Correlation and Experimentation

AP Psychology

What does it mean when we say two things are correlated?

Why do correlations permit prediction but not explanation?

Correlation coefficient Measures the relationship between two variables How well one variable predicts the other r

Closer to 1 = more strongly related + correlation means variables move in the same direction - correlation means variables move in different dorections

Correlation

Graphs of two variables that may or may not be related

Scatterplots

Are these Correlated?

Draw Best Fit Lines

Some correlations (What’s the causal factor?) Ice Cream Consumption Positively correlates with

violent crime # of years of marriage correlates with amount of gray

hair or amount of hair loss Problematic Causal Arrows

Low Self esteem <-> depression Parental love and acceptance <-> positive behavior

Correlation indicates the POSSIBILITY of a causal relationship but does not prove that one exists

Correlation and Causation

A perceived nonexistent correlation We are more likely to recall observations that confirm

the correlation (Troiler and Hamilton1986) More babies are born when the moon Is full Couples who adopt become more likely to conceive Sugar makes children hyper Arthritic patients can predict barometric changes

(Redelmeier and Trevsky 1996) Sensitivity to dramatic or unusual events

When we notice random coincidences, we forget that they are random

Illusory Correlations

Random sequences don’t often look random People expect lack of pattern in random sequences

(Kahneman and Tversky 1972) People favor randomness in picking lottery numbers,

even though repeated numbers are just as likely (Holtgraves and Skeel 1992)

Brains LOVE patterns We are startled by coincidences and tend to

remember them and think something special about them

(The called shot?)

Perceiving order in Random Events

How do experiments clarify or reveal cause and effect relationships? What is random assignment, and why is it important? Experiments isolate variables and control for other factors

Turning correlation into causation Breastfed infants have higher iq’s (Angelson et al 2001, Gale and Martin 1996,

Johnson et al 1996, Luca et al 1996, Mortensen et al 2002, Quinn et al 2001) Smarter mothers tend to breastfeed? Mother’s milk helps brain development? Correlation must be controlled for iq of mother, maternal age, and maternal

education How to isolate cause and effect

Manipulate the factors of interest Control for other factors Randomly assign the control and the test groups (Lucas 1998 breastmilk

study)

Experimentation

Illusory correlations can exist for treatment protocols Vitamin C for a cold (MY cold got better!) Subliminal tape before exam (I passed = it worked!) Antibiotics for sinus infections

Controlling for variables in experimental treatments Blind studies: patient does not know which treatment (if any)

he/she is getting Double blind studies: patient does not know which treatment

he/she is receiving and person administering treatment does not know which treatment he/she is administering

Placebo effect: sometimes people get better even if they are in the control group (Kirsh and Sapirstein 1998)

Evaluating Therapies

Experimental condition: people receiving the treatment Control condition: people receiving the placebo Random assignment

Each participant has an equal opportunity to be in each group Groups should be statistically identical

Treatment study designs Hormone replacement therapy increases health risks (Love 2002) Viagra Study (Goldstein et al 1998)

Independent Variable: variable that is manipulated (drug or placebo)

Dependent Variable: measurable outcome researcher is trying to manipulate

Operational definitions provide both a definition and a means of measurement for each variable

Experimental Design

Can Subliminal Tapes improve your life? The urban myth of the subliminal popcorn ad Multitude of subliminal messaging products that claim to feed you

positive reinforcements so that you can succeed in school, lose weight, quit smoking, or feel good about yourself.

1991 study by Greenwald, et al Students assigned to listen to memory improvement tapes every day for 5

weeks Half listened to “self-esteem,” half listened to memory Pre and post tested on actual self esteem and memory Pre and post tested on perception of self esteem and memory No change on actual test, big changes on perceptual test

General public does not understand importance of blind studies (Miller and Pifer 1996)

The Case of the Subliminal Tapes

Exit Ticket Why is experimental design sometimes interpreted as superior to other forms of

psychological study? Why is it important to have a control group?

Homework Define each of the following and provide an example

correlation coefficient scatterplot illusory correlation Experiment Double-blind procedure Placebo effect Experimental condition Control condition Random assignment Independent variable Dependent variable

How would you design an experiment to test whether or not doing crosswords improved a person’s verbal skills?

How would you design an experiment to test whether or not attractive people are more likely to be hired than unattractive people?

Exit Ticket and Homework