chap 9

design

• — of an experiment details an experimenter’s plan for testing a hypothesis.

• The — is the experiment’s structure or floor plan—not the experiment’s specific content

1. the number of independent variables in the hypothesis

2. the number of treatment conditions needed to fairly test the hypothesis

3. whether the same subjects are used in each of the treatment conditions

• The experimental design is largely determined by the experimental hypothesis.

• A researcher mainly selects an experimental design on the basis of three factors:

between-subjects design

• a subject participates in only one condition of the experiment.

• different people test each condition, so that each person is only exposed to a single user interface

within-subjects design

• the same person tests all the conditions

• all participants take part in every condition

random sampling

increases an experiment’s external validity.

10-20

You should have at — subjects in each treatment condition to detect a strong
treatment effect

effect size

• is a statistical estimate of the size or magnitude of a treatment effect.

• The larger the —, the stronger the relationship between the independent and
  dependent variables, and the fewer subjects needed to detect a treatment effect

• determines the number of subjects required to detect a treatment effect

power charts

Researchers determine the number of subjects required for an expected effect size using — or programs that incorporate these charts

two group design

• involves the creation of two separate groups of subjects.

• Two versions of the— are the two independent groups design and two matched groups design

two independent groups design

• a design where there is one IV with two levels and subjects are randomly assigned to one of the two conditions.

• This design includes the Experimental Group-Control Group design and Two-Experimental Groups design

random assignment

• involves assigning subjects to conditions so that each subject has an equal chance of participating in each condition.

• We use — to equally distribute subject variables between the treatment groups to prevent them from confounding an experiment

experimental condition

presents a value of the independent variable.

control condition

presents a zero level of the independent variable

experimental group

receives a level of the IV

control group

receives the same procedures, but receives no treatment.

two experimental groups design

• In a — we assign subjects to one of two levels of the independent variable.

• This design is appropriate if there is one independent variable with two levels and if we can assume that randomization will control extraneous variables.

two matched groups design

• In a — we:

• 1. match participants on a subject variable correlated with the DV, and

• 2. randomly assign them to one of two treatment conditions

matching

• is used to create groups that are equivalent on potentially confounding subject
  variables.

• Successful — prevents selection threat from undermining internal validity

multiple groups design

is a between-subjects design with more than two levels of an independent variable

multiple independent groups design

In a —, we randomly assign subjects to one of the treatment conditions

The hypothesis, prior research, pilot study results, and practical limits

can all help determine the number of treatments

pilot study

• is a trial run of the experiment that uses a few subjects.

• can help the experimenter refine the procedure or determine whether the experiment is promising.