Simple Experiments

Causal Claim

• Boldest claim a scientist can make• Verbs such as “associated with” and “related

to” replaced with “causes, influences, affects or makes”

• Must be based on sound experimental research

Experiments: The Basics

• Very specific meaning• Manipulation of at least one variable • Measurement of at least one variable• Control of possible threatening variables

Independent Variables

• Manipulated• At least two levels• Assign participants to at least one of the levels

(condition)• Plotted on the X axis• Examples:– Color, parenting type, amount of caffeine, minutes

of exercise a day

Dependent Variables

• Measured – Depends on IV

• Behavioral, physiological, self-reports, attitudes

• Determines kinds of statistics employed• Plotted on the Y axis

Control Variables

• Researchers need to be sure they are manipulating one variable at a time the IV

• Must hold all other factors/variables constant• Confound variables- may be the cause of the

change in the dependent variable. – Influences “internal validity”– Possible explanation other than the IV

Comparison Groups

• Control groups• Level of the IV that is intended to represent

“no treatment” or a neutral condition• AKA “placebo grou”

Other Important Factors

• Random selection

• Random assignment– Except when “matching “on variable

• Design

Design

• Refers to how the subjects are placed with regards to IV conditions/levels

• Depends on concerns for confound variables such as:– Fatigue– Practice– Variables known to be a potential confound

Within-Subjects Design

• All P’s exposed to all levels of the IV• Same P’s in each level• Advantage:– Less variability between groups

• Disadvantage:– Practice effect, fatigue, order effects; demand

characteristics• Use of counterbalancing present the levels

of the IV to in different orders

Counterbalancing

• Split participants into groups; each group receives one of the condition orders

• Example with IV that has 3 levels– ABC BCA– ACB CAB– BAC CBA

• Partial counterbalancing an option when too many levels

Between-Subjects Design

• Different P’s in each of the IV levels• Advantage:– No practice effects– No fatigue– No order effect

• Disadvantage:– Too much variability– Overcome by random selection and assignment– Large sample size

Matched-Groups Design

• Used when researcher concerned about known confounding variable (gender, IQ etc)

• Used to equate the groups so the effect of the IV is clearer