# Confidence intervals for means

1. May 10, 2010

### moonman239

Hi!

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. May 10, 2010

### diggy

The 95% confidence interval for a mean is between t_0.025 and t_0.975.

3. May 10, 2010

### moonman239

Okay thanks. The page I'm reading makes reference to another page, which then refers to an appendix, neither of which helped.

4. May 10, 2010

### moonman239

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

5. May 10, 2010

### WWGD

"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). 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.

HTH

6. May 11, 2010

### moonman239

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.

7. May 11, 2010

### moonman239

My table is not on the Internet - I'm reading a book called Probability and Statistics from Schaum's Outlines. (this is not homework)

8. May 11, 2010

### WWGD

O.K , let me go back home and check it out, I don't have it here with me.

9. May 12, 2010

### Redbelly98

Staff Emeritus
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
10. May 18, 2010

### moonman239

OK, how do I use it?

Last edited: May 18, 2010