Binomial distributions must also meet the following three criteria:
The number of observations or trials is fixed. In other words, you can only figure out the probability of something happening if you do it a certain number of times. This is common sense — if you toss a coin once, your probability of getting a tails is 50%. IF you toss a coin a 20 times, your probability of getting a tails is very, very close to 100%.
Each observation or trial is independent. In other words, none of your trials have an effect on the probability of the next trial.
The probability of success (tails, heads, fail or pass) is exactly the same from one trial to another.