How do you find the degree of freedom in statistics?

Question

A has an average of 47 minutes, and a standard deviation on 5 minutes. B has an average of 49 minutes, and a standard deviation of 3 minutes. I'm attempting to find a 95% confidence interval... in order to do so, I've found that the standard error is 1.304, now I'm trying to find df and the critical value. I know that df is 31.110482.. but how do I get that?

Answer 1

You want a 95% confidence interval for what?

This sounds like the kind of problem in which you try to determine whether two samples came from the same distribution (the likelihood thereof), but you haven't made it clear what question you are asking.

You may want to review

http://en.wikipedia.org/wiki/ Statistical_hypothesis_testing

about confidence intervals in statistical hypothesis testing, and

http://en.wikipedia.org/wiki/ Chi-square_distribution

about the Chi-Square distribution, which is parameterized by the number of "degrees of freedom".

If you are still stuck, please repost with more information.

Good luck.

Answer 2

The degrees of freedom is 19, not 31.110482.

I'm assuming you are running a students t-test to determine whether the two samples came from the same population, assuming equal sample sizes and unequal sample variance. I'm also assuming you used the pooled error method for calculating the standard error.  To get a standard error of 1.304, you'd have to have N1 and N2 = 20.

In a independent t test with equal sample size, your DF is equal to n - 1, so it would be 19.  

Good luck!

Answer 3

ANSWER: Indeterminate df (degrees of freedom)

Why??

Question does not state SAMPLE SIZE.