Microsoft Office Tutorials and References
In Depth Information
In order to help you to understand where the correlation number that ranges
from -1.0 to +1.0 comes from, we will walk you through the steps involved to use
the formula as if you were using a pocket calculator. This is the one time in this
book that we will ask you to use your pocket calculator to find a correlation, but
knowing how the correlation is computed step-by-step will give you the
opportunity to understand how the formula works in practice.
To do that, let’s create a situation in which you need to find the correlation
between two variables.
Suppose that you wanted to find out if there was a relationship between high
school grade-point average (HSGPA) and freshman GPA (FRGPA) for Biology
majors at a College of Science and Technology. You have decided to call HSGPA
the x-variable (i.e., the predictor variable) and FRGPA as the y-variable (i.e., the
criterion variable) in your analysis. To test your Excel skills, you take a random
sample of freshmen Biology majors at the end of their freshman year and record
their GPA. The hypothetical data for eight students appear in Fig. 6.7 . (Note: We
are using only one decimal place for these GPAs in this example to simplify the
mathematical computations).
Fig. 6.7 Worksheet data for
high school GPA and frosh
GPA (practical example)
Notice also that we have used Excel to find the sample size for both variables, X
and Y, and the MEAN and STDEV of both variables. (You can practice your Excel
skills by seeing if you get these same results when you create an Excel spreadsheet
for these data).
Now, let’s use the above table to compute the correlation r between HSGPA
and FRGPA using your pocket calculator.
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