Correlation and Causation in Research Psychology

1. Correlation and Causation in Research Psychology Bryce Maritano Job Talk at Shasta College July 25, 2007

2. Correlation • A correlation is a relationship between two variables (factors that change). Variables may include characteristics, attitudes, behaviors, or events. • Correlations are either positive (to +1.0), negative (to–1.0), or nonexistent (0.0).

3. Positive Correlation Positive Correlations: Both variables increase or decrease at the same time. A correlation coefficient close to +1.00 indicates a strong positive correlation. Examples: Height & Weight, Sit-ups & Abdominal muscles

4. Negative Correlation Negative Correlations: Indicates that as the amount of one variable increases, the other decreases (and vice versa). A correlation coefficient close to -1.00 indicates a strong negative correlation. Examples: altitude/Temp, flossing/decay

5. Nonexistent Correlation • No Correlation: no relationship between the two variables. • A correlation coefficient of 0 indicates no correlation. • Example: intelligence/happiness,

6. 2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 Relative risk of death 18.5 18.5- 20.5- 22.0- 23.5- 25.0- 26.5- 28.0- 30.0- 32.0- 35.0- 40 20.4 21.9 23.4 24.9 26.4 27.9 29.9 31.9 34.9 39.9 Body-mass index (BM I) Men Women Correlation of Obesity and Mortality

7. Accident frequency More sleep, fewer accidents Less sleep, more accidents 2,800 2,700 4,200 2,600 4000 2,500 3,800 2,400 3,600 Spring time change (hour sleep loss) Fall time change (hour sleep gained) Monday before time change Monday after time change Sleep Deprivation

8. Clever Hans • Clever Hans was a horse that was claimed to have been able to perform arithmetic and other intellectual tasks. • After formal investigation in 1907, psychologistOskar Pfungst demonstrated that the horse was not actually performing these mental tasks, but was watching the reaction of his human observers.

9. Children’s shoe size correlates with performance on spelling tests.

10. Causality • Causality or causation is defined as the relationship between one event (called cause) and another event (called effect) which is the consequence (result) of the first.

11. Flame from lamp (A) catches on curtain (B) and fire department sends stream of water (C) through window. Dwarf (D) thinks it is raining and reaches for umbrella (E), pulling string (F) and lifting end of platform (G). Iron ball (H) falls and pulls string (I), causing hammer (J) to hit plate of glass (K). Crashof glass wakes up pup (L) and mother dog (M) rocks him to sleep in cradle (N), causing attached wooden hand (O) to move up and down along your back.

12. Correlation does not imply causation • Although correlation is commonly confused with causation, correlational data does not indicate a cause-and-effect relationship. When a correlation is present, changes in the value of one variable reflect changes in the value of the other. The correlation does not imply that one variable causes the other variable, only that both variables are somehow related.

13. cum hoc ergo propter hoc • (Latin for "with this, therefore because of this") • The cum hoc ergo propter hoc logical fallacy can be expressed as follows: • A occurs in correlation with B. • Therefore, A causes B. • Example: • Sleeping with one's shoes on is strongly correlated with waking up with a headache. • Therefore, sleeping with one's shoes on causes headache.

14. Simpson’s Logic An episode of The Simpsons (Season 7, "Much Apu About Nothing") serves as a good example of this principle. Springfield had just spent millions of dollars creating a highly sophisticated "Bear Patrol" in response to the sighting of a single bear the week before. Homer: Not a bear in sight. The "Bear Patrol" is working like a charm! Lisa: That's specious reasoning, Dad. Homer: [uncomprehendingly] Thanks, honey. Lisa: By your logic, I could claim that this rock keeps tigers away. Homer: Hmm. How does it work? Lisa: It doesn't work. (pause) It's just a stupid rock! Homer: Uh-huh. Lisa: But I don't see any tigers around, do you? Homer: (pause) Lisa, I want to buy your rock.

15. Possible Explanations • Generally, if one factor (A) is observed to only be correlated with another factor (B), it is sometimes taken for granted that A is causing B even when no evidence supports this. This is a logical fallacy because there are at least four other possibilities: • B may be the cause of A, or • some unknown third factor is actually the cause of the relationship between A and B, or • the "relationship" is so complex it can be labeled coincidental (i.e., two events occurring at the same time that have no simple relationship to each other besides the fact that they are occurring at the same time). • B may be the cause of A at the same time as A is the cause of B (contradicting that the only relationship between A and B is that A causes B). This describes a self-reinforcing system.

16. Is There a Connection between Creativity and Mental Illness? • The rate of mental illness (in general) is slightly higher among those in the arts than those in other professions.

17. Autism rates are higher in cities with more rainfall and more cable television customers.

18. Experimental research • To study the effects that variables have on each other, an investigator must conduct an experiment. • Experimental research is concerned with how and why something happens. The goal of experimental research is to test the effect that an independent variable, which the scientist manipulates, has on a dependent variable, which the scientist observes. In other words, experimental research leads to conclusions regarding causation.

19. Dissociative Identity Disorder (Multiple Personality) correlates strongly with childhood abuse.

20. Dynamic System of 30 billion cells with trillions of connections

21. “Correlation is not causation but it sure is a hint.” Edward Tufte

22. Ice cream sales correlate with the number of people attacked by sharks. Therefore, ice cream causes shark attacks