What is the Spearman-Brown coefficient and its relation to the test and retest reliability?
Since this is not my area of expertise, I searched Google under the key words "Spearman-Brown coefficient" to get these possible sources:
http://faculty.chass.ncsu.edu/garson/PA765/reliab.htm
http://www.visualstatistics.net/Statistics/Principal%20Components%20of%20Reliability/PCofReliability.asp
(Broken Link Removed)
http://www.statsoft.com/textbook/streliab.html
In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.
I hope this helps.