Suppose we have some event that can either occur or not
occur. For example, if we flip a coin, it is heads or it isn't heads. We'll say that the
probability of the event occurring is
p.
Our experiment will be flipping a
coin. This is a fair coin, so the probability that the flip will be heads is
p=0.5.
We will proceed to flip the
coin n times. We'll say that `X_n` represents
thenth flip, and is 1 if the coin was heads, and zero
otherwise.
Then let `Y = X_1 + ... +
X_n`
That is, Y is the total number of heads
that occurred during our experiment ofn
flips.
Y then follows a Binomial Distribution
with n trials having
probability p.
In a binomial
distribution, the probability of getting exactly k heads
during ntrials is given by the
formula
`P(Y = k) =
((n),(k))p^k(1-p)^{n-k}`
In our example, suppose we wanted
to know the probability of flipping a coin 20 times and having exactly 5 heads. We
plug n=20, p=0.5, k=5 into the
above:
`P(Y = 5) = ((20),(5))0.5^5(1-0.5)^15 approx
0.015`
We evaluate the above, and find that the probability
of getting 5 heads out of 20 flips is approximately
0.015.
The important thing to take out of this example is
that a binomial distribution can be used to model a series of events that either do or
do not occur (with some probability).
Here are some more
examples:
1. The probability of rolling a 6 is 1/6. What is
the probability of rolling 3 sixes in 5 trials (n=5,
p=1/6)
2. The probability of me missing my bus to work is
1/100. What is the probability I'll miss the bus two days this work week? (n=5,
p=1/100)
3. The probability I'll get a phone call during
any given hour is 1/5. What's the probability I'll get 10 phone calls during the next 8
hours? (n=8, p=1/5).
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