Sampling
Distributions of
Proportions
•
•
The dotplot is a partial graph of the
Toss
a
penny
20
times
and
record
the
sampling distribution of all sample
number
of heads.
proportions
of sample size 20. If I
found all the possible sample
Calculate
the
proportion
of
heads
&
proportions – this would be
mark it on
the dot plotnormal!
on the board.
approximately
What shape do you think the dot plot
will have?
Sampling Distribution
• Is the distribution of possible values
of a statistic from all possible samples
of the same size from the same
population
We it’s
will use:
• In the case of the pennies,
the
p for the population
distribution of all possible
sample
proportion
proportions (p)
and
p-hat for the sample
proportion
Suppose we have a population of six
people: Alice, Ben, Charles, Denise,
Edward, & Frank
What is the proportion of females? 1/3
What is the parameter of interest in
this population?
gender
Draw samples of two from this population.
How many different samples are
possible?
6C2 =15
Find the 15 different samples that are possible &
find the sample proportion of the number of
females in each sample.
Ben & Frank
Charles & Denise
Charles & Edward
Alice & Charles.5
Charles & Frank
Alice & Denise
1
How does the mean
of &the
Denise
Edward
sampling distribution
(m&p-hat
) .5
Alice & Edward
.5
Denise
Frank
compare to the population
Edward & Frank
Alice & Frank
.5
mparameter
p-hat = p (p)?
Alice & Ben
.5
0
.5
0
0
.5
0
Ben mean
& Charles
0
Find the
& standard
deviation of all p-hats.
Ben & Denise
1
μ

Ben
ˆ& Edward
p
3
.5
&0
σ pˆ  0.29814
Formulas:
μpˆ  p
σ pˆ 
These are found on
the formula chart!
p 1  p 
n
Does the standard deviation of the
sampling distribution equal the
equation?
NO -
σ pˆ 
 
1 2
1 to calculate the
So
3 –3in order
 0.29814
standard
2
3 deviation of the
sampling distribution, we
WHY?
MUST be sure that our sample
Correction
factor
– multiply
size is
less than
10%
of the by
We are sampling more than 10% of our population!
If we
N n
population!
use the correction factor, we will see that weNare
 1 correct.
σ pˆ 
 
1 2
3 3  6  2  0.29814
2
6 1
Assumptions (Rules of Thumb)
• Sample size must be less than 10% of
the population (independence)
• Sample size must be large enough to
insure a normal approximation can be
used.
np > 10 & n (1 – p) > 10
Why does the second assumption insure
an approximate normal distribution?
Remember back to binomial distributions
Suppose n = 10 & p = 0.1
(probability of a success), a
histogram of this distribution
> 10 &skewed
n(1-p) >right!
10
isnp
strongly
insures that the sample size
Now
use
n
=
100
&
p
=
0.1
(Now
is large enough to have a
np normal
> 10!) While
the histogram is
approximation!
still strongly skewed right – look
what happens to the tail!
Based on past experience, a bank believes
that 7% of the people who receive loans
μpˆ  .07
will not make payments on time. The bank
recently approved 200
.93
.07loans.
σ pˆ 
 .01804
Yes
–
200
What are the mean and
standard
deviation
np = 200(.07) = 14
of the proportion of clients
in this group
n(1 - p) = 200(.93) = 186
who may not make payments on time?
Are assumptions met?
Ncdf(.10, 1E99, .07, .01804) =
What is the probability that over 10% of
.0482
these clients will not make payments on
time?
Suppose one student tossed a coin
200 times and found only 42% heads.
Do you believe that this is likely to
happen?


.5(.=5)200(.58)
np = 200(.42)
= 84 & n(1-p)
= 116


ncdf


,.
42
,.
5
,

.
0118
Since both > 10, I can use a normal
curve!

200
Find m & s using the

formulas.
No – since there is
approximately a 1% chance of
this happening, I do not
believe the student did this.
Assume that 30% of the students
at FHS wear contacts. In a
sample of 100 students, what is
the probability that more than 35%
of them wear contacts?
mp-hat = .3
& sp-hat = .045826
Check assumptions!
np = 100(.3) = 30 & n(1-p) =100(.7) = 70
Ncdf(.35, 1E99, .3, .045826) = .1376