lecture 10.4 bt
TRANSCRIPT
Today’s Agenda
Attendance / Announcements
Sections 10.4
Population vs. Sample
Sample Mean:
Sample Standard Deviation:
Population Mean:
Population Standard Deviation:
x
s
Analyzing Real World Data
Below are the scores for the Anatomy and
Physiology Final Exam (30 students)
79 51 67 50 78 62
89 83 73 80 88 48
60 71 79 89 63 55
98 71 40 81 46 50
61 61 50 90 75 61
Continuous Probability Distributions
Distributions for continuous random variables
Usually the result of measurement:
Height, time, distance,…
Usually concerned with the percentage of population
(probability) within a certain range
This is because a continuous random variable has an
infinite amount of values within any range, so we don’t
think in terms of probability for a specific value.
The Normal distribution
Considered one of the most important distribution in all
of statistics.
We’ve seen the idea of a “bell shaped and symmetric
curve.” This is the normal distribution……
The Normal Curve
The Normal Curve
The Normal Curve
The Normal Curve
The Normal Curve
Z-scores:
Standardizing Normal Curve
The standardized (or normalized) z-score
is basically “how many standard
deviations the value is from the mean”
xz
The Normal Curve
The following are synonymous when
it comes to the normal curve:
• Find the area under the curve …
• Find the percentage of the population …
• Find the probability that …
The Normal Curve
Using a z-Table to find probabilities
Note: Our z-table only gives area to the left
(or probabilities less than z)
Find Probability that z < 0.97
Z-scores: -2 -1 0 1 2 3-3
z = 0.97
Find area under
the curve to the left of z = 0.97
)97.0( zP
Using a Z-Table to find probabilities
Using a Z-Table to find probabilities
Find Probability
that z < 0.97
Since z > 0, use
positive side
Find Probability
that z < -2.91
Z-scores: -2 -1 0 1 2 3-3
-2.91
Find area under the
curve to the left of
z = -2.91
Using a Z-Table to find probabilities
Find Probability
that z < -2.91
* Since z < 0, use
negative side
Using a Z-Table to find probabilities
Not all Z-Tables are alike!
Using a Z-Table to find probabilities
But we can still use our z-table to find areas to the right (probability greater than), as well as areas between two values (probability between two values).
Find Probability
that z > 0.75
Z-scores: -2 -1 0 1 2 3-3
0.75
Find area under the
curve to the right
of z = 0.75)75.0( zP
Finding Area to the Right
Finding Area to the Right
Two Methods
Using the Complement
Using Symmetry
Complement Method
Z-scores: -2 -1 0 1 2 3-3
0.75
Find area under the
curve to the right
of z = 0.75
Find Probability
that z > 0.75
)75.0( zP
Complement Method
- Use fact that
area under entire
curve is 1.
- And that we
can find area to
the left
Z-scores: -2 -1 0 1 2 3-3
0.75
1)75.0()75.0( zPzP
Get
from
table Unknown
Complement Method
Z-scores: -2 -1 0 1 2 3-3
0.75
Find area under the
curve to the right
of z = 0.75
Find Probability
that z > 0.75
7734.0)75.0( zP
Find Probability
that z > 0.75
Z-scores: -2 -1 0 1 2 3-3
0.75
Find area under the
curve to the right
of z = 0.75
Complement Method
)75.0(1)75.0( zPzP
7734.01)75.0( zP
2266.0)75.0( zP
The Symmetry Method????Find Probability
that z > 0.75
Z-scores: -2 -1 0 1 2 3-3
0.75
Find area under the
curve to the right
of z = 0.75
Symmetry MethodUse symmetry of
the normal curve to
find area
Z-scores: -2 -1 0 1 2 3-3
0.75
Find area under the
curve to the right
of z = 0.75
- 0.75
2266.0)75.0( zP
2266.0)75.0( zP
Finding Area between two values
Just use difference of the
two areas
az bz
az
Finding Area between two values
az bzbz
So,
)()()( abba zzPzzPzzzP
)( bzzP
)( azzP
az
Difference of Area
Find Probability that
-1.25 < z < 0.75
Z-scores: -2 -1 0 1 2 3-3
0.75
Find area under the
curve between
z = -1.25 and 0.75
-1.25
)75.025.1( zP
)25.1()75.0( zPzP
1056.07734.0
6678.0
Finding Probabilities of Normal Distributions
1. For data that is normally distributed, find the percentage of data items that are:
a) below z = 0.6b) above z = –1.8c) between z = –2 and –0.5
Always draw sketch, and shade region!!!!
Finding Probabilities of Normal Distributions
2. Given a data set that is normally distributed, find the following probabilities:
a) P(0.32 ≤ z ≤ 3.18)b) P(z ≥ 0.98)
Working with Normal Distributions
1. Don’t confuse z with x !!
Before solving real world applications of
data that is normally distributed, we need to
first calculate any appropriate z-scores based on
the data. This is called normalizing the data.
Recall…
2. Make sure the data is normally distributed
xz
Systolic blood pressure readings are normally distributed with a mean of 121 and a standard deviation of 15. After converting each reading to its z-score, find the percentage of people with the following blood pressure readings:
a) below 142 )142( xP ?)( zP
z < 1.4
%92.919192.0 or
Systolic blood pressure readings are normally distributed with a mean of 121 and a standard deviation of 15. After converting each reading to its z-score, find the percentage of people with the following blood pressure readings:
b) above 131 )131( xP ?)( zP
z > 0.67
%14.252514.0 or
Systolic blood pressure readings are normally distributed with a mean of 121 and a standard deviation of 15. After converting each reading to its z-score, find the percentage of people with the following blood pressure readings:
c) between 142 and 154 )154142( xP ?)(? zP
1.4 < z < 2.2
%69.60669.0 or
The placement test for a college has scores that are normally distributed with = 500 and = 100. If the college accepts only the top 20% of examinees, what is the cutoff score on the test for admission?
(hint: you’ll need to use the table first, and work back)
z > ????
20.0????)( zP80.0????)( zP
Finding z-score from known probabilities
(or percentages)
39
845.0z80.0????)( zP
The placement test for a college has scores that are normally distributed with = 500 and = 100. If the college accepts only the top 20% of examinees, what is the cutoff score on the test for admission?
(hint: you’ll need to use the table first, and work back)
z > 0.845
20.0)845.0( zP80.0)845.0( zP
The placement test for a college has scores that are normally distributed with = 500 and = 100. If the college accepts only the top 20% of examinees, what is the cutoff score on the test for admission?
z > 0.845 So, what is minimum test score?
xz
100
500845.0
x 5.584x
Demonstrating Importance of z - scores
Lil’ Billy scores 60 on a vocabulary test and 80 on
a grammar test. The data items for both tests are
normally distributed. The vocabulary test has a
mean of 50 and a standard deviation of 5. The
grammar test has a mean of 72 and a standard
deviation of 6.
On which test did the student perform better?
Why?
Demonstrating Importance of z - scores
Lil’ Billy scores 60 on a vocabulary test and 80 on a grammar test.
The data items for both tests are normally distributed. The
vocabulary test has a mean of 50 and a standard deviation of 5.
The grammar test has a mean of 72 and a standard deviation of 6.
On which test did the student perform better? Explain why and
show all necessary work to support your conclusion.
Vocabulary (~Norm) Grammar (~Norm)
60vx 80gx
50v
5v
00.2vz
72g
6g
33.1vz
Classwork / Homework
• 10.4 Worksheet
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