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Math Help - Looking up Z-table "backwards"??

  1. #1
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    Exclamation Looking up Z-table "backwards"??

    Terribly stuck! How does one read a z-table 'backwards'?!

    In one question, I'm required to find a value z*, such that P(Z<z*)= 0.98 and P(0<Z<z*)= 0.48 -- by reading the table 'backwards'.

    And in the next one, in order to find z_{a/2}, I need to do the same.

    Any help would be greatly appreciated!
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  2. #2
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    Quote Originally Posted by trackies View Post
    Terribly stuck! How does one read a z-table 'backwards'?!

    In one question, I'm required to find a value z*, such that P(Z<z*)= 0.98 and P(0<Z<z*)= 0.48 -- by reading the table 'backwards'.

    And in the next one, in order to find z_{a/2}, I need to do the same.

    Any help would be greatly appreciated!
    This is difficult to explain this way but look at the attachment, which is a portion of the z-table. The two highlighted figures bracket p=0.98, so the z-score corresponding to 0.98 lies between 2.05 and 2.06, you can refine this by linear interpolation to get z~=2.054

    RonL
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  3. #3
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    Hmm. Okay. But how do I find P(0<Z<z*)=0.48 using this method?
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  4. #4
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    Quote Originally Posted by trackies View Post
    Hmm. Okay. But how do I find P(0<Z<z*)=0.48 using this method?
    P(0<Z<z^*)=P(Z<z^*)-0.5=0.48

    so:

    we seek z^* such that: p(Z<z^*)=0.98

    RonL
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  5. #5
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    Quote Originally Posted by CaptainBlack View Post
    P(0<Z<z^*)=P(Z<z^*)-0.5=0.48

    so:

    we seek z^* such that: p(Z<z^*)=0.98

    RonL
    As always, you make the toughest questions look easy
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