Chapter 10
Two Quantitative Variables
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Inference for Correlation: Simulation-Based Methods
Least-Squares Regression
Inference for Correlation and Regression: Theory-Based
Methods
Section 10.1 and 10.2: Summarizing Two Quantitative
Variables and Inference for Correlation (Simulation)
I was interested in knowing the relationship
between the time it takes a student to take a test
and the resulting score. So I collected some data …
Time
30
41
41
43
47
48
51
54
54
56
56
56 57 58
Score
100
84
94
90
88
99
85
84
94
100 65
64 65 89
Time
58
60
61
61
62
63
64
66
66
69
72
78 79
Score
83
85
86
92
74
73
75
53
91
85
62
68 72
Scatterplot
Put explanatory
variable on the
horizontal axis.
Put response
variable on the
vertical axis.
Describing Scatterplots

When we describe data in a scatterplot,
we describe the




Direction (positive or negative)
Form (linear or not)
Strength (strong-moderate-weak, we will let
correlation help us decide)
How would you describe the time and test
scatterplot?
Correlation






Correlation measures the strength and
direction of a linear association between
two quantitative variables.
Correlation is a number between -1 and 1.
With positive correlation one variable increases, on
average, as the other increases.
With negative correlation one variable decreases,
on average, as the other increases.
The closer it is to either -1 or 1 the closer the
points fit to a line.
The correlation for the test data is -0.56.
Correlation Guidelines
Strength of
Association
What this means
0.5 to 1.0
Strong
0.3 to 0.5
Moderate
The points will appear to be
nearly a straight line
When looking at the graph
the increasing/decreasing
pattern will be clear, but it
won’t be nearly a line
0.1 to 0.3
Weak
With some effort you will be
able to see a slightly
increasing/decreasing pattern
0 to 0.1
None
No discernible
increasing/decreasing pattern
Correlation
Value
Same Strength Results with Negative Correlations
Back to the test data

I was not completely honest. There were
three more test scores. The last three
people to finish the test had scores of 93,
93, and 97.
What does this
do to the
correlation?
Influential Observations


The correlation changed from -0.56 (a fairly
strong negative correlation) to -0.12 (a fairly
weak negative correlation).
Points that are far to the left or right and not in
the overall direction of the scatterplot can greatly
change the correlation. (influential observations)
Correlation

Correlation measures the strength and
direction of a linear association between
two quantitative variables.






-1 < r < 1
Correlation makes no distinction between
explanatory and response variables.
Correlation has no unit.
Correlation is not a resistant measure.
Correlation based on averages is usually higher
than that for individuals.
Correlation game
Inference for Correlation with Simulation
1. Compute the observed statistic. (Correlation)
2. Scramble the response variable, compute the
simulated statistic, and repeat this process many
times.
3. Reject the null hypothesis if the observed
statistic is in the tail of the null distribution.
Temperature and Heart Rate
Hypotheses
Null: There is no association between
heart rate and body temperature. (ρ = 0)
 Alternative: There is a positive linear
association between heart rate and body
temperature. (ρ > 0)

ρ = rho
Temperature and Heart Rate
Collect the Data
Tmp 98.3 98.2 98.7 98.5 97.0 98.8 98.5 98.7 99.3 97.8
HR
72
69
72
71
80
81
68
82
68
65
Tmp 98.2 99.9 98.6 98.6 97.8 98.4 98.7 97.4 96.7 98.0
HR
71
79
86
82
58
84
73
57
62
89
Temperature and Heart Rate
Explore the Data
r = 0.378
Temperature and Heart Rate

If there was no association between heart
rate and body temperature, what is the
probability we would get a correlation as
high as 0.378 just by chance?

If there is no association, we can break
apart the temperatures and their
corresponding heart rates. We will do this
by shuffling one of the variables. (The
applet shuffles the response.)
Shuffled r =-0.216
Temp
98.30
98.20
98.70
98.50
97
98.80
98.50
98.70
99.30
97.80
98.20
99.90
98.60
98.60
97.80
98.40
98.70
97.40
96.70
98
HR
81
73
57
86
79
72
71
68
58
89
82
69
71
72
62
84
82
80
65
68
Shuffled r =0.212
Temp
98.30
98.20
98.70
98.50
97
98.80
98.50
98.70
99.30
97.80
98.20
99.90
98.60
98.60
97.80
98.40
98.70
97.40
96.70
98
HR
58
86
79
73
71
82
57
80
71
72
81
82
89
62
84
72
68
68
69
65
Shuffled r =-0.097
Temp
98.30
98.20
98.70
98.50
97
98.80
98.50
98.70
99.30
97.80
98.20
99.90
98.60
98.60
97.80
98.40
98.70
97.40
96.70
98
HR
86
65
69
71
73
58
81
72
57
82
71
80
68
79
84
89
68
82
62
72
Temperature and Heart Rate

