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# Official Thread: OCR MEI S2/M1

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1. Whos looking forward to these....

Resources:

Formulae Booklet
http://www.mei.org.uk/files/pdf/formula_book_mf2.pdf

S2 90/100 UMS Grade Boundries from 2005-15 - v v small margin of error :/

90/100 UMS Grade Boundries for all other Modules (Including M1)

Good luck guys!

...if anyone has any extra resources could they comment and ill link em on here!
2. Got 55/72 on last year's paper, 60 UMS! Really not good grade boundaries, however I didn't do well in my other two further exams so if I worked harder my S2 may have been 60+
3. Not looking forward to these
4. Do we use ns^2/n-1 to estimate the population variance where s^2 is the sample variance or do we just use the sample variance as an estimate for the population variance?

(assuming the sample is large)
5. When using the normal distribuion do we take the value to 3dp or 4dp by interpolation. Im so confused any help would be appreciated. Like I saw on the 2009 Jan paper that they used something to 4dp like P(0.2345) instead of P(0.235). How??? Our tables only accurately go up to 3dp surely
6. Looking forward to these!
7. (Original post by Quido)
Looking forward to these!
What dp do you quote in the normal distribution cos our tables only go to 3dp
8. (Original post by 11234)
What dp do you quote in the normal distribution cos our tables only go to 3dp
You go to 3 dp most of the time. I think there are some exceptions but you probably wont get marked down. For example, if I get a z value of 0.2945, I would do the mean of 4 and 5 in the 'add' column. So it would be 0.6141 + (0.0015+0.0019)/2. The final answer would be 0.6158.
9. (Original post by Quido)
You go to 3 dp most of the time. I think there are some exceptions but you probably wont get marked down. For example, if I get a z value of 0.2945, I would do the mean of 4 and 5 in the 'add' column. So it would be 0.6141 + (0.0015+0.0019)/2. The final answer would be 0.6158.
Would they accept phi(0.295)
10. is there a general rule on how many significant figures / decimal places to go to? In the examiner's reports they talk about people over-specifying answers but I can't see any pattern in how many significant figures they use in mark schemes.
11. (Original post by 11234)
Would they accept phi(0.295)
I'm guessing they would because the tables only go to 3 dp so the number you put it should be the same, that makes logical sense to me.
12. Could someone explain how to do a question which asks to find a and b in:
P (a < X < b)=0.9 for example?

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13. (Original post by 11234)
Would they accept phi(0.295)
They accept 3dp, and say if 3dp was .214 and youve put 0.2145 to 4sf, they would accept. I noticed this in the June 10 paper I think, so you shouldnt worry about that. In terms of poission or normal distrib, i would out answer to 4dp to be safe, as ive not been told its wrong while sometimes using less could be.

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14. (Original post by zirak46)
Could someone explain how to do a question which asks to find a and b in:
P (a < X < b)=0.9 for example?

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Yeah could someone explain this please
15. (Original post by zirak46)
Could someone explain how to do a question which asks to find a and b in:
P (a < X < b)=0.9 for example?

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(Original post by G2S)
Yeah could someone explain this please
There are many approaches, but here's how I do these:

Since we need to find a range (a<x<b) such that 90% (p=0.9) of the data points fit within that range, I like to go for a symmetrical range.

Therefore the range from p=0.05 to p=0.95
In this range, 90% of the data is here.

Therefore, we have two things to solve:
1) P( x < a) = 0.05 (we want to find the value of a for which only 5% of the data is under it)
Solving this is pretty simple, simply being:
P( z < [a-mean]/s.d) = 0.05
And therefore [a-mean]/s.d = inverse-phi(0.05)
= -inverse-phi(0.95)
= -1.645
Then you can just solve a, since you would've been given the mean and the standard deviation in the question

2) P(x > b) = 0.05 (we want to find the value of b for which only 5% of the data is over it)
Solve the same as normal.

This will give two values (a and b) such that they collectively contain 90% (0.9) of the data. So say for example we get a = 135 and b=165
It would be true that P(135 < x < 165) = 0.9
Therefore 90% of the data is between 135 and 165

Hope this helped!
16. (Original post by klosovic)
is there a general rule on how many significant figures / decimal places to go to? In the examiner's reports they talk about people over-specifying answers but I can't see any pattern in how many significant figures they use in mark schemes.
Do probabilities to 4.d.p and other values to 3 or 4 significant figures depending on the value.

For example 0.487548375384 should be 0.4875
125.3258 should be 125.3
83.2142352 should be 83.2 or 83.21 depending on which you prefer, both get the marks.

Just never go beyond 4 s.f for solutions. However remember to never use rounded values in your calculations.
17. (Original post by klosovic)
Do we use ns^2/n-1 to estimate the population variance where s^2 is the sample variance or do we just use the sample variance as an estimate for the population variance?

(assuming the sample is large)
Use the sample variance as an estimate for the population variance unless given extra info or instructions otherwise.
18. (Original post by ComputerMaths97)
Use the sample variance as an estimate for the population variance unless given extra info or instructions otherwise.
Whats this for and is it in s2
19. (Original post by 11234)
Whats this for and is it in s2
Yes, it's for when you use sample data to test on a population. Just go through past papers and search for "population varience" and you'll find some examples
20. When using the normal tables with a z value to 4+ significant figures do you guys take an average from the 'add' column since my values always seem to be 1 out in the 4th decimal place but we were never taught to take an average.

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