# Quasi-Experiments - PowerPoint PPT Presentation

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Quasi-Experiments. Research Objective. Study relationships among variables for existing groups. Show direct cause & effect. Explain outcomes after the fact. Type of Design. Cross-Sectional Longitudinal. True Experiment Quasi-Experiment. Explanatory Case Study Exploratory Case Study.

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Quasi-Experiments

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## Quasi-Experiments

### Research Objective

Study relationships among variables for existing groups

Show direct cause & effect

Explain outcomes after the fact

### Type of Design

Cross-Sectional

Longitudinal

True Experiment

Quasi-Experiment

Explanatory Case Study

Exploratory Case Study

### What Are Quasi-Experiments?

• Quasi-experiments are just like experiments except for one thing.

• They violate the condition of random assignment of test subjects to treatment and control groups.

### Why Do This?

• There are two main reasons why researchers fail to achieve the random assignment assumption:

• It is difficult or impossible to achieve random assignment because the participants are embedded in groups

• Example: students in classes

• There are ethical reasons for assigning some participants to the treatment group

• Example: potentially life-saving new drug

### The Classic Experiment

• Select representative sample

• Randomly assign to control and treatment groups

• Measure the outcome (dependent) variable for both groups

• Do something to the treatment group

• Measure the outcome variable again for both groups

### Quasi-Experiment

Did you select a representative sample of test cases from a single homogeneous population?

YES

No

Not a quasi-experiment

Not a quasi-experiment

Not a quasi-experiment

No

No

Did you ASSIGN test subjects to treatment & control groups NON-randomly?

YES

Did you then do something to treatment groups that you did not do to the control group?

YES

Quasi-Experiment

### Non-Equivalent Group Designs

• One of the most common reasons for going to a quasi-experiment is that your potential test subjects are naturally embedded in some larger groups.

• Students in classes

• Hospital patients in wards

• Participants in an intervention program

• Monkeys in troops

### Subjects Embedded in Groups

• There is a new set of experiential learning materials about environmental education for high school students.

• You believe that this way of learning about the environment is more apt to produce change in behavior than traditional methods.

• Things like recycling, turning off the lights, not letting the water run while you brush your teeth.

• You get three of seven science teachers in a school district to agree to try the new approach.

• The other 4 will use the old approach.

• Your test subjects are students in their classes.

• You will measure behavior (how often they recycle, etc.) before and after the education, new or traditional.

### The Problem

• This looks just like a true pre-post measurement experiment.

• The problem is that your test subjects, the students, come in groups.

• You did not assign Student A to control and Student B to treatment.

### The Threat

• The big problem with this design is that you cannot be really sure that the control and treatment groups are equivalent.

• For example, what if the 3 teachers who agree to try the new approach are just better teachers and their students are therefore more motivated?

## Key Concepts

What’s the same and what’s different between a true and a quasi-experiment.

### The DIFFERENCE

• Quasi-experiments differ from true experiments in only one way: there is no random assignment of individual participants to treatment and control groups.

• This makes it much harder to control non-treatment variance.

• It greatly increases threats to internal validity.

• It can also increase threats to external validity.

• It reduces explanatory power.