Components of a causal relationship

• Does a change in X cause a change in Y?

• There are 3 components:

• 1) Co‑variation of events

• 2) Time‑order relationship

• 3) Elimination of alternative causes.

Independent Variable

• The presumed "cause" of a behavioral effect or change

• Manipulated (varied) by experimenter

• IV has several levels selected by experimenter

• Occurs, or can be "set up" before DV is measured

• "Independent" of what the subject does.

Dependent Variable

• Some measure of behavior that is a measure of the effect of the IV(cause)

• What is recorded by the experimenter

• The behavior occurs after IV is varied, and DV measures the behavior

• "Depends" on manipulation of the IV

• DV does not have levels.

Confounding Variable

• Any variable that is a potential cause for the experimental effect, other than the IV

• Any variable whose values change systematically across levels of the IV.

Control variable

• Variable whose values remain the same across levels of the IV (eg, room temp, light levels, time-of-day, etc).

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.

Subject variable

• A personal characteristic (eg, height, weight, gender, ethnicity, socio-economic status, etc).

Control group

• The group that receives “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 performances of the experimental groups.

Experimental groups

• The groups that receive non-zero values of the IV

• Eg, the drug groups in a drug study

• 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

• Tends to be more general, more vague.

Operational Definition • Specifies the operations or procedures necessary

to measure the variable

• Very precise

• Not general or vague at all

• Tells how the variable was measured

• There may be many OD’s for a single CD.

ODs and CDs - Example 1

• Conceptual - Amount of alcohol

• Operational - # of beers in 1 hour (0,1,2,3)

• Operational - grams of alc./kg body weight

• Operational - BAC (mg alc./deciliter blood).

ODs and CDs - Example 2

• Conceptual - Helping behavior

• Operational - # of people who help a “victim”

• Operational - duration of helping behavior

• Operational - # seconds before helping occurs (latency).

EXR-intermediate scenarios

Complex designs

• More than one IV

• Eg, Left/Right and 1, 5, or 10 spaces fr. 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• “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”

EXR-horn honks and abstracts

Designing experiments• Two general types of designs• Between-subjects (between groups or independent

groups) = each group gets one level of the IV • Within-subjects (within-group or 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 - more efficient in terms of # of subjects• Within - zero variability (ind diff) between levels.

Order effects • Order effects (practice effects) = experiencing

one level affects behavior in another level

• Eg, does content (biology text vs. novel) affect proofreading speed? Order is Biology-Novel

• Eg, practice, boredom, fatigue

• Order effects cannot occur in between-subjects and are controlled in within-subjects by randomization or counterbalancing.

Differential carryover effects

• (carryover effects, differential/asymmetrical transfer effects)

• 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 is due to which level precedes which.

FIG: Order effects in proofreading

Group 1Biology

1

(no practice) (practice)Novel

2

Group 2Biology

1

(no practice) (practice)Novel

2

FIG: 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.

Group 1

Neutralinstructions

1

(no practice) (practice)

2

Group 2

1

(no practice) (practice)

2

Specialinstructions

Neutralinstructions

Specialinstructions