Microsoft Office Tutorials and References
In Depth Information
3.1.5 Finding the Value for t in the Confidence Interval Formula
Objective : To find the value for t in the confidence interval
formula
The correct formula for the confidence interval about the mean for different
sample sizes is the following:
X
ts
e
(3.2)
:
:
To use this formula, you find the sample mean, X , and add to it the value of
t times the s.e. to get the upper limit of this 95% confidence interval. Also, you take
the sample mean , X , and subtract from it the value of t times the s.e. to get the lower
limit of this 95% confidence interval. And, you find the value of t in the table given
in Appendix E of this topic in the following way:
Objective : To find the value of t in the t-table in Appendix E
Before we get into an explanation of what is meant by “the value of t,” let’s give
you practice in finding the value of t by using the t-table in Appendix E.
Keep your finger on Appendix E as we explain how you need to “read” that table.
Since the test in this chapter is called the “confidence interval about the mean
test,” you will use the first column on the left in Appendix E to find the critical value
of t for your research study (note that this column is headed: “ sample size n” ).
To find the value of t , you go down this first column until you find the sample
size in your research study, and then you go to the right and read the value of t for
that sample size in the “critical t column” of the table (note that this column is the
column that you would use for the 95% confidence interval about the mean).
For example, if you have 14 people in your research study, the value of t is 2.160.
If you have 26 people in your research study, the value of t is 2.060.
If you have more than 40 people in your research study, the value of t is always
1.96.
Note that the “critical t column” in Appendix E represents the value of t that you
need to use to obtain to be 95% confident of your results as “significant” results.
Throughout this topic, we are assuming that you want to be 95% confident in the
results of your statistical tests . Therefore, the value for t in the t-table in Appendix E
tells you which value you should use for t when you use the formula for the 95%
confidence interval about the mean.
Now that you know how to find the value of t in the formula for the confidence
interval about the mean, let’s explore how you find this confidence interval using
Excel.
Search JabSto ::




Custom Search