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Math Help - 90% and 95 % intervals

  1. #1
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    90% and 95 % intervals

    This is my question in the math book.
    construct the 90% and the 95% confidence intervals for the population mean.


    from a random sample of 36 days in a recent year, the closing stock prices for Tonka toys had a mean of $ 19.31 and a stardard deviation of $ 2.37

    Do I look at the z table? Thank you,
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by mkcar View Post
    This is my question in the math book.
    construct the 90% and the 95% confidence intervals for the population mean.


    from a random sample of 36 days in a recent year, the closing stock prices for Tonka toys had a mean of $ 19.31 and a stardard deviation of $ 2.37

    Do I look at the z table? Thank you,
    To be honest without more information or some assumptions this question
    cannot be answered.

    If we assume that the closing prices have a normal distribution then we
    would be looking at the t table with 35 degrees of freedom (as 35 is a small
    sample for these purposes). But without this assumption all we can say
    nothing.

    Now it might be that you are expected to make some assumtion but without
    seeing your notes we don't know if you are supposed to treat 36 as a large
    number and so use the z table, or assume the closing prices are normal, but
    36 is small and so use the t table.

    RonL
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