The Basics of Experimentation

• In an experiment, variables are measurable elements that can vary or take on different values along dimension.
• Validity means the operational definition accurately manipulates the independent variable or measures the dependent variable.
• An experiment is confounded when the value of an extraneous variable systematically changes along with the independent variable.
• Main Features of Psychological Experiment

  1. We manipulate the antecedent conditions to create at least two different treatment conditions.
  2. We expose subjects to different treatment conditions so that we can measure the effects of those conditions on behavior.
  3. We record the responses or behaviors of subjects under various conditions and them compare them using statistics.
  4. We then assess whether our predictions are confirmed.
• An operational definition specifies the exact meaning of a variable in an experiment by defining it in terms of observable operations, procedures, and measurements.
• An independent variable (IV) is the variable (antecedent condition) an experimenter intentionally manipulates.
• Reliability refers to the consistency of experimental operational definitions and measured operational definitions.
• A dependent variable is the outcome measure the experimenter uses to assess the change in behavior produced by the independent variable.
• Each IV and DV has 2 definitions: (1) conceptual definition (used in everyday language); (2) operational definition (used in carrying out the experiment).
• The experimental hypothesis expresses a potential relationship between 2 kinds of variables – the IV and the DV.
• There are 2 kinds of operational definitions: experimental and measured.
• Levels of an independent variable are the values of the IV created by the experimenter.
• Interrater reliability is the degree to which observers agree in their measurement of the behavior.
• Confounding variable is an outside influence that changes the effect of IV and DV.
• Construct validity is how accurately an operational definition represents a construct.
• An experimental operational definition specifies the exact procedure for creating values of the independent variable.
• Subject mortality threat occurs when subjects drop out of experimental conditions at different rates.
• Face validity is the degree to which the validity of a manipulation or measurement technique is self-evident.
• The dependent variable depends on the value of the independent variable.
• Main Features of Psychological Experiment

  1. When conducted skillfully, an experiment allows us to make cause and-effect statements about behavior.
  2. If behavior changes as the antecedent conditions change, we can say that the differences in antecedent conditions caused the changes in behavior
• History threat occurs when an event outside the experiment threatens internal validity by changing the dependent variable.
• Test-retest reliability means the degree to which a person's scores are consistent across two or more administrations of a measurement procedure.
• The need for operational definitions becomes easily apparent when we zero in on variables that are actually hypothetical constructs or concepts.
• Internal validity is the degree to which changes in the dependent variable across treatment conditions were due to the independent variable.
• A measured operational definition specifies the exact procedure for measuring the dependent variable.
• Statistical regression threat occurs when subjects are assigned to conditions on the basis of extreme scores, the measurement procedure is not completely reliable, and subjects are retested using the same procedure to measure change on the dependent variable
• An experiment requires at least two levels.


  Independent Variable
• 4 types of validity:

  1. Face validity
  2. Content validity
  3. Construct validity
  4. Internal validity
• Randomization (aim is random distribution of confounders between study groups)
• Internal validity establishes a cause-and-effect relationship between the independent and dependent variables.
• Constructs cannot be observed directly, but we can infer their existence from behaviors that we can observe.


  Operational Definition
• Maturation threat is produced when physical or psychological changes in the subject threaten internal validity by changing the DV.
• Restriction (restrict entry to study of individuals with confounding factors – risks bias in itself)
• Content validity means how accurately a measurement procedure samples the content of the dependent variable.
• 2 types of Operational Definition:

  1. Experimental operational definition
  2. Measured operational definition
• Matching (of individuals or groups, aim for equal distribution of confounders)
• Confounding occurs when an extraneous variable systematically changes across the experimental conditions.
• 2 types of Reliability:

  1. Interrater reliability
  2. Test-retest reliability
• Stratification (confounders are distributed evenly within each stratum)
• Because we manipulate the IV and measure its effects on the DV, the latter are sometimes called dependent measures.