Homework for scatterplots and correlation
Here are two formulas for finding r:
r
n
x
x
s
y
y
s
i
x
i
y
=



∑
1
1
r
n
x y
n x y
s
s
i
i
x
y
=

 ⋅ ⋅
⋅
∑
1
1
The first formula is the one given in the book.
The second one is easier to use.
Be
careful  both formulas are affected by roundingoff error.
1. For each data set below make a scatterplot and find r.
Before you start, look at the
numbers I
have chosen and notice that each set consists of the same 5 numbers.
(That means
that the averages and standard deviations are the same for all four sets.)
The only
difference is in the way the numbers have been paired.
a.
X
Y
b.
X
Y
c.
X
Y
d.
X
Y
1
1
1
1
1
1
1
6
2
2
2
2
2
4
2
4
2
2
2
4
2
2
2
2
4
4
4
2
4
6
4
2
6
6
6
6
6
2
6
1
2.
In this problem, use the same 5 values of X and Y that I used in question 1.
a. Find a combination that will give a value of r that is negative, but closer to 0
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 Spring '08
 Staff
 Statistics, Formulas, Correlation, Probability, Standard Deviation, Mean, standard deviations

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