The U.S. Geological Survey compiled historical data about Old Faithful Geyser (Yellowstone National Park) from 1870 to 1987. Letx₁be a random variable that represents the time interval (in minutes) between Old Faithful eruptions for the years 1948 to 1952. Based on8840observations, the sample mean interval wasx₁=63.4minutes. Letx₂be a random variable that represents the time interval in minutes between Old Faithful eruptions for the years 1983 to 1987. Based on23,117observations, the sample mean time interval wasx₂=71.2minutes. Historical data suggest that₁=8.42minutes and₂=12.27minutes. Let₁be the population mean ofx₁and let₂be the population mean ofx₂.

(a) Compute a95%confidence interval for₁-₂. (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 the95%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.

a) Because the interval contains only positive numbers, we can say that the interval length between eruptions has gotten shorter.

b) Because the interval contains both positive and negative numbers, we can not say that the interval length between eruptions has gotten longer.

c) We can not make any conclusions using this confidence interval.

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