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    Can someone help me figure out the steps in between these two please?Name:  normal.jpg
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    Here's what I know:
    on the RHS, it's 1-0.95 = 0.05. But which sign has that changed as a consequence? the >= outside the brackets or the <= inside- ie from P(Z<= ...) to P(Z>..)?
    I know that it's divided by 2 since the normal dist. is symmetric about it's mean.
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    (Original post by zomgleh)
    ...
    You may find it useful to sketch the areas on a diagram of the normal distribution, and hopefully all will be clear.

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    (Original post by ghostwalker)
    You may find it useful to sketch the areas on a diagram of the normal distribution, and hopefully all will be clear.

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    Okay, done that and I think I get it. Just one more thing though, why has there been a change of sign from P(Z>z) < 0.025 and sqrt(n)/7.39 >=1.97 (the value for that prob)? Is it solely because the q. says ' find the smallest n such that'...?
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    (Original post by zomgleh)
    Name:  1111.jpg
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    Okay, done that and I think I get it. Just one more thing though, why has there been a change of sign from P(Z>z) < 0.025 and sqrt(n)/7.39 >=1.97 (the value for that prob)? Is it solely because the q. says ' find the smallest n such that'...?
    If we rearrange to use the cumulative distribution, here are the details.

    P(Z>z) < 0.025

    Means 1-P(Z<=z) < 0.025

    So P(Z<=z) >0.975

    Hence z >= 1.97 (although it's 1.96 in my book, and that's the value I've seen elsewhere).

    and sqrt(x)/7.39 >= 1.97
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    (Original post by ghostwalker)
    If we rearrange to use the cumulative distribution, here are the details.

    P(Z>z) < 0.025

    Means 1-P(Z<=z) < 0.025

    So P(Z<=z) >0.975

    Hence z >= 1.97 (although it's 1.96 in my book, and that's the value I've seen elsewhere).

    and sqrt(x)/7.39 >= 1.97
    it makes sense this way, thanks, however we're only give the areas in the upper tail of the normal distribution. So P(Z>z) < .025
    =z<1.97 so it doesn't make sense why the sign changes to z>... in the last bit? And yes, I got 1.96 too but I'm guessing it's because the question asks for the smallest z that satisfies P(Z>z)<.025 ? is that right do you think?
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    (Original post by ghostwalker)
    If we rearrange to use the cumulative distribution, here are the details.

    P(Z>z) < 0.025

    Means 1-P(Z<=z) < 0.025

    So P(Z<=z) >0.975

    Hence z >= 1.97 (although it's 1.96 in my book, and that's the value I've seen elsewhere).

    and sqrt(x)/7.39 >= 1.97
    using another example-- why has the sign changed from P(Z>z) <= .025

    Is it simply because the table I'm using gives the upper tail of the distribution?

    to z>=1.96 ?Name:  222.jpg
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    (Original post by zomgleh)
    using another example-- why has the sign changed from P(Z>z) <= .025

    Is it simply because the table I'm using gives the upper tail of the distribution?

    to z>=1.96 ?Name:  222.jpg
Views: 41
Size:  16.6 KB
    No, it has nothing to do with how you look up the tables - that's a later process.

    This is purely a mathematical one.

    Looking at my previously worked example, you need to think about what these things mean. Then it should be clear what's going on. I usually try and visualise for the normal distribution.

    As to 1.97, rather than 1.96 - just seems wrong.
 
 
 
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