Chapter 7Confidence Intervals for a
Population Mean ; t distributions
• t distributions• t confidence intervals for a
population mean • Sample size required to
estimate
The Importance of the Central Limit Theorem
When we select simple random samples of size n, the sample means we find will vary from sample to sample. We can model the distribution of these sample means with a probability model that is
,Nn
Since the sampling model for x is the normal model, when we standardize x we get the
standard normal z
n
xz
nxSD
)( that Note
If is unknown, we probably don’t know either.
The sample standard deviation s provides an estimate of
the population standard deviation
For a sample of size n,
the sample standard deviation s is:
n − 1 is the “degrees of freedom.”
The value s/√n is called the standard error of x , denoted
SE(x).
nxSD
)(
2)(1
1 xxn
s i
nsxSE )(
Standardize using s for
Substitute s (sample standard deviation) for
n
xz ssss sss s
n
xz
Note quite correct
Not knowing means using z is no longer correct
t-distributionsSuppose that a Simple Random Sample of size n is drawn
from a population whose distribution can be approximated by
a N(µ, σ) model. When is known, the sampling model for
the mean x is N(, /√n).
When is estimated from the sample standard deviation s, the sampling model for the mean x follows a t distribution t(, s/√n) with degrees of freedom n − 1.
is the 1-sample t statistic
t x s n
Confidence Interval Estimates CONFIDENCE
INTERVAL for
where: t = Critical value from t-
distribution with n-1 degrees of freedom
= Sample mean s = Sample standard
deviation n = Sample size
For very small samples (n < 15), the data should follow a Normal model very closely.
For moderate sample sizes (n between 15 and 40), t methods will work well as long as the data are unimodal and reasonably symmetric.
For sample sizes larger than 40, t methods are safe to use unless the data are extremely skewed. If outliers are present, analyses can be performed twice, with the outliers and without.
nstx
x
t distributions Very similar to z~N(0, 1) Sometimes called Student’s t
distribution; Gossett, brewery employee Properties:i) symmetric around 0 (like z)ii)degrees of freedom
if > 1, E( ) = 0if > 2, = - 2, which is alwaysbigger than 1.
t
-3 -2 -1 0 1 2 3
Z
0 1 2 3-1-2-3
z = x - x
x
t =
x - s
, s = sn
x
x
x
Student’s t Distribution
-3 -2 -1 0 1 2 3
Z
t
0 1 2 3-1-2-3
nx-x = z
ns - x =t x
Student’s t Distribution
Figure 11.3, Page 372
-3 -2 -1 0 1 2 3
Z
t1
0 1 2 3-1-2-3
ns
x - x =t
Student’s t Distribution
Figure 11.3, Page 372
Degrees of Freedoms = s2
s = (X X)
n -12
i2
i=1
n
-3 -2 -1 0 1 2 3
Z
t1
0 1 2 3-1-2-3
t7
Student’s t Distribution
Figure 11.3, Page 372
ns
x - x =t Degrees of Freedoms = s2
s = (X X)
n -12
i2
i=1
n
Degrees of Freedom1 3.0777 6.314 12.706 31.821 63.6572 1.8856 2.9200 4.3027 6.9645 9.9250. . . . . .. . . . . .
10 1.3722 1.8125 2.2281 2.7638 3.1693. . . . . .. . . . . .
100 1.2901 1.6604 1.9840 2.3642 2.62591.282 1.6449 1.9600 2.3263 2.5758
0.80 0.90 0.95 0.98 0.99
t-Table: text- inside back cover
90% confidence interval; df = n-1 = 10
118125.1 :interval confidence%90 sx
0 1.8125
Student’s t Distribution
P(t > 1.8125) = .05
-1.8125.05.05
.90
t10
P(t < -1.8125) = .05
Comparing t and z Critical Values
Conf.level n = 30
z = 1.645 90% t = 1.6991
z = 1.96 95% t = 2.0452
z = 2.33 98% t = 2.4620
z = 2.58 99% t = 2.7564
Example – An investor is trying to estimate the return
on investment in companies that won quality awards last year.
– A random sample of 41 such companies is selected, and the return on investment is recorded for each company. The data for the 41 companies have
– Construct a 95% confidence interval for the
mean return.
18.875.14 sx
40141freedom of degrees18.875.14
sx
awards.quality that wincompaniesfor investmenton return
mean population thecontains 17.36) (12.14,interval that theconfident 95% are We
36.17,14.1261.275.144118.80211.275.14
2.0211 t table,- tfrom
nstx
1.. nfdnstx
Example Because cardiac deaths increase after heavy
snowfalls, a study was conducted to measure the cardiac demands of shoveling snow by hand
The maximum heart rates for 10 adult males were recorded while shoveling snow. The sample mean and sample standard deviation were
Find a 90% CI for the population mean max. heart rate for those who shovel snow.
15,175 sx
Solution 1.. nfdnstx
shovelers snowfor rateheart maximummean thecontains 183.70) (166.30,
interval that theconfident 90% are We)70.183,30.166(
70.817510
158331.1175
1.8331 ttable,- t theFrom1015,175
nsx
EXAMPLE: Consumer Protection Agency
Selected random sample of 16 packages of a product whose packages are marked as weighing 1 pound.
From the 16 packages: a. find a 95% CI for the mean weight
of the 1-pound packages b. should the company’s claim that the
mean weight is 1 pound be challenged ?
1.10 pounds, .36 poundx s
EXAMPLE
95% CI, n=16, df=15, x=1.10s=.36critical value of t is 2.1315
becomes
.361.10 (2.1315) 1.10 .19 .91, 1.2916
Since 1 pound is in the interval, the company'sclaim appears reasonable.
tsx tn
1.. nfdnstx