Exam 7: Analyzing Proportions

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The binomial test uses data to test whether a sample proportion matches the null expectation for the proportion.

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Consider a set of nine separate water samples from a region where the prevalence of bacterial contamination is 30%. Assuming the binomial distribution is appropriate, what is the probability that exactly three of the samples indicate contamination?

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The term (104)\left(\begin{array}{c}10 \\4\end{array}\right) is stated verbally as "ten choose four."

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Consider a binomial distribution with a sample size of 10 and a success probability of 0.9. What is the probability that there are more than 8 successes?

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When calculating binomial probabilities, we use the term "success" for the desired outcome of each individual trial.

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Imagine a surgery that is known to have a 10% chance of serious side effects. An internal hospital review shows that three out of eight of a particular doctor's patients have these side effects. If we conducted a binomial test of whether this doctor's patients are experiencing unusually low or high rates of side effects, what would our conclusion be?

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Consider the claim that 60% of the members of a population of bacteria have a plasmid conferring antibiotic resistance. If we collected some random bacterial samples and the P-value of a binomial test was 0.15, what would our conclusion be?

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What is the 95% confidence interval, using the Agresti-Coull method, for the proportion when there are 7 observed successes out of a total of 50 trials?

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Consider a situation in which a group of ecologists are studying the frequency of symbiotic relationships between acacia trees and ant colonies in Bolivia. Previous studies in Colombia have shown the frequency of symbiotic relationships there to be 30%, and the ecologists are interested in whether the same frequency holds in Colombia. They locate 20 trees and determine that 11 of the trees appear to have a symbiotic relationship with an ant colony. Use a binomial test and the calculation of 95% confidence intervals to address this question: Is there sufficient evidence to determine whether the frequency of symbiotic acacia trees in Colombia differs from 30% or not?

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If a study with a total sample size of 14 measures 7 successes, in how many different sequences could these successes have occurred??

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Consider the claim that 60% of the members of a population of bacteria have a plasmid conferring antibiotic resistance. If we collected some random bacterial samples and the P-value of a binomial test was 0.015, what would our conclusion be?

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The value of 99! / 97! is which of the following?

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Consider a situation in which a group of physiologists are studying the sizes of wings in seagulls. They are interested in whether the gulls show asymmetry or symmetry with respect to the wing sizes. They humanely trap (and subsequently release) 14 seagulls, and after measuring the wings they determine that 10 had larger right wings than left wings and 4 had larger left wings than right wings. Use a binomial test and the calculation of 95% confidence intervals to address this question: Is there sufficient evidence to determine whether the population of seagulls is symmetric with respect to their wing sizes or not?

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The value of (117)\left(\begin{array}{c}11 \\7\end{array}\right) is which of the following?

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When calculating the standard error of a proportion, if the sample size is small or the population proportion is close to 0 or 1, then the Wald method is preferable to the Agresti-Coull method.

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Consider a bird has laid a clutch of six eggs that all hatch. Assuming the binomial distribution is appropriate, what is the probability that there is an equal number of male and female chicks that hatch?

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Demonstrate with a hypothetical numerical example how the Agresti-Coull method generates a narrower confidence interval than the Wald method.

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What is the 95% confidence interval, using the Wald method, for the proportion when there are 7 observed successes out of a total of 50 trials?

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If a binomial test returns a P-value greater than 0.05, we typically interpret this as meaning there is a match between the data and our expectations arising from using the binomial distribution to model the population.

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Draw a bar chart showing the sampling distribution for the proportion of successes for sample size 3 where the population proportion of successes is 0.6. Clearly label each axis and be precise.

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