1. Correlation vs. Cause and Effect Relationships
The “Neglect of Common Cause” Fallacy

2. Definition: Correlation
A causal, complementary, parallel, or reciprocal relationship, especially a structural, functional, or qualitative correspondence between two comparable entities
EX: a correlation between drug abuse and crime.
Statistics The simultaneous change in value of two numerically valued random variables
EX: the positive correlation between cigarette smoking and the incidence of lung cancer; the negative correlation between age and normal vision.
The American Heritage® Dictionary of the English Language, Fourth Edition. Copyright © 2000 by Houghton Mifflin Company.

3. Correlation and Causality
Just because two quantities are correlated statistically, doesn’t mean one causes the other
EX: Whenever ice cream sales increase, so does the number of reported rapes (and vice-versa)
If two quantities are not correlated, then they can NOT be related by cause and effect.

4. Logical Flaws
Common mistake is to assume that if A occurs in correlation with B, then A causes B.
Two most common reasons this fails:
B causes A
Both A and B are caused by a third factor
the so-called “Common cause”
ignoring this possibility is called “Neglect of a Common Cause”

5. Example: Neglect of Common Cause (silly)
Observed Correlation: Every time I sleep with my shoes on, I wake up with a headache.
Assumed Cause and Effect: Sleeping with shoes on causes headaches.
Possible NoCC explanation: Both are caused by drinking too much alcohol.

6. Example: Neglect of Common Cause
Observed Correlation: Every time I eat chocolate, I get acne.
Assumed Cause and Effect: Chocolate triggers acne.
Possible NoCC explanation: Both are caused by stress.

7. Chocolate Acne
Stress

8. Example: Neglect of Common Cause
Observed Correlation: Children who watch a lot of violent imagery in movies and video games are more likely to commit violent crimes as adults.
Assumed C and E: Watching violence leads to violent behavior.
Possible NoCC explanation: Both watching and doing violence are caused by anger issues.

9. Movies Crime
Anger

10. Example: Neglect of Common Cause
Observed Correlation: Teenagers who use marijuana are more likely to try cocaine/heroine/meth as adults (The Gateway Drug concept)
Assumed C and E: Marijuana uses promotes hard drug use.
Possible NoCC explanation: Both are caused by rebellious personality, lousy homelife, feeling of helplessness …

11. Marijuana Heroin
Rebellious

12. Explain these: Drug use is much more common among poor people.
Blacks represent 10% of the population at large, but 50% of the prison population.
Smoking is much less common among college graduates.
Average income among Hispanics is lower than among whites.

13. Poverty Drug use
Helplessness
Racism
Unemployment  Addiction