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Agenda for This Unit Experimental Design Causal Reasoning in Experiments Notation Factorial Designs Main Effects Interaction Effects

Experimental Design Defined: a study of the effects of variables manipulated by a researcher in a situation in which all other variables are controlled, completed for the purpose of establishing causal relationships

Experimental Design Distinguishing Carefully Designed Studies from Experiments Manipulation of Variables Cause-effect Conclusions

Finding True Causes in Experiments The Challenge of Control A Case Study

Notation O - observation X - experimental variable R - randomization

Designs Reading Designs Pre-Experimental vs. True Experimental Designs

Randomization controlling for extraneous variables random assignment random selection

Comparing Designs

Internal Validity Defined: the degree to which the researcher can make an unequivocal statement of experimental effect Sources Mnemonic device: He said my tush is sagging extra inches.

External Validity Defined: the degree to which research findings can be generalized to other similar circumstances Sources

Other Sources of Invalidity Law of the Instrument Experimenter Effects (Demand Characteristics) Ignoring Initial Differences between Control and Experimental Groups

Factorial Designs Defined: experimental designs using more than one independent variable

One Factor Designs X1X1 - +

A Two Factor Design A 2 x 2 Design X1X X2X2

A 3 x 2 Design X1X1 - + X2X2 LOWMO HI

A Three Factor Design X1X X2X2 - + X3X3

The Relationship Between Factorial and Simple Designs X1X X2X2

X1X X2X2 R X O R O R X O R O R X O R O R X O R O

Identification of Offset Control Groups X1X X2X2 R X O R O R X O R O R X O R O R X O R O

Identification of Offset Control Groups X1X X2X2 R X O R O R X O R O R X O R O

Designs in Which Control Groups are Included X1X X2X2 R X O R O R X O R O R X O R O

Main Effects Defined: dependent variable effects from independent variables separately

A Main Effect Example X1X X2X Amount of Attitude Change Advocated Source Character

A Main Effect Example X1X X2X Amount of Attitude Change Advocated Source Character

A Main Effect Example X1X X2X Amount of Attitude Change Advocated Source Character

Diagrams of Main Effects Men Women Variable 1 D.V.: touching

Diagrams of Main Effects Men Women Variable 1 D.V.: touching

Diagrams of Main Effects Low High Variable 1 Amount of Attitude Change Advocated D.V.: Attitude Change

Diagrams of Main Effects Low High Variable 1 Variable 2 (Low) Amount of Attitude Change Advocated Source Character D.V.: Attitude Change

Diagrams of Main Effects Low High Variable 1 Variable 2 (High) Variable 2 (Low) Amount of Attitude Change Advocated Source Character D.V.: Attitude Change

Effects X1X X2X

Effects X1X X2X

Effects X1X X2X

Diagrams of Main Effects Low High Variable 1

Diagrams of Main Effects Low High Variable 1 Variable 2 (Low)

Diagrams of Main Effects Low High Variable 1 Variable 2 (Low) Variable 2 (High)

Interaction Effects Defined: dependent variable effects from independent variables taken together Forms: Ordinal (in the same direction as the main effects of variables involved) Disordinal (not in the same direction as the main effects of the variables involved)

An Interaction Effect Example X1X X2X

An Interaction Effect Example X1X X2X

An Interaction Effect Example X1X X2X

Diagram of the Interaction Effect Low High Variable 1

Diagram of the Interaction Effect Low High Variable 1 Variable 2 (Low)

Diagram of the Interaction Effect Low High Variable 1 Variable 2 (High) Variable 2 (Low)

Another Interaction Effect Example X1X X2X Sex of Clinician Male Female Type of Stuttering: Clonic Blocking

Another Interaction Effect Example X1X X2X Sex of Clinician Male Female Type of Stuttering: Clonic Blocking 30

Another Interaction Effect Example X1X X2X Sex of Clinician Male Female Type of Stuttering: Clonic Blocking 30

Diagram of the Interaction Effect Low High Variable 1 Male Female Sex of Clinician

Diagram of the Interaction Effect Low High Variable 1 Variable 2 (Low) Male Female Sex of Clinician Type of Stuttering: Clonic

Diagram of the Interaction Effect Low High Variable 1 Variable 2 (Low) Variable 2 (High) Male Female Sex of Clinician Type of Stuttering: Clonic Blocking

Interpreting Ordinal Interactions acceptable to look at the independent variables separately permissible to interpret main effects for independent variables involved in the interaction

Interpreting Disordinal Interactions must look at both independent variables together not permissible to interpret main effects for independent variables involved in the interaction

OK to Interpret Main Effects Low High Variable 1 Variable 2 (High) Variable 2 (Low)

Not OK to Interpret Main Effects Low High Variable 1 Variable 2 (Low) Variable 2 (High)

Effects: Example Low High Variable 2

Effects: Example Low High Variable 2 Variable 1 (Low)

Effects: Example Low High Variable 2 Variable 1(High) Variable 1 (Low)

Low High Variable 2 Effects: Example 2

Low High Variable 2 Effects: Example 2 Variable 1 (Low)

Low High Variable 2 Effects: Example 2 Variable 1 (High) Variable 1 (Low)