Experimental Design Presented By: Amber Atwater & Charlott Livingston

“When you start in science, you are brainwashed into believing how careful you must be and how difficult it is to discover things. There’s something that might be called the “graduate student syndrome”; graduate students hardly believe they can make a discovery.” ~Francis Crick~

Key R = random assignment R = random assignment O = observation (either pre-test or post-test) O = observation (either pre-test or post-test) X = treatment X = treatment = there is not a symbol for a control group so it is represented by a blank = there is not a symbol for a control group so it is represented by a blank = a dotted line between two groups of symbols indicates that the two groups are intact (not random) = a dotted line between two groups of symbols indicates that the two groups are intact (not random) X 1 = experimental treatment X 1 = experimental treatment X 0 = control condition or comparison treatment X 0 = control condition or comparison treatment

Scientific Method Choose a topic Choose a topic Identify the problem Identify the problem Research the problem Research the problem Develop a hypothesis Develop a hypothesis Design the Experiments Design the Experiments Test the hypothesis by doing experiments Test the hypothesis by doing experiments Analyze the results Analyze the results Formulate conclusions Formulate conclusions

Note About the Validity Threats to internal and external validity that effect the generalization of an experiment Threats to internal and external validity that effect the generalization of an experiment In addition, influences known as confounds, which are sources of confusion, should be minimized. In addition, influences known as confounds, which are sources of confusion, should be minimized.

Internal Validity History History Maturation Maturation Instrumentation Instrumentation Testing Testing Statistical Regression Statistical Regression Selection Interaction Selection Interaction

External Validity Selection Bias Selection Bias Reactive effects of experimental arrangements Reactive effects of experimental arrangements Reactive effect of testing (pretest sensitization) Reactive effect of testing (pretest sensitization) Multiple treatment interference Multiple treatment interference

Basic Experimental Methods True experimental design True experimental design Quasi-experimental design Quasi-experimental design Pre-experimental design Pre-experimental design

True Experimental Design There are three true experimental designs

Design 1: pre-test post-test randomized control group design R O X O R O O Threats to Internal and External Validity: No known threats No known threats

Design 2: post-test only randomized control group design R X O R O Threats to Internal and External Validity: Pretest sensitivity Pretest sensitivity

Design 3: Solomon randomized four group design R O X O R O O R X O R O Threats to Internal and External Validity No known threats No known threats

Quasi-Experimental Design There are two quasi-experimental designs

Design 7: nonequivalent control group design O X O O O Threats to Internal and External Validity: Mortality Mortality Selection Selection Interaction of selection Interaction of selection history history

Design 8: equivalent time samples design X 0 O X 1 O X 0 O X 1 O Threats to Internal and External Validity: Multiple treatment interference Multiple treatment interference

Pre-Experimental Design There are three pre-experimental designs

Design 4: one group pre-test post-test O X O Threats to Internal and External Validity: History History Maturation Maturation Instrumentation Instrumentation Testing Testing Statistical regression Statistical regression

Design 5: one-shot case study X O Threats to Internal and External Validity: No pretest = no comparison No pretest = no comparison

Design 6: static group comparison X O X O O Threats to Internal and External Validity: Selection Selection No pretest = no comparison No pretest = no comparison

Confounds A confound is a source of confusion regarding the explanation for a given difference Hawthorn Effect John Henry Effect Placebo Effect Blind Procedures Double-blind Experiments Demand Characteristics

References Campbell, D., & Stanley, J. (1963). Experimental and quasi-experimental designs for research. Dallas: Houghton Mifflin Company. Heffner, C. L. (2004). Research Methods. Retrieved February 13, 2009, from All Psych Online Website: Key, J. P. (1997). Experimental Research and Design. Retrieved February 13, 2009, from Oklahoma State University Website: Patten, M. L. (2009). Understanding research methods: an overview of the essentials seventh edition. Glendale, CA.: Pyrczak. Sridhar, M. S. (2008). Research Methodology: Part 4 – Experimental Design. Retrieved February 13, 2009, from Scribd Website: The Scientific Method (n.d.). Retrieved February 13, 2009 from website