This gives you a recap of the key points of correlation that should hopefully be familiar to you from last year.

1. Correlation recap
Dr David Playfoot
d.r.playfoot@swansea.ac.uk

2. Research questions
• Most psychological research boils down to
two simple questions
1. If I change x does something happen to
y?
2. Are x and y related?
Research questions

3. Correlation
• “Are x and y related?”
1. Do people who spend more time in the
library get better marks?
2. Are intelligence and attractiveness
related?
Correlation

4. Things to remember
• We do not manipulate anything in a
correlational design.
• There is no IV and no DV
• Correlation is usually used when the variables
are continuous, or making distinct groups is
inappropriate.
Correlation

5. Measuring relationships
• In its simplest form:
• Where does this person’s score fall on variable x? Better than
most people? Worse than most people? About average?
• Where does this person’s score fall on variable y? Better than
most people? Worse than most people? About average?
• Compare the two. Do it for lots of people.
Correlation

6. Measuring relationships
If people who are “better than most people” on variable x are
also “better than most people” on variable y
POSITIVE CORRELATION
Correlation

7. Measuring relationships
If people who are “worse than most people” on variable x are
also “worse than most people” on variable y
POSITIVE CORRELATION
Correlation

8. Measuring relationships
If people who are “better than most people” on variable x are
“worse than most people” on variable y
NEGATIVE CORRELATION
Correlation

9. Measuring relationships
If people who are “worse than most people” on variable x are
“better than most people” on variable y
NEGATIVE CORRELATION
Correlation

10. Measuring relationships
• There are two correlation tests we’ll use
• Pearson’s Product Moment
• Spearman’s Rank
Correlation

11. Measuring relationships
• Does the correlation based on z scores per
variable
• i.e. is the score of the participant
above or below the mean for all
participants?
• Says “mean” so must be parametric
Pearson’s

12. Measuring relationships
• Does the correlation based on ranks
• i.e. is the best person at one thing also
the best at another?
• Non-parametric test
Spearman’s

13. THE DIRECTION OF THE CORRELATION IS DETERMINED
BY WHETHER PEOPLE ARE DOING THE SAME THING ON
BOTH VARIABLES, NOT BY WHETHER THEIR SCORES
ARE BETTER/WORSE THAN AVERAGE
Remember

14. Measuring relationships
• We can calculate the degree of correlation
between any two variables as long as they can
be paired.
• Correlation is a only descriptive measure
• Unless we also calculate whether the
relationship is strong enough to be significant
Correlation

15. Measuring relationships
• The strength of a correlation is expressed on a
scale from + 1 through 0 and on to -1.
• This figure is called the correlation coefficient
• The closer to 1, the stronger the relationship
• Ignore the sign when you are assessing the
strength and the significance.
Correlation