What is my formula?

#1
I just took 15 practice questions for the bar exam and I missed all 15 (don't worry for me, this question set was on a subject I have yet to learn :) ). This made my chuckle becuase I thought it was pretty unlikely. There are four answer choices for each question. I know that on average I should at least answer 3.75 correctly. (.25 x 15). But at the outset, what was the probability that I would miss all 15? in other words, if there is a 25% chance of an event occuring over 15 occurrences and that event never occurs, what is the probability? I'm not sure what the formula is.
 

katxt

Active Member
#2
This is binomial. Follow the pattern below for similar problems.
Try in Excel =BINOMDIST(0,15,25%,0) - that is 0 out of 15 each 25% chance. (don't worry about the final 0)
 
#3
Awersome, thanks! So, if i were trting to figure out the probability of five, seven, etc., occurances, i would just replace those numbers with the zeros? Or does it not work that way?
 

katxt

Active Member
#4
Only the first zero. The final zero can be 0 or 1. The 1 indicates "or less" or cumulative in stats speak
=BINOMDIST(5,15,25%,0) means exactly 5 out of 15
=BINOMDIST(0,15,25%,1) means 5 or less out of 15