Experimental Research

• Control Variables • The process of eliminating the influence of extraneous variables that could be mistaken for the effect of the independent variable.
• Manipulation • The operation performed with the independent variable to study its effect on the dependent variable. Also called the treatment.
• Observation and Measurement • The researcher observes to determine if the hypothesized change has occurred. Some changes can be observed directly, whereas other changes are measured indirectly.
• Law of the Single Independent Variable • Rule stating that to unambiguously determine the effect of an independent variable on a dependent variable, the independent variable must be the only difference between the experimental and control groups before the dependent variable is measured
• Law of the Single Significant Variable • One of the basic assumptions of the experimental method. If all significant variables are made equal, then any difference observed after introducing a new variable can be attributed to the new variable.
• Experimental Comparison:

  1. Experimental group
  2. Control group
  3. Comparison group
• Experimental group • The group in a research study that receives the experimental treatment
• Control group • The group in a study that does not receive the experimental treatment; it is compared with the experimental group to determine the effects of the treatment.
• Comparison group • The group in a study that receives no treatment or a treatment different from the treatment received by the experimental group.
• Experimental Design - A plan for an experiment that specifies what independent variables will be applied, the number of levels of each, how subjects are assigned to groups, and the dependent variable.
• Experimental Design - It establishes the conditions for the comparisons required to test the hypotheses of the experiment.
• Experimental Design - It enables the experimenter, through statistical analysis of the data, to make a meaningful interpretation of the results of the study.
• Campbell and Stanley (1963) • They defined two general categories of validity of research designs: internal validity and external validity
• Internal validity - The validity of the inferences about whether the effect of variable A (the treatment) on variable B (the outcome) reflects a causal relationship
• Statistical conclusion validity - The validity of the inferences about the covariation between treatment and outcome
• Construct validity - The validity of the inferences about psychological constructs involved in the subjects, settings, treatments, and observations used in the experiment
• External validity - The validity of the inference about whether the cause–effect relationship holds up with other subjects, settings, and measurements
• Internal validity - The extent to which observed differences on the dependent variable in an experiment are the result of the independent variable, not of some uncontrolled extraneous variable(s).
• Threats to Internal Validity: History - Specific events or conditions, other than the experimental treatment, may occur between the beginning of the treatment and the posttest measurement and may produce changes in the dependent variable.
• Threats to Internal Validity: Maturation - Naturally occurring changes in subjects due to the passage of time, which may threaten internal validity.
• Threats to Internal Validity: Testing - Taking a test once may affect the subjects’ performance when the test is taken again, regardless of any treatment.
• Instrumentation - A threat to internal validity due to a change in the instruments used during an experimental study.
• Statistical regression - The tendency for subjects who scored extremely high or extremely low on a pretest to score closer to the mean on a posttest.
• Selection bias - A threat to internal validity in quantitative research occurring when there are important differences between the experimental and control groups before the experiment begins.
• Experimental mortality (attrition) - A threat to internal validity occurring when there is a differential loss of participants from the comparison groups in an experiment
• Selection–maturation interaction - An internal validity problem that occurs when subjects in the experimental and control groups mature at different rates, so they are no longer equivalent.
• . Experimenter effect - A threat to internal validity referring to unintentional effects that the researcher may have on the study.
• Subject Effects - Attitudes developed by subjects in response to the research situation
• Hawthorne effect - An effect on the dependent variable resulting from the treatment group’s knowledge that the members are participating in an experiment.
• Random Assignment - The experimenter with an available supply of subjects has the task of dividing them into tw
• Diffusion - A threat to internal validity occurring when participants in an experimental group communicate information about the treatment to subjects in the control group in such a way as to influence the latter’s behavior on the dependent variable.
• Randomized Matching - A procedure in which subjects are first paired (matched) on relevant variables and then a chance procedure is used to assign one member of each pair to the experimental group and the other to the control group.
• Homogeneous Selection - Employing subjects for an experiment who are as alike as possible in regard to relevant characteristics, to reduce the influence of extraneous variables.
• Building Variables into the Design - This method not only controls the extraneous gender variable, but also yields information about its effect on the dependent variable as well as its possible interaction with the other independent variable(s).
• Statistical Control - Analysis of covariance (ANCOVA) is a statistical technique used to control for the effect of an extraneous variable known to be correlated with the dependent variable
• Using Subjects as Their Own Controls - A procedure used to avoid threats to internal validity by assigning all subjects to all experimental treatments.
• Statistical Conclusion Validity - The validity of the inferences about the covariation between two variables in an experimental study.
• Manipulation of the construct - The construct was not properly manipulated in the study; faulty manipulation may lead to incorrect inferences.
• Measure of the construct - The measures used were inappropriate (poor operational definition), so the construct was not accurately measured.
• Reactivity to the experimental situation - Subjects’ perceptions of the experimental situation become part of the treatment construct being tested. Recall the Hawthorne effect from the discussion of internal validity.