In three shuffles, we obtained correlations
of -0.216, 0.212 and -0.097. These are
all a bit closer to 0 than our sample
correlation of 0.378

Let’s scramble 1000 times and compute
1000 simulated correlations.
Temperature and Heart Rate


Notice our null
distribution is
centered at 0 and
somewhat
symmetric.
We found that
530/10000 times
we had a
simulated
correlation
greater than or
equal to 0.378.
Temperature and Heart Rate
With a p-value of 0.053 we don’t have
strong evidence there is a positive linear
association between body temperature
and heart rate. However, we do have
moderate evidence of such an association
and perhaps a larger sample give a
smaller p-value.
 If this was significant, to what population
can we make our inference?

Temperature and Heart Rate
Let’s look at a different data set
comparing temperature and heart rate
(Example 10.5A) using the Analyzing Two
Quantitative Variables applet.
 Pay attention to which variable is
explanatory and which is response.
 The default statistic used is not
correlation, so we need to change that.

Work on exploration 10.2 to see if there is
an association between birth date and the
1970 draft lottery number.
Least Squares Regression
Section 10.3
Introduction
If we decide an association is linear, it’s
helpful to develop a mathematical model
of that association.
 Helps make predictions about the
response variable.
 The least-squares regression line is
the most common way of doing this.

Introduction
Unless the points form a perfect line,
there won’t be a single line that goes
through every point.
 We want a line that gets as close as
possible to all the points

Introduction

We want a line that minimizes the vertical
distances between the line and the points
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These distances are called residuals.
The line we will find actually minimizes the
sum of the squares of the residuals.
This is called a least-squares regression
line.
Are Dinner Plates Getting
Larger?
Example 10.3
Growing Plates?
There are many recent articles and TV
reports about the obesity problem.
 One reason some have given is that the
size of dinner plates are increasing.
 Are these black circles the same size, or is
one larger than the other?

Growing Plates?

They appear to be the same size for
many, but the one on the right is about
20% larger than the left.

This suggests that people will put more
food on larger dinner plates without
knowing it.
Portion Distortion

There is name for this phenomenon:
Delboeuf illusion
Growing Plates?
Researchers gathered data to investigate
the claim that dinner plates are growing
 American dinner plates sold on ebay on
March 30, 2010 (Van Ittersum and Wansink,
2011)
 Size and year manufactured.

Year
size
1950
10
1956
10.8
1957
10.1
1958
10
1963
10.6
1964
10.8
1969
10.6
1974
10
1975
10.5
1978
10.1
Year
Size
1980
10.38
1986
10.8
1990
10.4
1995
11
2004
10.8
2004
10.1
2007
11.5
2008
11
2008
11.1
2009
11
Growing Plates?
Both year (explanatory variable) and size
(response variable) are quantitative.
 Each dot represents one plate in this
scatterplot.
 Describe the
association
here.

Growing Plates?



The association appears to be roughly linear
The least squares regression line is added
How can we describe this line?
Growing Plates?
The regression equation is 𝑦 = 𝑎 + 𝑏𝑥:
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
a is the y-intercept
b is the slope
x is a value of the explanatory variable
ŷ is the predicted value for the response
variable
For a specific value of x, the
corresponding distance y − 𝑦 is a residual
Growing Plates?

The least squares line for the dinner plate
data is 𝑦 = −14.8 + 0.0128𝑥
Or 𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟 = −14.8 + 0.0128(𝑦𝑒𝑎𝑟)
 This allows us to predict plate diameter for
a particular year.
 Substitute 2000 for year and we predict a
diameter of -14.8 + 0.0128(2000) = 10.8
in.

Growing Plates?
𝑦 = −14.8 + 0.0128𝑥
 What is the predicted diameter for a plate
manufactured in the year 2001?


How does this compare to our prediction
for the year 2000 (10.8 in.)?


-14.8 + 0.0128(2001) = 10.8128 in.
0.0128 larger
Slope b = 0.0128 means that diameters
are predicted to increase by 0.0128 inches
per year on average
Growing Plates?
Slope is the predicted change in the
response variable for one-unit change in
the explanatory variable.
 Both the slope and the correlation
coefficient for this study were positive.
 The slope and correlation coefficient will
always have the same sign.

Growing Plates?


