Experimental Design

Chapter 1

Research Strategies and the Control of Nuisance Variables

Introduction

Experimental design – A plan for assigning subjects to experimental

conditions and the statistical analysis associated with the plan

– Identifies the independent, dependent, and nuisance variables and indicates the way in which the randomization and statistical aspects of an experiment are to be carried out

Formulation of Plans for Collection and Analysis of Data

Acceptable research hypotheses Distinction between independent and

dependent variables Selection of independent variable

– Quantitative independent variables– Qualitative independent variables

Selection of the dependent variable– Multivariate designs

Nuisance variables– Bias

Research Strategies

Experiments Quasi-experiments Surveys Case studies Naturalistic observation

Other Research Strategies

Ex post facto studies– Retrospective and prospective studies

Longitudinal and cross-sectional studies Longitudinal-overlapping and time-lag studies Time-series and single-subject studies

Threats to Valid Inference Making

Four categories of threats to valid inference making

1. Statistical conclusion validity

2. Internal validity

3. Construct validity of causes and effects

4. External validity

Threats to Statistical Conclusion Validity

1. Low statistical power2. Violated assumptions of statistical tests3. Fishing for significant results and the error

rate problem4. Reliability of measures5. Reliability of treatment implementation6. Random irrelevancies in the experimental

setting7. Random heterogeneity of respondents

Threats to Internal Validity

1. History• Events other than the treatment that occur

between the time the treatment level is assigned and the DV is measured

2. Maturation Processes not related to treatment that occur

within the subject

3. Testing Repeated testing may result in familiarity

4. Instrumentation Changes in calibration of a measuring instrument

Threats to Internal Validity

5. Statistical regression Occurs when the measurement of the DV is not

perfectly reliable

6. Selection Prior differences in subjects

7. Mortality Loss of subjects

8. Interactions with selection Selection-history effects or Selection-maturation

effects

Threats to Internal Validity

9. Ambiguity about the direction of causal influence

10. Diffusion or imitation of treatments If the subjects in different levels can communicate,

differences may be compromised

11. Compensatory rivalry by respondents receiving less desirable treatments

12. Resentful demoralization of respondents receiving less desirable treatments

Threats to External Validity

1. Interaction of testing and treatment

2. Interaction of selection and treatment

3. Interaction of setting and treatment

4. Interaction of history and treatment

5. Reactive arrangements

6. Multiple-treatment interference

Other Threats to Valid Inference Making

Experimenter-expectancy effect Demand characteristics Subject-predisposition effects

– Cooperative-subject effect– Screw you effect– Evaluation apprehension– Faithful subjects

Placebo effect

Controlling Nuisance Variables

General Approaches to Control– Experimental control– Statistical control

Some Specific Approaches to Control– Blind procedures– Deception– Unobtrusive experimentation– Multiple researchers

Controlling Nuisance Variables

Some Specific Approaches to Control– Debriefing– Experimenter-expectancy control groups– Unrelated-experiment technique– Quasi-control group– Yoked control procedure

Ethical Treatment of Subjects

1. Responsibility of researcher

2. Informed consent

3. Protection from harm

4. Protection of rights

5. Research deception

6. Confidentiality

7. Debriefing

8. Accurate and reporting