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CHAPTER 6: Independent Groups Designs 187• Control is the essential ingredient of experiments; experimental control isgained through manipulation, holding conditions constant, and balancing.• An experiment has internal validity when it fulfills the three conditionsrequired for causal inference: covariation, time-order relationship, andelimination of plausible alternative causes.• When confounding occurs, a plausible alternative explanation for theobserved covariation exists, and therefore, the experiment lacks internalvalidity. Plausible alternative explanations are ruled out by holdingconditions constant and balancing.Key ConceptA true experiment involves the manipulation of one or more factors and themeasurement (observation) of the effects of this manipulation on behavior. Asyou saw in Chapter 2, the factors the researcher controls or manipulates arecalled the independent variables. An independent variable must have at least twolevels (also called conditions). One level may be considered the “treatment”condition and a second level the control (or comparison) condition. Often, morethan two levels are used for additional comparisons between groups. The measuresused to observe the effect (if any) of the independent variables are calleddependent variables. One way to remember the distinction between these twotypes of variables is to understand that the outcome (dependent variable)depends on the independent variable.Experiments are effective for testing hypotheses because they allow us to exercisea relatively high degree of control in a situation. Researchers use controlin experiments to be able to state with confidence that the independent variablecaused the observed changes in the dependent variable. The three conditionsneeded to make a causal inference are covariation, time-order relationship, andelimination of plausible alternative causes (see Chapter 2).Covariation is met when we observe a relationship between the independentand dependent variables of an experiment. A time-order relationship isestablished when researchers manipulate an independent variable and thenobserve a subsequent difference in behavior (i.e., the difference in behavior iscontingent on the manipulation). Finally, elimination of plausible alternativecauses is accomplished through the use of control procedures, most importantly,through holding conditions constant and balancing. When the three conditions for acausal inference are met, the experiment is said to have internal validity, and wecan say the independent variable caused the difference in behavior as measuredby the dependent variable.RANDOM GROUPS DESIGN• In an independent groups design, each group of subjects participates inonly one condition of the independent variable.• Random assignment to conditions is used to form comparable groups bybalancing or averaging subject characteristics (individual differences) acrossthe conditions of the independent variable manipulation.• When random assignment is used to form independent groups for the levelsof the independent variable, the experiment is called a random groups design.