Question

The following data represent petal lengths (in cm) for independent random samples of two species of Iris. Petal length (in cm) of Iris virginica: x1; n1 =

35 5.1 5.8 6.5 6.1 5.1 5.5 5.3 5.5 6.9 5.0 4.9 6.0 4.8 6.1 5.6 5.1 5.6 4.8 5.4 5.1 5.1 5.9 5.2 5.7 5.4 4.5 6.4 5.3 5.5 6.7 5.7 4.9 4.8 5.9 5.1

Petal length (in cm) of Iris setosa: x2; n2 =

38 1.6 1.8 1.4 1.5 1.5 1.6 1.4 1.1 1.2 1.4 1.7 1.0 1.7 1.9 1.6 1.4 1.5 1.4 1.2 1.3 1.5 1.3 1.6 1.9 1.4 1.6 1.5 1.4 1.6 1.2 1.9 1.5 1.6 1.4 1.3 1.7 1.5 1.6

(a) Use a calculator with mean and standard deviation keys to calculate x1, s1, x2, and s2. (Round your answers to two decimal places.)

x1 =

s1 =

x2 =

s2 =

(b) Let μ1 be the population mean for x1 and let μ2 be the population mean for x2. Find a 99% confidence interval for μ1 − μ2. (Round your answers to two decimal places.)

lower limit

upper limit

(c) Explain what the confidence interval means in the context of this problem. Does the interval consist of numbers that are all positive? all negative? of different signs? At the 99% level of confidence, is the population mean petal length of Iris virginica longer than that of Iris setosa?

Because the interval contains only positive numbers, we can say that the mean petal length of Iris virginica is longer.

Because the interval contains only negative numbers, we can say that the mean petal length of Iris virginica is shorter.   

Because the interval contains both positive and negative numbers, we cannot say that the mean petal length of Iris virginica is longer.

(d) Which distribution did you use? Why?

The Student's t-distribution was used because σ₁ and σ₂ are unknown.

The standard normal distribution was used because σ₁ and σ₂ are unknown.

The Student's t-distribution was used because σ₁ and σ₂ are known.

The standard normal distribution was used because σ₁ and σ₂ are known.

Do you need information about the petal length distributions? Explain.

Both samples are large, so information about the distributions is not needed

.Both samples are large, so information about the distributions is needed.

Both samples are small, so information about the distributions is needed.

Both samples are small, so information about the distributions is not needed.

Answer 1

a) From the given data

x1 = 5.50

s1 = 0.58

x2 = 1.49

s2 = 0.21

b)

c) Correct answer: Option (A)

Because the interval contains only positive numbers, we can say that the mean petal length of Iris virginica is longer.

d) Correct answer: Option (A)

The Student's t-distribution was used because σ₁ and σ₂ are unknown.

e)

Both samples are large, so information about the distributions is not needed