Educational Psychology / Unit 2 EDP 0117 worked example, 60 to 90 seconds
Reading a Correlation
The idea
To read a correlation, always take three steps. First, the sign: a plus means the two variables rise together, a minus means one rises as the other falls. Second, the size: the closer r is to one, in either direction, the stronger the relationship, and values near zero mean little or none. Third, the limit: however strong, a correlation shows that variables move together, never that one causes the other.
In practice
Worked example: days of short sleep and grade point average correlate at minus point two nine. The sign says more short nights go with lower grades; the size says the link is weak.
Where people slip
A negative correlation is not the absence of one: minus point eight is a strong relationship running in the opposite direction, while zero is no relationship at all. Scatterplots show strength and direction at a glance, so check the picture as well as the number.
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