| Question : Suppose that in NY State, 35% of voters are Democrats, 30% are Republicans, and 11% are independent. You are conducting a poll by randomly calling registered voters. In your first four calls, what is the probability that you talk to:
a)all Republicans?
b)exactly 2 Republicans?
c)no Independents?
d)all Independents?
e)at least one independent? Answer : You can use the properties of the binomial distribution to answer these questions. You would use the binomial cdf -cumulative distribution function and the pmf probability mass function, depending on if the question is asking you to provide the probability of a cumulative range (cdf) or just the probability of exactly one value (pmf). The formula for the Binomial pmf is:
n= number of trials p= probability of selecting x = number of successes n-x = number of failure 1-p = probability of failure The n over the x means n choose x and is the number of ways to get x successes out of n trials.
To answer your questions:
Stop for a second, and be sure it intuitively makes sense. Since there aren't a lot of independents (11 in 100) compared to Democrats (35 in 100) we should not expect to be reaching a lot of independents in a sample of 4. We should expect the probability of selecting all independents to be much lower than selecting 0 out of 4, which is what we're seeing. At least one independent : Finally this one has you use the cdf, which is just adding up (accumulating) the values from the pmf in choosing 1, 2, 3 or 4 independents. For choosing 1 its =BINOMDIST(1,4,0.11,FALSE) = .31. Rinse and repeat for 2, 3 and 4. Then add them all up. You should get around a 37% chance of selecting at least one independent in 4 calls. | How helpful was this answer?
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