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In: Statistics and Probability

The U.S. Geological Survey compiled historical data about Old Faithful Geyser (Yellowstone National Park) from 1870...

The U.S. Geological Survey compiled historical data about Old Faithful Geyser (Yellowstone National Park) from 1870 to 1987. Let x1 be a random variable that represents the time interval (in minutes) between Old Faithful eruptions for the years 1948 to 1952. Based on 9280 observations, the sample mean interval was x1 = 62.0 minutes. Let x2 be a random variable that represents the time interval in minutes between Old Faithful eruptions for the years 1983 to 1987. Based on 24,170 observations, the sample mean time interval was x2 = 69.6 minutes. Historical data suggest that σ1 = 8.35 minutes and σ2 = 12.76 minutes. Let μ1 be the population mean of x1 and let μ2 be the population mean of x2.

(a) Compute a 99% confidence interval for μ1 – μ2. (Use 2 decimal places.)

lower limit    
upper limit    

(b) Comment on the meaning of the confidence interval in the context of this problem. Does the interval consist of positive numbers only? negative numbers only? a mix of positive and negative numbers? Does it appear (at the 99% confidence level) that a change in the interval length between eruptions has occurred? Many geologic experts believe that the distribution of eruption times of Old Faithful changed after the major earthquake that occurred in 1959.

Because the interval contains only positive numbers, we can say that the interval length between eruptions has gotten shorter.Because the interval contains both positive and negative numbers, we can not say that the interval length between eruptions has gotten longer.    We can not make any conclusions using this confidence interval.Because the interval contains only negative numbers, we can say that the interval length between eruptions has gotten longer.

Solutions

Expert Solution

(a) The 99% confidence interval for is

where

n1= 9280, n2= 24170

For 99% confidence , zc = 2.58

Thus , 99% confidence interval for is

= (-7.91 , -7.29)

Lower limit = -7.91

Upper limit = - 7.29

(b) The confidence interval consists of negative numbers only

Because the confidence interval contains only negative numbers we can say that interval length between eruptions has gotten longer.

orchestra answered 2 years ago
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