Experimental 
Designs
................1900.........................1925..............................1950.................................1975.............................2000.....................
Psychological experiments
Procedure for comparing groups (McCall)
Statistical Advancement
e.g. Chi-square & significance of difference
Campbell and Stanley identify 15 types of experimental designs
1963
Types of basic designs and threats elaborated upon
1979
Experimental Design Timeline
More complex and sophistocated experiments, involving many variables
hello
Experimental 
Designs
RANDOM ASSIGNMENT
In an experiment you test an idea to determine whether it influences an outcome or dependent variable
EXPERIMENTAL DESIGN PROCESS
Determine if an experiment is the appropriate research approach for your study
Determine activities with which to “experiment” 
Assign individuals to experience the experiment (and have some individuals experience something different)
Determine whether those who experienced the activities (or practice or procedure) performed better on some outcome than those who did not experience the same thing
What is it?
How does it work?
When do we use it?
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We use experiments when we want to establish possible cause and effect between independent and dependent variables
or
when we have two or more groups to study
THREATS TO VALIDITY
Internal
External
History
Maturation
Regression
Selection
Mortality
Interaction with selection
Diffusion of treatments
Compensatory equalization
Compensatory rivalry
Resentful demoralization
Testing
Instrumentation
Interaction of selection and treatment
Interaction of setting and treatment
Interaction of history and treatment
The degree to which the findings are generalizable to a population
Problems that threaten our ability to draw correct cause-and-effect inferences that arise because of our procedures or participant experiences
GROUP COMPARISONS
Process of a researcher obtaining scores for individuals or groups on the dependent variable and comparing the means and variance both within the group and between the groups
Phase 1: Relationship Picture
Phase 2: Timeline Picture
Phase 3: Statistical Comparisons
Error Correction Treatment
Spelling Accuracy
Test 1         Test 2          Test 3
6 weeks      6 weeks       6 weeks

6 weeks      6 weeks       6 weeks

6 weeks      6 weeks       6 weeks
Class A (control group)
Class B
(comparison group)
Class C
(experimental group)
CONTROL OVER EXTRANEOUS VARIABLES
Influences in the selection of participants, the procedures, the statistics, or the design likely to affect the outcome and provide an alternative explanation for our results than what was expected
Pretests and Posttests
PRETEST
POSTTEST
INTERVENTION
Covariates
No Covariate
Covariate Introduced
Independent Variable
Dependent Variable
Independent Variable
Dependent Variable
Covariate
Variables that the researcher controls for using statistics and that relate to the dependent variable but that do not relate to the independent variable
Matching of Participants
This is the process of identifying one or more personal characteristics that influence the outcome and assigning individuals with that characteristic equally to the experimental and control groups
MATCHING BASED ON GENDER
Homogeneous Samples
This is the process of selecting people who vary little in their personal characteristics
Similar:
Age
Gender
Racial Group
GPA
Degree Program
Thus, these characteristics or attributes are controlled in the experiment
Blocking Variables
This is a variable the researcher controls before the experiment starts by dividing (or "blocking") the participants into subgroups (or categories) and analyzing the impact of each subgroup on the outcome
The researcher forms homogeneous subgroups
Then the investigator randomly assigns individuals to the control and experimental groups using each category of the variable
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2.
The process of assigning individuals at random to groups or to different groups in an experiment
Random assignment equates groups and distributes variability between or among groups and conditions
Equating the groups
Random selection
Randomly selecting a sample from a population
MANIPULATING
TREATMENT CONDITIONS
  Identify a treatment variable
  Identify the conditions (or levels) of the variable
  Manipulate the treatment conditions
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2.
3.
-this is the independent variable
-these are the levels of the independent variable
-do something different for each condition
EDFI 6420
Research in Education
Amanda Kochmit
Matt Priest
Lindsay Whitman Drewes