how do you find the interval
Interval between what and what? Since I am not sure what you are asking, I searched Google under the key words "find interval" and "find interval math" to get these possible sources:
http://www.yawiktionary.com/i/1148366673828.html
http://mathweb.mathsci.usna.edu/faculty/turnerjc/339f99/CImedian.htm
http://www.cvgs.k12.va.us/DIGSTATS/main/descriptv/d_confidencemath.htm
http://en.wikipedia.org/wiki/Interval_(mathematics)
I hope this helps. Thanks for asking.