3.4 Cause and Effect3.4 Cause and Effect

Usually the main reason for a correlational Usually the main reason for a correlational study is to find evidence of a cause-and-study is to find evidence of a cause-and-effect relationshipeffect relationship

Cause and Effect Relationship: when a Cause and Effect Relationship: when a change in the independent variable, x, change in the independent variable, x, produces a change in the dependent produces a change in the dependent variable, y.variable, y.

Examples of cause and effect Examples of cause and effect relationships:relationships:

Physical activity will cause your heart rate Physical activity will cause your heart rate to increaseto increase

Increasing the temperature on the Increasing the temperature on the thermostat will cause the room thermostat will cause the room temperature to increasetemperature to increase

More hours studied will cause your marks More hours studied will cause your marks to increaseto increase

3.4 Cause and Effect3.4 Cause and Effect

An external variable that causes two An external variable that causes two variables to change in the same wayvariables to change in the same way

Crimes and churches example:Crimes and churches example:

Common Cause FactorCommon Cause Factor3.4 Cause and Effect Relationships

Churches

Crim

es POPULATION is the common cause variable

The dependent and independent variable are The dependent and independent variable are reversed in the process of establishing causalityreversed in the process of establishing causality

For instance, suppose that statistics show a For instance, suppose that statistics show a positive correlation between gun ownership and positive correlation between gun ownership and violent crime, namely, the higher number of guns violent crime, namely, the higher number of guns owned, the higher the rate of violent crime. It owned, the higher the rate of violent crime. It would be tempting to jump to the conclusion that would be tempting to jump to the conclusion that gun ownership causes violent crime, but the gun ownership causes violent crime, but the causal relationship may be the exact reverse. causal relationship may be the exact reverse. High rates of violent crime may cause fearful High rates of violent crime may cause fearful citizens to purchase guns for protectioncitizens to purchase guns for protection

Reverse Cause and Effect Reverse Cause and Effect RelationshipRelationship

3.4 Cause and Effect Relationships

A correlation exists without any casual A correlation exists without any casual relationship between the variablesrelationship between the variables

For example, there has been an increase in the For example, there has been an increase in the number of reality shows on TV over the last five number of reality shows on TV over the last five years. Over the same time period, there has years. Over the same time period, there has been an increase in the number of females been an increase in the number of females enrolled in engineering programsenrolled in engineering programs

These two variable have a positive correlation, These two variable have a positive correlation, however, they are purely coincidental however, they are purely coincidental

Accidental RelationshipAccidental Relationship3.4 Cause and Effect Relationships

A correlation does not seem to be accidental A correlation does not seem to be accidental even though no cause and effect relationship even though no cause and effect relationship exists exists

Suppose you found a correlation between Suppose you found a correlation between people’s fitness levels and the amout of action people’s fitness levels and the amout of action movies that they watch. Seems logical to make movies that they watch. Seems logical to make a conclusion that a physically fit individual may a conclusion that a physically fit individual may prefer adventure movies, however, it would be prefer adventure movies, however, it would be very difficult to prove that one variable affects very difficult to prove that one variable affects the otherthe other

Presumed RelationshipPresumed Relationship3.4 Cause and Effect Relationships

Extraneous VariableExtraneous Variable An external variable that complicates the nature of the An external variable that complicates the nature of the

cause and effect relationship between two variablescause and effect relationship between two variables

3.4 Cause and Effect Relationships

PERIOD 1 TEST SCORE vs STUDY TIME

y = 0.0292x + 0.7163r = 0.3984

0.00%

20.00%

40.00%

60.00%

80.00%

100.00%

120.00%

0 2 4 6 8 10

HOURS STUDIED (hours)

CH

2 TE

ST

MA

RK

(%)

Seems logical that studytime will affect test scores,however, there are manyextraneous variables thatcomplicate the cause andeffect relationship.

-Sleep hours before test-Attendance in class-Other courses-Part time job hours-Ability to take tests-Etc..

Extraneous VariableExtraneous Variable

In order to reduce the effect of extraneous variables, In order to reduce the effect of extraneous variables, researchers often compare an experimental group to a researchers often compare an experimental group to a control group.control group.

The two groups are as similar as possible so that any The two groups are as similar as possible so that any extraneous variable will have about the same effect on extraneous variable will have about the same effect on either groupeither group

Researchers will vary the independent variable for the Researchers will vary the independent variable for the experimental group but not the control groupexperimental group but not the control group

Any difference in the dependent variable for the two Any difference in the dependent variable for the two groups can then be attributed to the changes in the groups can then be attributed to the changes in the independent variable and not the extraneous variablesindependent variable and not the extraneous variables

3.4 Cause and Effect Relationships