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

    9
    ReputationRep:
    The question is find P(X < 8.6), with mean 8 and standard deviation 5.

    So I standardised it and got that z = 0.12

    I looked up the z score for 0.12 on the table and it says 0.4522 which is wrong. The answers 0.5478, am I reading the table wrong??? Because it doesn't say 0.5478 anywhere even though I know that's the answer...

    This is the table. Thanks!



    Posted from TSR Mobile
    • Community Assistant
    • Welcome Squad
    Offline

    20
    ReputationRep:
    (Original post by JellyCat99)
    The question is find P(X < 8.6), with mean 8 and standard deviation 5.

    So I standardised it and got that z = 0.12

    I looked up the z score for 0.12 on the table and it says 0.4522 which is wrong. The answers 0.5478, am I reading the table wrong??? Because it doesn't say 0.5478 anywhere even though I know that's the answer...

    This is the table. Thanks!

    Posted from TSR Mobile
    Your table is not appearing. It should indeed be 0.5478.

    Your answer is 1-0.5478 though which is P(X>8.6). Perhaps your table is giving you values that are P(Z>z) rather than the conventional P(Z<z)? I'd have to see the table in order to confirm.


    Name:  dist.PNG
Views: 139
Size:  34.9 KB
    Offline

    10
    ReputationRep:
    You are using the wrong Standard Normal Distribution table.

    You are using the "reverse" of the table you should be using.

    Note in this image (the table you may had been using):


    Image source:
    https://onlinecourses.science.psu.ed...rmalBTable.gif

    You require the Standard Normal Distribution Table where:
     \lim_{x \to - \infty} F_{X} (x) = 0 , (where F_{X} is the cumulative distribution function).

    i.e. the area under the curve is swept out FROM THE LEFT.

    The second post, by RDKGames, shows what you need to do.

    (Note that:
     P(Z \leq 0) = 0.5 ,
    so that:
     P(Z \leq 0.12) ,
    should be greater then 0.)
    • Thread Starter
    Offline

    9
    ReputationRep:
    (Original post by simon0)
    You are using the wrong Standard Normal Distribution.

    You are using the "reverse" of the table you should be using.

    Note in this image (the table you may had been using):


    Image source:
    https://onlinecourses.science.psu.ed...rmalBTable.gif

    You require the Standard Normal Distribution Table where:
     \lim_{x \to - \infty} F_{X} (x) = 0 , (where F_{X} is the cumulative distribution function).

    i.e. the area under the curve is swept out FROM THE LEFT.

    The second post, by RDKGames, shows what you need to do.

    (Note that:
     P(Z \leq 0) = 0.5 ,
    so that:
     P(Z \leq 0.12) ,
    should be greater then 0.)
    Yeah I had a closer look at the table they give us and you're completely right, I'd just never had to use a table with the probabilities shown in this style before. Thanks!


    Posted from TSR Mobile
 
 
 
  • 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
    Are unpaid trial work shifts fair?
    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.