Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    15
    ReputationRep:
    answered
    Offline

    8
    ReputationRep:
    well z = \frac{x-\mu}{\sqrt{\sigma^2}}, where \mu is the mean and \sigma^2 is the variance which you have already worked out. Rearranging this you get x = z\sqrt{\sigma^2} + \mu. Then just substitute in the values already calculated for the mean and variance and the critical value for z at 90% (which is 1.282) and you get x!
    • Thread Starter
    Offline

    15
    ReputationRep:
    (Original post by sonofdot)
    well z = \frac{x-\mu}{\sqrt{\sigma^2}}, where \mu is the mean and \sigma^2 is the variance which you have already worked out. Rearranging this you get x = z\sqrt{\sigma^2} + \mu. Then just substitute in the values already calculated for the mean and variance and the critical value for z at 90% (which is 1.282) and you get x!
    i was told that z = x minus the mean over just the standard deviation, not the standard deviation squared?
    Offline

    8
    ReputationRep:
    (Original post by jermaindefoe)
    i was told that z = x minus the mean over just the standard deviation, not the standard deviation squared?
    It is (I put it was over the square root of the variance, which is the same as the standard deviation. Sorry if that confused you )
    • Thread Starter
    Offline

    15
    ReputationRep:
    (Original post by sonofdot)
    It is (I put it was over the square root of the variance, which is the same as the standard deviation. Sorry if that confused you )
    how did you get 1.282 for the value of z :eek:

    sorry for being a pain in the arse but i want to learn this
    Offline

    8
    ReputationRep:
    (Original post by jermaindefoe)
    how did you get 1.282 for the value of z :eek:

    sorry for being a pain in the arse but i want to learn this
    From my normal distribution tables (I guess it depends on your exam board, but near the back of my OCR formula book there is a page headed "the normal distribution function" At the bottom, there are critical values for the normal distribution, which tells you the value of z for different "special" probabilities. At 0.90 (90%), the value for z is 1.282 )
    • Thread Starter
    Offline

    15
    ReputationRep:
    hmmm i dont know if i have one of those i just had a table for the normal distribution that you look through for the values, it isn't for "special" probabilities though?
    Offline

    8
    ReputationRep:
    (Original post by jermaindefoe)
    hmmm i dont know if i have one of those i just had a table for the normal distribution that you look through for the values, it isn't for "special" probabilities though?
    well in that case, you can look through the values for the probability until you find the one closest to 0.9000, then read off the z-value for that probablility

    I've even drawn you a nice picture

    Name:  normal01.jpg
Views: 44
Size:  188.9 KB
    • Thread Starter
    Offline

    15
    ReputationRep:
    thanks very much for that

    so can i just clarify so i can write this up in my notes so i can revise for the exam with it

    when i need to find the area up to a certain point (x) i need to

    rearrange the forumla to make x the subject
    look through the table to find the closest possible to the boundary given (in this case 90%, 0.9)
    substitute this value with the others, such as the mean and the standard deviation, and work out from there?

    what would the rearranged formulae look like if it were to not use the variance, and just the SD instead? as there would be no square root would there, then i can remember that and the points above for the exam

    rep comming ur way
    Offline

    8
    ReputationRep:
    (Original post by jermaindefoe)
    thanks very much for that

    so can i just clarify so i can write this up in my notes so i can revise for the exam with it

    when i need to find the area up to a certain point (x) i need to

    rearrange the forumla to make x the subject
    look through the table to find the closest possible to the boundary given (in this case 90%, 0.9)
    substitute this value with the others, such as the mean and the standard deviation, and work out from there?

    what would the rearranged formulae look like if it were to not use the variance, and just the SD instead? as there would be no square root would there, then i can remember that and the points above for the exam

    rep comming ur way
    Yep, those notes all look fine, and thanks for the rep
    • Thread Starter
    Offline

    15
    ReputationRep:
    what would the rearranged formulae look like if it were to not use the variance, and just the SD instead?
    Offline

    8
    ReputationRep:
    (Original post by jermaindefoe)
    what would the rearranged formulae look like if it were to not use the variance, and just the SD instead?
    x = z\sigma + \mu where \sigma is the standard deviation
    • Thread Starter
    Offline

    15
    ReputationRep:
    cheers mate

    you dont no how much i appreciate that! i am struggling with the statistics a little in computer science as the lectures move so fast, having explanations like that make it so much easier to understand
    • Thread Starter
    Offline

    15
    ReputationRep:
    well that makes the answer

    17035.2

    which is too much isnt it
    Offline

    8
    ReputationRep:
    (Original post by jermaindefoe)
    well that makes the answer

    17035.2

    which is too much isnt it
    Are you sure?

    x = 1.282\sigma + \mu



x = 1.282 \times \sqrt{7840} + 7000



x = 113.5 + 7000



x = 7113.5
    • Thread Starter
    Offline

    15
    ReputationRep:
    just seen what i am doing wrong i need to square root the variance
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    Brussels sprouts
    Useful resources

    Make your revision easier

    Maths

    Maths Forum posting guidelines

    Not sure where to post? Read the updated guidelines here

    Equations

    How to use LaTex

    Writing equations the easy way

    Student revising

    Study habits of A* students

    Top tips from students who have already aced their exams

    Study Planner

    Create your own Study Planner

    Never miss a deadline again

    Polling station sign

    Thinking about a maths degree?

    Chat with other maths applicants

    Can you help? Study help unanswered threads

    Groups associated with this forum:

    View associated groups
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

    Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

    Quick reply
    Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.