PSTAT 109: Lab 8
Fall 2015
Instructions: When you finish all problems, ask your TA to check your work.
Problem 1 (Compute CI for small sample using t-distribution given sample statistics).
From a population of normally distributed IQ scores, a random sample of 25 scores yields
¯
sample mean as X = 98.4 and sample deviation Sx = 14.2. Find a 98% confidence interval
for the true mean score µ of this population. Use Excel’s TINV function to obtain your
answer.
Problem 2 (Compute CI using t-distribution given raw data). Standardized test scores are
often found to be normally distributed. The data set below is a random sample of combined
SAT scores from a group of sophomores picked from a random university. Compute the
sample mean, sample deviation and find a 95% confidence interval for the mean SAT score
of sophomores at this university.
1270
1220
1160
950
1070
1110
1220
1150
1440
850
1280
1020
880
1080
1200
970 720 1350 1070
1210 1170 910 1320
1030 1070 1120
1020 860 980
1400 920 800
Use Excel’s TINV function to obtain your answer.
Problem 3 (Compare CI using z-score and t-score). A random sample of size 400 from a
¯
normally distributed population yields X = 980.5 and Sx = 56.3. Compare the results of
finding a 99% confidence interval for µ when using z-scores rather than t-scores. Use Excel’s
TINV and NORMINV functions to obtain your answer.
