"Calculate Errors for leaf in a Decision Tree C4.5?"

elfor

Hi guys, I am trying to understand how Quinlan calculates the errors for leaf when pruning a decision tree. I have read his book on the subject and he says:

"For a confidence level CF, the upper limit on this probability can be found from the confidence limits for the binomial distribution. This upper limit here is written U_cf(E,N)"

where E is incorrectly classified events and N is total events in the leaf. He has a example with a confidence level of 25%, N=6 and E=0 and he calculates the error to U_25%(0,6) = 0.206. Could anyone explain how this is actually calculated? I have had no luck searching for it. Thank you for any help!