Microsoft Office Tutorials and References
In Depth Information
6.1.1 Understanding the Formula for Computing a Correlation
Objective : To understand the formula for computing the
correlation r
The formula for computing the correlation r is as follows:
X
Y
n 1 S ð
1
X
Þð
Y
Þ
r
¼
(6.1)
S x S y
This formula looks daunting at first glance, but let’s “break it down into its steps”
to understand how to compute the correlation r.
6.1.2 Understanding the Nine Steps for Computing
a Correlation, r
Objective : To understand the nine steps of computing a
correlation r
The nine steps are as follows:
Step Computation
Result
1.
Find the sample size n by noting the number of students
8
2.
Divide the number 1 by the sample size minus 1 (i.e., 1 / 7)
0.14286
3.
For each student , take the HSGPA and subtract
the mean HSGPA
for the 8 students and call this X
X (For example,
for student # 6, this would be: 2.6 – 2.86)
0.26
Note: With your calculator, this difference is
0.26, but when Excel uses 16
decimal places for every computation, this result could be slightly different for each
student
4.
For each student , take the FRGPA and subt ract the mean
FRGPA for the 8 students and call this Y
Y (For example,
for student # 6, this would be: 2.3 – 2. )
0.44
5.
Then , for each student , multiply ( X
X ) times
( Y
Y ) (For example, for student # 6 this would
be: (
0.26) x (
0.44)
þ
0.1144
6.
Add the results of ( X
X ) times (Y – Y ) for the 8 students
þ
1.09
Steps 1-6 would produce the Excel table given in Fig. 6.8 .
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