MATH 1410/6.1 and 6.2 pp
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Transcript MATH 1410/6.1 and 6.2 pp
CHAPTER SIX
Confidence
Intervals
Section 6.1
Confidence Intervals for the
MEAN (Large Samples)
Estimating Vocab
Point Estimate: a single value estimate for a
population parameter.
Interval Estimate: a range of values used to
estimate a population parameter.
Level of Confidence (c): the probability that the
interval estimate contains the population
parameter
The level of confidence, c, is the area under the
curve between 2 z-scores called Critical Values
Find the critical value zc necessary
to construct a confidence interval
at the given level of confidence.
C = 0.85
C = 0.75
More Vocab!
Find the margin of error for the
given values.
C = 0.90
s = 2.9 n = 50
C = 0.975
s = 4.6 n = 100
Confidence Intervals for the
Population Mean
Construct a C.I. for the Mean
1. Find the sample mean and sample size.
2. Specify σ if known. Otherwise, if n > 30
, find the sample standard deviation s.
3. Find the critical value zc that
corresponds with the given level of
confidence.
4. Find the margin of error, E.
5. Find the left and right endpoints and
form the confidence interval.
Construct the indicated confidence
interval for the population mean.
44. A random sample of 55 standard hotel rooms
in the Philadelphia, PA area has a mean nightly
cost of $154.17 and a standard deviation of
$38.60. Construct a 99% confidence interval for
the population mean. Interpret the results.
46. Repeat Exercise 44, using a standard
deviation of s = $42.50. Which confidence
interval is wider? Explain.
Sample Size: given c and E…
Example from p 308
54. A soccer ball manufacturer wants to estimate
the mean circumference of mini-soccer balls within
0.15 inch. Assume the population of circumferences
is normally distributed.
A) Determine the minimum sample size required to
construct a 90% C.I. for the population mean.
Assume σ = 0.20 inch.
B) Repeat part (A) using σ = 0.10 inch.
C) Which standard deviation requires a larger
sample size? Explain.
Section 6.2
Confidence Intervals for the
MEAN (Small Samples)
The t – Distribution (table #5)
Used when the sample size n < 30 , the
population is normally distributed, and σ
is unknown.
t – Distribution is a family of curves.
Bell shaped, symmetric about the mean.
Total area under the t - curve is 1
Mean, median, mode are equal to 0
Uses Degrees of Freedom (d.f. =n–1)
d. f. are the # of free choices after a the
sample mean is calculated.
To find the critical value, tc , use the t
table.
Find the critical value, tc for c = 0.98, n = 20
Find the critical value, tc for c = 0.95, n = 12
Confidence Intervals and
t - Distributions
1. Find the sample mean, standard
deviation, and sample size.
2. ID the degrees of freedom, level of
confidence and the critical value.
3. Find the margin of error, E.
4. Find the left and right endpoints and
for the confidence interval.
Construct the indicated C.I.
Use a Normal or a t – Distribution to
construct a 95% C.I. for the population
mean. (from page 317)
36. In a random sample of 13 people,
the mean length of stay at a hospital
was 6.2 days. Assume the population
standard deviation is 1.7 days and the
lengths of stay are normally
distributed.
Find the sample mean and standard
deviation, the construct a 99% C.I. for the
population mean. (from p 316)
28. The weekly time spent (in hours) on
homework for 18 randomly selected high
school students:
12.0 11.3 13.5 11.7 12.0 13.0
15.5 10.8 12.5 12.3 14.0 9.5
8.8 10.0 12.8 15.0 11.8 13.0