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S1 Jan 2012 Edexcel Post Exam Discussion Thread - Solutions in the first post

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  • View Poll Results: Having seen the solutions, how many marks do you think you got?
    71-75
    54
    29.51%
    66-70
    31
    16.94%
    61-65
    35
    19.13%
    56-60
    23
    12.57%
    51-55
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    46-50
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    4.92%
    41-45
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    1.09%
    36-40
    4
    2.19%
    31-35
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    1.09%
    26-30
    1
    0.55%
    21-25
    0
    0%
    20 or less
    9
    4.92%

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    are u sure about question 2 with the probablility question doesn't p(a) include the intersection so 2/3 - 1/4 should give you p(A'nb) then you just fill in the ven diagram from there
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    the answer to p(b) was 7/12 with P(a) 1/4 p(ANB) 1/6 IT works now adding this up gives you the union of 2/3 1/4 + 7/12 - 1/6
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    (Original post by alz333)
    are u sure about question 2 with the probablility question doesn't p(a) include the intersection so 2/3 - 1/4 should give you p(A'nb) then you just fill in the ven diagram from there
    A'nB means not A AND B this is the 15 and only the 15 region in the Venn
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    is anyone posting a mark scheme?
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    (Original post by alz333)
    the answer to p(b) was 7/12 with P(a) 1/4 p(ANB) 1/6 IT works now adding this up gives you the union of 2/3 1/4 + 7/12 - 1/6
    Because A and B are independent the intersection is P(A) x P(B) therefore if you substitute this into the conditional rule you get:
    P(A) x P(B) = P(A) + P(B) - P(AuB)

    You don't know P(B) so you input values and rearrange to find P(B) as ou have eveything else. So Arsey is correct :/
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    (Original post by sdfgs)
    is anyone posting a mark scheme?
    Seriously?
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    I'm confused with Arsey's answer for Q5f.
    I thought coding does not affect the product moment correlation?
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    (Original post by Arsey)
    A'nB means not A AND B this is the 15 and only the 15 region in the Venn
    For the last question, if I worked out the probability of w> 234.4 using tables then multiplied that by 0.8 to get a 4 decimal place answer which rounds to 0.16 but then forgot to multiply it by two would I get any method marks? I know I should have just used the simpler values but surely the values I got from the table are more accurate ?
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    (Original post by That Was So Funny!)
    For the last question, if I worked out the probability of w> 234.4 using tables then multiplied that by 0.8 to get a 4 decimal place answer which rounds to 0.16 but then forgot to multiply it by two would I get any method marks? I know I should have just used the simpler values but surely the values I got from the table are more accurate ?
    I am not sure what you mean, why did you work out P(w>234.4) in the first place?
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    (Original post by MushyMorshy)
    I'm confused with Arsey's answer for Q5f.
    I thought coding does not affect the product moment correlation?
    5f has nothing to do with coding
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    (Original post by vishalb)
    But that means that the question wasn't phrased very well
    It could've been interpreted in 2 different ways?
    I completely agree with you.

    This question is very naughty.

    From the calculations it would appear that the fake 5 year old coin weighing 20g is an outlier. Statistically, removing an outlier from a data set should increase the correlation between the two sets of data (It should make any formula generated more reliable).

    So we would say PMCC increases, hence my solution.

    However, given that the value of PMCC was -0.908, it would get closer to -1. So the value of the negative number should decrease. That said, you can understand why someone would say it is getting bigger, the negative number is getting bigger....

    I have no idea how they will mark this one.

    Furthermore, the data given is not possible. I appreciate it states 'without further calculation' but if you were to remove the fake coin, reduce your data set to 9 coins, calculate new values of Stt, Sww, Stw.

    Stw changes to a positive value, meaning that there is now evidence of positive correlation, so PMCC WOULD INCREASE Numerically (if not statistically)

    Sww is negative, which is impossible.


    With such a small data set, 1 extreme piece of data will have a massive impact, so having an original PMCC so close to -1 doesn't make any sense in the first place.
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    (Original post by Arsey)
    I am not sure what you mean, why did you work out P(w>234.4) in the first place?
    I think the question was to work out the probability of the 2 items being picked and one being within the given range. So the prob of the given range was 0.2 and the prob of before the given range was 0.5 as it was half the diagram which left 0.3 which is the value I should have used. 0.3 was the value of p(w>234.4) but I used the tables instead being an idiot and got a more accurate value for that to 4 decimal places. Then I multiplied it by 0.8 to get a 4 decimal place answer which rounded to 0.16 but didn't multiply it by 2
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    Arsey, will i lose all 9 marks even if a attempted to do number 2? I got mixed up between mutually exclusive and independent..I assumed they were separate. Did i just lose all 9 marks?
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    And if i rounded my 0.026 to 0.03 for the regression line ?
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    (Original post by Sarah456)
    Arsey, will i lose all 9 marks even if a attempted to do number 2? I got mixed up between mutually exclusive and independent..I assumed they were separate. Did i just lose all 9 marks?
    i think so , as all your other answers will be wrong too.
    sorry
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    Thats just being too mean =/. Damnit...I wish i could get marks for attempting..
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    (Original post by Sarah456)
    Thats just being too mean =/. Damnit...I wish i could get marks for attempting..
    You would get method marks depending on what the mark scheme says, and in some cases there is something where you get a mark even if your wrong because of the question before as long as it makes some kind of sense.

    You most prob get some of the 9 marks but depends on generosity of the markers
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    (Original post by That Was So Funny!)
    I think the question was to work out the probability of the 2 items being picked and one being within the given range. So the prob of the given range was 0.2 and the prob of before the given range was 0.5 as it was half the diagram which left 0.3 which is the value I should have used. 0.3 was the value of p(w>234.4) but I used the tables instead being an idiot and got a more accurate value for that to 4 decimal places. Then I multiplied it by 0.8 to get a 4 decimal place answer which rounded to 0.16 but didn't multiply it by 2
    i one of the previous question they said area= 0.2 if within range
    So prob not in range is 1-0.2=0.8

    Then it was just
    In range x Not in range + Not in Rangex In range Or one of those multiplied by 2

    I did it but forgot the x2
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    (Original post by Sarah456)
    Arsey, will i lose all 9 marks even if a attempted to do number 2? I got mixed up between mutually exclusive and independent..I assumed they were separate. Did i just lose all 9 marks?
    no idea, depends on what you did.
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    What about S2 MS Arsey??

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