Criteria for Establishing Causal Relationships Concomitant variation Temporal ordering of variables...

Criteria for Establishing Causal Relationships

• Concomitant variation

• Temporal ordering of variables

• Control over other possible causal factors

Threats to Internal Validity

Experiment

• A research investigation in which conditions are controlled

• One independent variable is manipulated (sometimes more than one)

• Its effect on a dependent variable is measured

• To test a hypothesis

Basic Issues of Experimental Design

• Manipulation of the Independent Variable

• Selection of Dependent Variable

• Assignment of Subjects (or other Test Units)

• Control Over Extraneous Variables

The experimenter has some degree of control over the independent variable. The variable is independent because its value can be manipulated by the experimenter to whatever he or she wishes it to be.

Experiment Treatment

Alternative manipulations of the independent variable

being investigated

Independent Variable

• The experimenter controls independent variable.

• The variable’s value can be manipulated by the experimenters to whatever they wish it to be.

Manipulation of Independent Variable

• Classificatory vs. continuous variables

• Experimental and control groups

• Treatment levels

• More than one independent variable

Experimental Treatments

• The alternative manipulations of the independent variable being investigated

Dependent Variable

• Its value is expected to be dependent on the experimenter’s manipulation

• Criterion or standard by which the results are judged

Dependent Variable

• Selection–e.g... sales volume, awareness,

recall,

• Measurement

Test Units

• Subjects or entities whose response to the experimental treatment are measured or observed.

Two Types of Experimental Error

• Constant errors• Random errors

Field versus Laboratory Experiments

Controlling Extraneous Variables

• Elimination of extraneous variables

• Constancy of conditions

• Order of presentation

• Blinding

• Random assignment

How May an Experimenter control forExtraneous Variation?

• Eliminate Extraneous Variables

• Hold Conditions Constant

• Randomization

• Matching Subjects

Demand Characteristics

• Experimental procedures that intentionally hint to subjects something about the experimenter’s hypothesis

Demand Characteristics

• Guinea pig effect

• Hawthorne effect

Field vs. Laboratory Experiment

Laboratory Experiment Field Experiment

Artificial-Low Realism

Few ExtraneousVariables

High control

Low Cost

Short Duration

Subjects Aware ofParticipation

Natural-High Realism

Many ExtraneousVariables

Low control

High Cost

Long Duration

Subjects Unaware ofParticipation

Control Groups

Isolate extraneous variation

When does an Experiment have Internal Validity?

Internal Validity - The ability of an experiment to answer the question whether the experimental treatment was the sole cause of changes in a dependent variable

Did the manipulation do what it was supposed to do?

Factors Influencing Internal Validity

• History

• Maturation

• Testing

• Instrumentation

• Selection

• Mortality

Isolating Extraneous Variationwith a Control Group

• History Effects

• Maturation Effects

• Mortality Effects

Type of Extraneous Variable Example

History - Specific events in theenvironment between the Beforeand After measurement that are beyond the experimenter’s control

Maturation - Subjects changeduring the course of the experiment

Testing - The Before measure alertsor sensitizes subject to nature of experiment or second measure.

A major employercloses its plant intest market area

Subjects become tired

Questionnaireabout the traditionalrole of women triggers enhanced awareness of womenin an experiment.

Instrument - Changes ininstrument result in response bias

Selection - Sample selectionerror because of differentialselection comparison groups

Mortality - Sample attrition; some subjects withdraw from experiment

New questions aboutwomen are interpreteddifferently from earlierquestions.

Control group and experimental group isself-selected groupbased on preference forsoft drinks

Subjects in one groupof a hair dying study marry rich widows and move to Florida

Increasing Internal Validity

• Control group

• Random assignment

• Pretesting and posttesting

• Posttest only