The y-intercept is where the regression line
crosses the y-axis or the predicted response
when the explanatory variable equals 0.
We had a y-intercept of -14.8 in the dinner plate
equation. What does this tell us about our dinner
plate example?


How can it be negative?


Dinner plates in year 0 were -14.8 inches.
The equation works well within the range of values given
for the explanatory variable, but fails outside that range.
Our equation should only be used to predict the
size of dinner plates from about 1950 to 2010.
Growing Plates?

Predicting values for the response variable
for values of the explanatory variable that
are outside of the range of the original
data is called extrapolation.
Predicting Height from
Footprints
Exploration 10.3
Inference for the Regression
Slope: Theory-Based Approach
Section 10.5
Beta vs Rho
Testing the slope of the regression line is
equivalent to testing the correlation.
 Hence these hypotheses are equivalent.

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
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Ho: β = 0
Ho: ρ = 0
H a: β > 0
Ha: ρ > 0
(Slope)
(Correlation)
Sample slope (b) Population (β: beta)
Sample correlation (r) Population (ρ: rho)
When we do the theory based test, we will be
using the t-statistic which can be calculated
from either the slope or correlation.
Introduction

We have seen in this chapter that our null
distributions are again bell-shaped and
centered at 0 (for either correlation or
slope as our statistic). This is also true if
we use the t-statistic.
Validity Conditions
Under certain conditions, theory-based
inference for correlation or slope of the
regression line use t-distributions.
 We are going to use the theory-based
methods for the slope of the regression
line. (The applet allows us to do
confidence intervals for slope.)
 We would get the same p-value if we used
correlation as our statistic.

Validity Conditions
Validity Conditions for theory-based
inference for slope of the regression
equation.
1. The values of the response variable at
each possible value of the explanatory
variable must have a normal distribution
in the population from which the sample
was drawn.
2. Each normal distribution at each x-value
must have the same standard deviation.

Validity Conditions

Suppose the graph represents a
scatterplot for an entire population where
the explanatory variable is heart rate and
the response is body temperature.
Normal distributions of
temps for each BPM
Each normal distribution
has the same SD
Validity Conditions
How realistic are these conditions?
 In practice, you can check these
conditions by examining your scatterplot.
 Let’s look at some scatterplots that do not
meet the requirements.

Smoking and Drinking

The relationship between number of drinks
and cigarettes per week for a random
sample of students at Hope College.
The dot at (0,0)
represents 524
students
Are conditions met?
Smoking and Drinking

Since these conditions aren’t met, we
shouldn’t apply theory-based inference
When checking conditions for
traditional tests use the following logic.
Assume that the condition is met in your
data
 Examine the data in the appropriate
manner
 If there is strong evidence to contradict
that the condition is met, then don’t use
the test.

Validity Conditions

What do you do when validity conditions
aren’t met for theory-based inference?


Use the simulated-based approach
Another strategy is to “transform” the
data on a different scale so conditions are
met.

The logarithmic scale is common
Predicting Heart Rate
from Body Temperature
Example 10.5A
Heart Rate and Body Temp
Earlier we looked at the relationship
between heart rate and body temperature
with 130 healthy adults
 𝑃𝑟𝑒𝑑𝑖𝑐𝑡𝑒𝑑 𝐻𝑒𝑎𝑟𝑡 𝑅𝑎𝑡𝑒 = −166.3 + 2.44 𝑇𝑒𝑚𝑝
 r = 0.257

Heart Rate and Body Temp



We tested to see if we had convincing
evidence that there is a positive association
between heart rate and body temp in the
population using a simulation-based
approach. (We will make it 2-sided this
time.)
Null Hypothesis: There is no association
between heart rate and body temperature in
the population. β = 0
Alternative Hypothesis: There is an
association between heart rate and body
temperature in the population. β ≠ 0
Heart Rate and Body Temp

We get a very small
p-value (≈ 0.006)
since anything as
extreme as our
observed slope of
2.44 is very rare
Heart Rate and Body Temp
We can also approximate a 95%
confidence interval
observed statistic + 2 SD of statistic
2.44 ± 2(0.850) = 0.74 to 4.14
 What does this mean?
 We’re 95% confident that, in the
population of healthy adults, each 1º
increase in body temp is associated with
an increase in heart rate of between
0.74 to 4.14 beats per minute

Heart Rate and Body Temp
The theory-based approach should work
well since the distribution has a nice bell
shape
 Also check the scatterplot

Heart Rate and Body Temp

Let’s do the theory-based test using the
applet.



We will use the t-statistic to get our theorybased p-value.
We will find a theory-based confidence interval
for the slope.
After we are done with that, do
Exploration 10.5: Predicting Brain Density
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