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    I wonder if anyone can help me understand the basics here.

    Course : Edexcel AS - S1 Module.
    Lookup Table percenatge points : 0.3 = 0.5244

    I want to understand each eventuality and how I should remember to do the right thing.

    Assume X - N (mean, 4)

    I am thinking about all the possibilities for a question that would involve the lookup, to find the mean, so these might be, e.g. :

    1. P (X > 52) = 0.7
    2. P (X < 52) = 0.7
    3. P (X > 52) = 0.3
    4. P (X < 52) = 0.3

    ... and the normalised bits are :

    1. P (Z > (52-mean)/4) = 0.7
    2. P (Z < (52-mean)/4) = 0.7
    3. P (Z > (52-mean)/4) = 0.3
    4. P (Z < (52-mean)/4) = 0.3

    ... it's all about changing the sign and knowing why I am doing it that I get a bit confused with, so and help in talking this through would be appreciated.

    Cheers.
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    (Original post by Charries)
    I wonder if anyone can help me understand the basics here.

    Course : Edexcel AS - S1 Module.
    Lookup Table percenatge points : 0.3 = 0.5244

    I want to understand each eventuality and how I should remember to do the right thing.

    Assume X - N (mean, 4)

    I am thinking about all the possibilities for a question that would involve the lookup, to find the mean, so these might be, e.g. :

    1. P (X > 52) = 0.7
    2. P (X < 52) = 0.7
    3. P (X > 52) = 0.3
    4. P (X < 52) = 0.3

    ... and the normalised bits are :

    1. P (Z > (52-mean)/4) = 0.7
    2. P (Z < (52-mean)/4) = 0.7
    3. P (Z > (52-mean)/4) = 0.3
    4. P (Z < (52-mean)/4) = 0.3

    ... it's all about changing the sign and knowing why I am doing it that I get a bit confused with, so and help in talking this through would be appreciated.

    Cheers.
    I'll do 4 as an example and you can work your way through the rest.


    P (X < 52) = 0.3 => P (Z < (52-mean)/4) = 0.3

    but we know that P(Z< 0.5244) = 0.3

    And this means that

    (52-mean)/4 = 0.5244 => mean = 52 - 4*0.5244

    Hope this has helped.
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    (Original post by Beansontoast yum)
    I'll do 4 as an example and you can work your way through the rest.


    P (X < 52) = 0.3 => P (Z < (52-mean)/4) = 0.3

    but we know that P(Z< 0.5244) = 0.3

    And this means that

    (52-mean)/4 = 0.5244 => mean = 52 - 4*0.5244

    Hope this has helped.
    When standardising, shouldn't it be P (Z < (52-mean)/2) (2 not 4) Standard Deviation not Variance
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    Thanks - I understand now.
 
 
 
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