STAT 121 Exam 2025 Questions Verified Answers
Consider the following interpretation of 90% confidence level: "The probability that the
mean of all axle diameters lies somewhere in the interval (0.98 cm, 1.02 cm) is 0.90." Is this
interpretation of 90% confidence level correct or incorrect? Why or why not?
Incorrect. It states "probability on one specific calculated interval" rather than "confidence
in the procedure".
Consider the following interpretation of 90% confidence level: "90% of all possible
confidence intervals computed using the same procedure used to obtain (0.98, 1.02) will
contain the value of μμ." Is this interpretation of 90% confidence level correct or incorrect?
Why or why not?
Correct. It is stated in terms of the confidence interval procedure - not one specific,
calculated interval.
The BYU Testing Center is known for being the location with the most prayers said per capita.
Researchers asked a random sample of 47 BYU students to estimate the number of prayers
they said in the BYU Testing Center last semester. The researchers are willing to consider
these 47 BYU students as an SRS from the population of all BYU students. Suppose it is known
that the number of prayers said in the BYU Testing Center has standard deviation of σσ = 8.
Researchers plan to use these data to estimate the mean number of prayers said by students
last semester in the BYU Testing Center with 99% confidence. Suppose the confidence
interval turned out to be (8.001, 13.829). What is an appropriate interpretation of this
interval?
We are 99% confident that the true mean number of prayers said in the BYU Testing Center
by all BYU students last semester is between 8.001 and 13.829 prayers.
, What does the expression z∗[(σ)/√n] represent?
margin of error
True or False: The margin of error, m, represents the maximum estimation error for a
given level of confidence.
True
What does the expression (σ)/√n represent?
standard deviation of x bar
Suppose that the mean amount of money spent by college students on textbooks each
semester in a sample of 100 students is $325. A 98% confidence interval was
constructed based on the sample data. What confidence interval could it be?
(317, 333)
How can we reduce the margin of error without compromising the level of confidence?
Increase sample size
For a fixed sample size, increasing our confidence level does what to our margin of error?
Increases margin of error
A researcher wishes to estimate the mean weight of full-grown male grizzly bears. He wishes
to construct a 95% confidence interval with a margin of error of 3 pounds. Suppose the
population standard deviation, σσ, is 12.3 pounds. How many bears must be in his sample?
65 bears
A medical researcher at a sports medicine clinic uses 35 volunteers from the clinic to study
the average number of hours the typical American exercises per week. It is known that hours
of exercise are normally distributed, and past data give him a standard deviation of 1.2
hours. Is it safe to compute a confidence interval for μμ?
No sample is not random
Consider the following interpretation of 90% confidence level: "The probability that the
mean of all axle diameters lies somewhere in the interval (0.98 cm, 1.02 cm) is 0.90." Is this
interpretation of 90% confidence level correct or incorrect? Why or why not?
Incorrect. It states "probability on one specific calculated interval" rather than "confidence
in the procedure".
Consider the following interpretation of 90% confidence level: "90% of all possible
confidence intervals computed using the same procedure used to obtain (0.98, 1.02) will
contain the value of μμ." Is this interpretation of 90% confidence level correct or incorrect?
Why or why not?
Correct. It is stated in terms of the confidence interval procedure - not one specific,
calculated interval.
The BYU Testing Center is known for being the location with the most prayers said per capita.
Researchers asked a random sample of 47 BYU students to estimate the number of prayers
they said in the BYU Testing Center last semester. The researchers are willing to consider
these 47 BYU students as an SRS from the population of all BYU students. Suppose it is known
that the number of prayers said in the BYU Testing Center has standard deviation of σσ = 8.
Researchers plan to use these data to estimate the mean number of prayers said by students
last semester in the BYU Testing Center with 99% confidence. Suppose the confidence
interval turned out to be (8.001, 13.829). What is an appropriate interpretation of this
interval?
We are 99% confident that the true mean number of prayers said in the BYU Testing Center
by all BYU students last semester is between 8.001 and 13.829 prayers.
, What does the expression z∗[(σ)/√n] represent?
margin of error
True or False: The margin of error, m, represents the maximum estimation error for a
given level of confidence.
True
What does the expression (σ)/√n represent?
standard deviation of x bar
Suppose that the mean amount of money spent by college students on textbooks each
semester in a sample of 100 students is $325. A 98% confidence interval was
constructed based on the sample data. What confidence interval could it be?
(317, 333)
How can we reduce the margin of error without compromising the level of confidence?
Increase sample size
For a fixed sample size, increasing our confidence level does what to our margin of error?
Increases margin of error
A researcher wishes to estimate the mean weight of full-grown male grizzly bears. He wishes
to construct a 95% confidence interval with a margin of error of 3 pounds. Suppose the
population standard deviation, σσ, is 12.3 pounds. How many bears must be in his sample?
65 bears
A medical researcher at a sports medicine clinic uses 35 volunteers from the clinic to study
the average number of hours the typical American exercises per week. It is known that hours
of exercise are normally distributed, and past data give him a standard deviation of 1.2
hours. Is it safe to compute a confidence interval for μμ?
No sample is not random
