. (Original post by Jamian)
It says on the extra points "Also accept £40760, £40800 etc"

I honestly can't imagine them severely penalizing you in any case like this, you've done the maths correctly... the only case I remember seeing things like this mattering are when you're dealing with £s and pennies.
that was only for the mean though, what about SD? and I used that value of mean in 1000s for the other parts of the question but the markscheme didnt allow for it there

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