This presentation describes the types of experimental research.

1. Experimental Research

2. Experiments
 Begin with a Hypothesis
 Modify Something in a Situation
 Compare Outcomes
 Cases or People are Termed “Subjects”

3. Random Assignment
 Probability of Equal Selection
 Allows Accurate Prediction
 An Alternative to Random
Assignment is Matching

4. Parts of the Classic Experiment
 Treatment or Independent Variable
 Dependent Variable
 Pretest
 Posttest
 Experimental Group
 Control Group
 Random Assignment

5. Variations on Experimental Design
 Pre-experimental Design
 One-shot Case Study
 One-group Pretest-Posttest Design
 Static Group Comparison
 Quasi-Experimental and Special Designs

6. Types of Validity
 External Validity
 Do the results apply to the broader
population?
 Internal Validity
 Is the independent variable responsible for the
observed changes in the dependent variable?

7. Confounding Variables That Threaten
Internal Validity
 Maturation
 Changes due to normal growth or
predictable changes
 History
 Changes due to an event that occurs during
the study, which might have affects the
results

8. Confounding Variables That Threaten
Internal Validity
 Instrumentation
 Any change in the calibration of the measuring
instrument over the course of the study
 Regression to the Mean
 Tendency for participants selected because of extreme
scores to be less extreme on a retest
 Selection
 Any factor that creates groups that are not equal at the
start of the study

9. Confounding Variables That Threaten
Internal Validity
 Attrition

Loss of participants during a study; are the
participants who drop out different from those
who continue?
 Diffusion of treatment
 Changes in participants” behavior in one
condition because of information they
obtained about the procedures in other
conditions

10. Subject Effects
 Participants are not passive
 They try to understand the study to help them to
know what they “should do”
 This behavior termed “subject effects”
 Participants respond to subtle cues about what is
expected (termed “demand characteristics”)
 Placebo effect: treatment effect that is due
to expectations that the treatment will work

11. Experimenter Effects
 Any preconceived idea of the
researcher about how the experiment
should turn out
 Compensatory effects

12. Types of Control Procedures
 General control procedures (applicable to
virtually all research)
 Control over subject and experimenter effects
 Control through the selection and assignment
of participants
 Control through specific experimental design

13. Principles of Experimental Design
 Control the effects of lurking
variables on the response, most
simply by comparing two or more
treatments
 Randomize
 Replicate

14. Randomization
 The use of chance to divide experimental
units into groups is called randomization.
 Comparison of effects of several treatments
is valid only when all treatments are applied
to similar groups of experimental units.

15. How to randomize?
How to randomize?
 Flip a coin or draw numbers out of a hat
 Use a random number table
 Use a statistical software package or
program
 Minitab
 www.whfreeman.com/ips

16. Statistical Significance
 An observed effect so large that it
would rarely occur by chance is called
statistically significant.

17. A few more things…
A few more things…
 Double-blind: neither the subjects nor the
person administering the treatment knew
which treatment any subject had received
 Lack of realism is a major weakness of
experiments. Is it possible to duplicate the
conditions that we want?