Experimental DesignExperimental Design

Presented By: Presented By:

Amber Atwater &Amber Atwater &

Charlott LivingstonCharlott 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~

KeyKey R = random assignmentR = random assignment O = observation (either pre-test or post-test)O = observation (either pre-test or post-test) X = treatmentX = treatment = there is not a symbol for a control group = there is not a symbol for a control group

so it is represented by a blankso it is represented by a blank ----- = a dotted line between two groups of ----- = a dotted line between two groups of

symbols indicates that the two groups are symbols indicates that the two groups are intact (not random)intact (not random)

XX11 = experimental treatment = experimental treatment XX00 = control condition or comparison = control condition or comparison

treatmenttreatment

Scientific MethodScientific Method Choose a topicChoose a topic Identify the problemIdentify the problem Research the problemResearch the problem Develop a hypothesisDevelop a hypothesis Design the ExperimentsDesign the Experiments Test the hypothesis by doing experimentsTest the hypothesis by doing experiments Analyze the resultsAnalyze the results Formulate conclusionsFormulate conclusions

Note About the ValidityNote About the Validity

Threats to internal and external Threats to internal and external validity that effect the generalization validity that effect the generalization of an experimentof an experiment

In addition, influences known as In addition, influences known as confounds, which are sources of confounds, which are sources of confusion, should be minimized.confusion, should be minimized.

Internal ValidityInternal Validity

HistoryHistory MaturationMaturation InstrumentationInstrumentation TestingTesting Statistical RegressionStatistical Regression Selection InteractionSelection Interaction

External ValidityExternal Validity

Selection BiasSelection Bias Reactive effects of experimental Reactive effects of experimental

arrangementsarrangements Reactive effect of testing (pretest Reactive effect of testing (pretest

sensitization)sensitization) Multiple treatment interferenceMultiple treatment interference

Basic Experimental MethodsBasic Experimental Methods

True experimental designTrue experimental design Quasi-experimental design Quasi-experimental design Pre-experimental designPre-experimental design

True Experimental DesignTrue Experimental Design

There are three true experimental There are three true experimental designsdesigns

Design 1: pre-test post-test Design 1: pre-test post-test randomized control group randomized control group design design

R O X OR O X O

R O OR O OThreats to Internal and External Validity:Threats to Internal and External Validity:

No known threatsNo known threats

Design 2: post-test only Design 2: post-test only randomized control group randomized control group designdesign

R X OR X O

R OR OThreats to Internal and External Validity:Threats to Internal and External Validity:

Pretest sensitivityPretest sensitivity

Design 3: Solomon randomized Design 3: Solomon randomized four group designfour group design

R O X OR O X OR O OR O OR X OR X OR OR O

Threats to Internal and External ValidityThreats to Internal and External Validity No known threatsNo known threats

Quasi-Experimental DesignQuasi-Experimental Design

There are two quasi-There are two quasi-experimental designs experimental designs

Design 7: nonequivalent control Design 7: nonequivalent control group designgroup design

O X OO X O-------- -------- O OO O

Threats to Internal and External Validity:Threats to Internal and External Validity:

MortalityMortality SelectionSelection Interaction of selectionInteraction of selection historyhistory

Design 8: equivalent time samples Design 8: equivalent time samples designdesign

XX00O XO X11O XO X00O XO X11OO

Threats to Internal and External Validity:Threats to Internal and External Validity:

Multiple treatment interferenceMultiple treatment interference

Pre-Experimental DesignPre-Experimental Design

There are three pre-There are three pre-experimental designsexperimental designs

Design 4: one group pre-test post-Design 4: one group pre-test post-testtest

O X OO X O

Threats to Internal and External Validity:Threats to Internal and External Validity: HistoryHistory MaturationMaturation InstrumentationInstrumentation TestingTesting Statistical regressionStatistical regression

Design 5: one-shot case studyDesign 5: one-shot case study

X OX O

Threats to Internal and External Validity:Threats to Internal and External Validity: No pretest = no comparisonNo pretest = no comparison

Design 6: static group comparisonDesign 6: static group comparison

X OX O

----------------

OOThreats to Internal and External Validity:Threats to Internal and External Validity:

SelectionSelection No pretest = no comparisonNo pretest = no comparison

ConfoundsConfounds•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

ReferencesCampbell, 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: http://allpsych.com/researchmethods/experimentaldesign.html”

Key, J. P. (1997). Experimental Research and Design. Retrieved February 13, 2009, from Oklahoma State University

Website: http://www.okstate.edu/ag/agedcm4h/academic/aged5980a/5980/newpage2.html

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: http://www.scribd.com/doc/1555464/Research-Methodology-Part-4-Experimental-Design

The Scientific Method (n.d.). Retrieved February 13, 2009 from website

http://www.biology.ed.ac.uk/research/groups/jdeacon/statistics/tress2.html#DESIGN%20OF%20EXPERIMENTS