Experimental Research

Experiments

Begin with a Hypothesis

Modify Something in a Situation

Compare Outcomes

Cases or People are Termed “Subjects”

Random Assignment

Probability of Equal Selection

Allows Accurate Prediction

An Alternative to Random Assignment is Matching

Parts of the Classic Experiment Treatment or Independent Variable

Dependent Variable

Pretest

Posttest

Experimental Group

Control Group

Random Assignment

Variations on Experimental Design

Pre-experimental Design

One-shot Case Study

One-group Pretest-Posttest Design

Static Group Comparison

Quasi-Experimental and Special Designs

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?

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

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

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

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

Experimenter Effects

Any preconceived idea of the researcher about how the experiment should turn out

Compensatory effects

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

Principles of Experimental Design

Control the effects of lurking variables on the response, most simply by comparing two or more treatments

Randomize

Replicate

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.

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

Statistical Significance

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

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?