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Experimental & Quasi Experimental Design

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by

Gretchen Snethen

on 4 December 2013

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Transcript of Experimental & Quasi Experimental Design

Experimental and Quasi-Experimental Design
Let's get back to experiments!
Review!
Experiments... Manipulation
Quasi-Experimental Design
Manipulation of independent variable
No randomization
Within group vs. between group
Common types
Non-equivalent group designs
Pre-post designs
Interrupted time series design
True Experimental Design
Randomization!
What do experimental designs bring to RT?
Benefits of quasi experimental designs?
Benefits of true experimental design?
Generalizability
Efficacy vs. effectiveness
Strengths of quasi v. true experimental designs for each of these
Descriptive studies
Surveys
Observations
Qualitative
Interviews
Thick Description
Often understanding existing phenomena
Case studies
Intervention studies with 1 or few cases
Often qualitative description of outcomes
SSD
AB, ABBA, ABAB etc...
Variables
Independent
Dependent
Controlling (or descriptive)
BBT Example

Independent:
Dependent:
Descriptive:
Non-equivalent group design
Post with non-equivalent control group
G1 X(intervention) O(bservation)
G2 (no interventio) O(bservation)



Pre-post design
One group
G1 O1 X O2

Pre-post with non-equivalent control group
G1 O1 X O2
G2 O1 O2


Interrupted Time Series Design
Time series: Set of measurements taken at intervals over time.
Attendance, behavior, etc...
Intervention "interrupts" time series


Single group interrupted time series

G1 O1 O2 O3 O4 X O5 O6 O7 O8

Control group interrupted time series

G1 O1 O2 O3 O4 X O5 O6 O7 O8

G2 O9 O10 O11 O12 O13 O14 O15 O16


Random assignment vs. Random sampling
Research Randomizer examples
http://www.randomizer.org/lesson2.htm
http://www.randomizer.org/lesson3.htm

Experimental Group
Control Group
Assigned to the intervention group
Subjects may or may not know they are in the tx group (subject blinding)
Researchers may or may not know group assignment (researcher blinding)
Importance of understanding co-occurring treatments?
Different types of control groups
No-treatment control condition
Placebo treatment (information only)
Treatment as usual
Wait list control
Blinding may occur as well
Placebo groups allow for blinding
Between Group Design
Within-Subjects Experiments
Each participant tested under each condition
Benefits
Maximum control of extraneous variables
Participants in all conditions have the same characteristics (same people!)
Issues
Carryover effect
Practice effect
Fatigue effect
Context effect
Counterbalancing
Testing participants in different orders (ABC, ACB, BAC, BCA, CAB, CBA)
Scientific notation
G = Group of subjects
(Subscripts 1 thru N indicate different groups)
X = treatment or independent experimental variable (Subscripts 1 though n indicate different treatments)
O = observation, a measure that records observation of a pre or posttest (i.e, the dependent variable), subscripts indicate multiple observations and is equated with time at which observation is taken
R = Random assignment

Limitations
History
Selection
Statistical Regression
Testing
Instrumentation
Mortality

Maturation
Diffusion of treatment
Experimenter Bias
Statistical conclusions
Full transcript