Experimental PsychologyPSY 433

Chapter 3

Experiments -- Designs

Two Kinds of Influence on DV

Systematic – affects the DV in the same way each time. Can introduce bias into results. When it occurs due to the manipulation of the

IV we call it an “effect.” Random – varies and thus affects the DV

differently from observation to observation. Can introduce “noise” into results. Typically makes it more difficult to observe

systematic influences.

Confounding Variable

Any variable, besides the IV, that can influence the DV.

A potential cause for the experimental effect, other than the IV. An alternative explanation for observed

findings in a study. Any variable whose values change

systematically across levels of the IV.

Control Variable

A variable whose values remain the same across levels of the IV (e.g., room temp, light levels, time-of-day, etc).

A goal in experimentation is to control as many variables as possible, to eliminate their potential effect on the DV. Eliminates both confounds and noise.

Except for the IV, each subject should have as closely similar an experience in the experiment as possible.

Random Variable

Variable whose values vary randomly in an unbiased way across levels of the IV.

Random variables are usually created by the process of random assignment to levels in the experiment.

Subject Variable

A personal characteristic Sex, height, weight, age, education, ethnicity,

socio-economic status, etc. Sometimes called a “demographic” variable. Data describing subject characteristics should

be collected in every study. Can be controlled or left to vary

unsystematically (through random assignment to levels).

Control Group

The group that receives no intervention, “zero” or “the absence of” the IV.

Eg, the placebo group in a drug experiment. The group that serves as a baseline to

compare with the behavior of the experimental groups.

Experimental Groups (Levels)

The groups that receive non-zero values of the IV, the groups receiving some intervention. The drug groups in a drug study. Also called “levels” of the IV.

The performances of these groups are compared with the performance of the control group.

Conceptual Definition

Definition of a variable at the conceptual or idea level.

Tends not to be very precise, more general. May be expressed in terms of constructs

(intervening variables).

Operational Definition

Specifies the operations or procedures necessary to measure the variables relevant to the conceptual definition.

Very precise. Not general or vague at all. Tells how the dependent variable was

measured. There may be many operational definitions

(and DVs) for a single conceptual definition.

ODs and CDs - Example 1

Conceptual - Amount of alcohol consumed Operational - # of beers in 1 hour (0,1,2,3) Operational - grams of alcohol/kg body weight Operational – blood alcohol content, BAC (mg

alcohol/deciliter blood).

ODs and CDs - Example 2

Conceptual - Helping behavior Operational - # of people who help a “victim”

in a defined context. Operational - duration of helping behavior. Operational - # seconds before helping

occurs (latency).

Complex Designs

More than one IV Left/Right and 1, 5, or 10 spaces from

center More efficient than single IV experiments Gives more information Allows analysis of main effects and

interactions.

Complex Designs - Terminology

An IV is called a factor number of numbers = how many IVs there are values of numbers = how many levels each IV has

Examples: “2 X 2 design” (two IVs, each with 2 levels) “2 X 3 design” (first IV has 2 levels, second IV has 3

levels) “2 X 8 design” (first IV has 2 levels, second IV has 8

levels) “2 X 2 X 4 design” (first IV has 2 levels, second IV

has 2 levels, third IV has 4 levels).

Main Effects

There is one potential main effect for each IV A 2 X 8 design has two possible main

effects A 2 X 2 X 4 has three possible main

effects A main effect is present if an IV had a

significant effect on the experiment’s outcome (regardless of the effects of the other IVs).

Interactions

Please memorize: “An interaction occurs if the effect of one IV varies depending on the level of the other IV”

Designing Experiments

Two general types of designs Between-subjects (independent groups) =

each group gets one level of the IV  Within-subjects (repeated measures) =

each subject gets all levels of the IV Equivalency of groups at each level is built-

in for within-subjects and achieved by random assignment for between-subjects

Within designs require fewer subjects because there is less variability between levels.

Order Effects

Order effects (practice effects) = experiencing one level affects behavior in another level Effects of practice, boredom, fatigue

Example: Does content (biology text vs. novel) affect proofreading speed? Order is Biology-Novel

Order effects are controlled in within-subjects designs by randomizing or counterbalancing the presentation orders.

Differential Carryover Effects

Carryover effects, differential/asymmetrical transfer effects occur when experiencing one level affects performance on the next.

The effect of the first level on the second level differs depending on which comes first. Effect of B following A ≠ effect of A following

B Confound occurs when one level

consistently precedes the other.

Order Effects in Proofreading

Group 1Biology

1

(no practice) (practice)Novel

2

Group 2Biology

1

(no practice) (practice)Novel

2

Differential Carryover Effects in Problem Solving

Group 1 Neutralinstructions

1

(no practice) (practice)

2

Group 2

1

(no practice) (practice)

2

Specialinstructions

Neutralinstructions

Specialinstructions

Other Considerations

Mixed designs (some between, some within)

Small-n designs Matched groups designs Demand characteristics = cues that tell

subjects how they should behave (eg, drug studies)

Blind and double-blind procedures Internal and external validity Quasi experiments.