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Confidence intervals for means

  1. May 10, 2010 #1

    I'm reading up on confidence intervals for means. This is leaving my mind boggled. I caught the part where the interval = tc*(sample standard deviation/the square root of n). What's boggling my mind though is the variable tc. I see that t refers to a T distribution. But, I can't figure out how t0.975 could give you a 95% confidence interval.
  2. jcsd
  3. May 10, 2010 #2
    The 95% confidence interval for a mean is between t_0.025 and t_0.975.
  4. May 10, 2010 #3
    Okay thanks. The page I'm reading makes reference to another page, which then refers to an appendix, neither of which helped.
  5. May 10, 2010 #4
    So how do I get the value of Tc for a certain confidence interval?
  6. May 10, 2010 #5


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    "So how do I get the value of Tc for a certain confidence interval? "

    Look at the t-table (not stuttering; it is a table of t-values:smile:). It then depends if
    your interval is 1-sided or 2-sided. Unfortunately, there are different table formats
    too. If your table deals with standard errors (which you use in case you don't know
    the pop. standard deviation) .

    If you have a table that tells you the percent of the data between a given number
    and the mean, and you want a 2-sided interval, from a sample of size N, then do this:

    First go to the row N-1 , since you have N-1 degrees of freedom when your sample
    size is N. Then look up the column value c/2 on the table, e.g, if c=0.95 ( for a 95% confidence interval). That is your t_c value.

    If you give me more details, and a link to the type of table you are using, we
    can work out a specific example.

  7. May 11, 2010 #6
    My table has the Tc values from top to bottom and degrees of freedom from left to right. Must be one-sided because your directions don't agree with my table.
  8. May 11, 2010 #7
    My table is not on the Internet - I'm reading a book called Probability and Statistics from Schaum's Outlines. (this is not homework)
  9. May 11, 2010 #8


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    O.K , let me go back home and check it out, I don't have it here with me.
  10. May 12, 2010 #9


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    FYI, one can google the following: student t distribution table

    Here is the first hit:
    http://www.math.unb.ca/~knight/utility/t-table.htm [Broken]​
    I like that it shows both the one-tailed and two-tailed values.
    Last edited by a moderator: May 4, 2017
  11. May 18, 2010 #10
    OK, how do I use it?
    Last edited: May 18, 2010
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