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Experimental & Quasi-Experimental Designs. Dr. Guerette. Purpose. Used in explanation and evaluation research purposes. Used in hypothesis testing. The Classical Experiment. Independent and Dependent Variables Pre-testing and Post-testing

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Presentation Transcript
  • Used in explanation and evaluation research purposes.
  • Used in hypothesis testing.
the classical experiment
The Classical Experiment
  • Independent and Dependent Variables
  • Pre-testing and Post-testing
    • Subjects are measured on the dependent variable (pre-test), exposed to the independent variable and are measured again on the dependent variable (post-test).
the classical experiment1
The Classical Experiment
  • Experimental and Control Groups
    • In the classical experiment the experiment group receives the independent variable and the control group does not. Each is measured on the dependent variable and differences are identified.
the classical experiment2
The Classical Experiment
  • Selection of subjects
    • Identify the population for your study and decide how the subjects will be selected.
  • Randomization is the typical method of selection of subjects. Randomization removes bias.
the classical experiment3
The Classical Experiment
  • Double-Blind Experiment
    • One way to ensure that the researcher will not allow bias to enter into the study is to use a double-blind experiment where neither the researcher nor the subjects know which group they are in.
  • Eliminates Hawthorne Effect
    • e.g. reactivity by research subjects
the classical experiment4
The Classical Experiment

Experimental O O X O O

Control O O O O

Random Assignment

threats to internal validity




Statistical Regression

Selection Bias

Experimental Mortality

Causal Time Order

Diffusion or Imitation of Treatment

Compensatory Treatment

Compensatory Rivalry


Threats to Internal Validity
  • The ability of the researcher to take findings from an experiment and apply them to the real world.
  • The strength of the experimental design is the ability to control for causal processes. This is also its weakness.
  • Construct Validity
    • Deals with the ability to generalize from what is observed in an experiment to actual causal processes in the real world.
  • External Validity
    • Deals with whether results from experiments in one environment would be similar to those found in a different environment.
  • Statistical Conclusion Validity
    • Deals with the size of the sample and how representative that sample is of the target population. This can be an issue in experiments since by their very nature they do not lend themselves to large samples.
variations on classical experiments
Variations on Classical Experiments
  • When a study does not lend itself to the classical experiment, changes can be made that will allow for modifications in procedures, such as the post-test only design.
  • The variations are called Quasi-experimental designs.
quasi experimental designs
Quasi-Experimental Designs
  • These are techniques that apply when random selection and assignment of subjects is not possible.
  • These varieties include
    • Cohort designs
    • Time-series designs
    • Variable oriented research (e.g. multiple regressions)
quasi experimental designs1
Quasi-Experimental Designs
  • There are an infinite number of ways to vary the number and composition of groups, subject, observations and the types of experimental stimuli used.
quasi experimental designs2
Quasi-Experimental Designs

Pre-post tests with treatment only;

no comparison group

Experimental O O X O O

quasi experimental designs3
Quasi-Experimental Designs

Post test only;

Either with or without comparison group

Experimental X O O

quasi experimental designs4
Quasi-Experimental Designs

Non-random assignment

Experimental O O X O O

Comparison O O O O

quasi experimental designs5
Quasi-Experimental Designs

Interrupted time series with removed intervention

Experimental O O X O O O X O O

quasi experimental designs6
Quasi-Experimental Designs

Non-random assignment Interrupted time series with switched intervention

Group 1 O O X O O O O

Group 2 O O O O X O O

in class exercise
In Class Exercise
  • You have been asked to conduct a study of the relationship between watching CNN, and fear of violent crime. Using the “classical” experimental design, describe how you will conduct this research, assuming you have access to 200 FIU students who volunteered for the study. Be sure to discuss possible threats to validity, and how the design of your experiment will help to rule out such threats.