Physical Variables

Social Variables

Personality Variables

Context Variables

What are physical variables?

Physical Variables

Physical variables are aspects of the testing situation that need to be controlled:

day of the week

experimental room

lighting

Explain elimination.

Physical Variables

Elimination completely removes extraneous physical variables from the experimental situation (e.g., soundproofing a room).

Removal of extraneous physical variables prevents them from operating differently across different treatment conditions.

How does constancy of conditions work?

Physical Variables

Constancy of conditions controls extraneous physical variables by keeping all aspects of the treatment conditions identical, except for the independent variable.

For example, test all subjects in the same roomor at the same time of day.

How does balancing work?

Physical Variables

Balancing controls extraneous physical variables by equally distributing their effects across treatment conditions.

For example, running half of the subjects in each condition in the morning and half in the evening.

In which order should you use these techniques?

Physical Variables

1. Eliminate extraneous variables whenever possible.

2. Keep conditions constant where elimination is not possible.

3. Balance the effects of extraneous variableswhen constancy of conditions is not possible.

What are social variables?

Social Variables

Social variables are aspects of the relationships between subjects and experimenters that can influence experimental results.

These include demand characteristics and experimenter bias.

Explain demand characteristics.

Social Variables

Demand characteristics are cues within the experimental situation that demand or elicit specific participant responses.

Example: students cue professors to wrap up their lectures by packing their binders, books, and water bottles, and by looking at the door.

How can demand characteristics threaten internal validity?

Social Variables

Demand characteristics can confound an experiment if they vary across experimental conditions.

Subjects may act to confirm what they think is the experimental hypothesis.

What is a single-blind experiment?

Social Variables

In a single-blind experiment, subjects are not told their treatment condition.

For example, in a single-blind drug study, the experimental and control groups might receive capsules that look and taste identical.

How do single-blind experiments control demand characteristics?

Social Variables

When subjects are not told their treatment condition, this eliminates cues that might alter their behavior.

What is the placebo effect?

Social Variables

The placebo effect is when a subject receives an inert treatment and improves because of positive expectancies.

How do cover stories control demand characteristics?

Social Variables

A cover story is a false plausible explanation of the experimental procedures to disguise the research hypothesis from the subjects.

They should be used sparingly, since they are a form of deception.

What is experimenter bias?

Social Variables

Experimenter bias is any behavior by the experimenter that can confound the experiment.

For example, an experimenter might provide more attention to subjects in one condition than another.

What is the Rosenthal effect?

Social Variables

The Rosenthal effect is the phenomenon in which experimenters treat subjects differently based on their expectations and their resulting actions influence subject performance.

This is also called the Pygmalion effect and self-fulfilling prophecy.

What is the Rosenthal effect?

Social Variables

For example, teachers might give more attention and feedback to high aptitude students than to low aptitude students.

The Rosenthal effect can confound an experiment, producing results consistent with the experimenter’s expectations.

Why is a double-blind design superior to a single-blind design in controlling experimenter bias?

Social Variables

Single-blind experiments only control demand characteristics, since subjects are blinded to their condition.

Double-blind experiments control both demand characteristics and experimenter bias, since both the experimenter and subjects are blinded.

How might an experimenter's personality affect experimental results?

Personality Variables

Research on experimenter personality shows that when experimenters are warm and friendly, subjects learn more, talk more, earn better test scores, and are eager to please.

Hostile or authoritarian experimenters obtain inferior subject performance.

How can experimenters control personality variables?

Personality Variables

Employ multiple experimenters to run an equal number of subjects in each of the experimental conditions (balancing).

Treat “experimenter” as an independent variable in statistical analysis. If an interaction is found, then the experiment was confounded.

How can experimenters control personality variables?

Personality Variables

When there is a single experimenter, minimize face-to-face contact and closely follow the script.

Videotape sessions to confirm consistent performance.

How do volunteers differ from nonvolunteers?

Personality Variables

Volunteers are more sociable, score higher in social desirability, hold more liberal social and political attitudes, are less authoritarian, and score higher on intelligence tests than nonvolunteers.

What are context variables?

Context Variables

Context variables are extraneous variables produced by experimental procedures created by the research setting environment, like assignment of participants to conditions.

When might subjects select the experiment?

Context Variables

When we allow subjects to sign up for experiments whose titles differ in their appeal:

“The Memory Test Experiment” “The Heavy Metal Music Experiment”

However, this could result in a biased sample threatening external validity.

Why shouldn’t you run your friends in your experiment?

Context Variables

Selecting your friends might bias your sample, threatening external validity.

Both you and your friends might act differently in your experiment than strangers.

Summarize the folklore about subjects.

Context Variables

Subjects who sign up late in the semester may be less motivated and may behave differently than those who sign up earlier in the semester.

Rosenthal speculated that the differences seen at the start and end of an experiment may be just as likely due to changes in the experimenter.