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274 PART III: Experimental MethodsHalpern and Bower theorized that musical training led musicians to “chunk”musical notation into meaningful musical units, thereby reducing the amountof information they needed to remember in order to reproduce the notationfor a simple melody. Furthermore, if this process were responsible for the differencebetween the memory performance of musicians and nonmusicians,then the difference between musicians and nonmusicians should be greaterfor melodies with good musical structure than for melodies with poor musicalstructure. Halpern and Bower manipulated the independent variable of musicalstructure to test their theory. To do this, they used three different types ofmelodies to test their groups of musicians and nonmusicians. They preparedsets of simple melodies whose notations had similar visual structures but thatwere good, bad, or random in musical structure.The critical test in Halpern and Bower’s experiment was whether theywould obtain an interaction effect between the two independent variables:musical training and type of melodies. Specifically, they expected that thedifference in memory performance between musicians and nonmusicianswould be largest for the melodies exhibiting good structure, next largest forthe melodies exhibiting bad structure, and smallest for the random melodies.The results of Halpern and Bower’s experiment conformed exactly to theirpredictions.The obtained interaction effect allowed Halpern and Bower to rule outmany alternative hypotheses for the difference in memory performancebetween musicians and nonmusicians. Such characteristics as amount andtype of general education, socioeconomic status, family background, andgood memory ability are not likely to explain why there is a systematicrelationship between the structure of the melodies and the size of the differencein memory performance between musicians and nonmusicians. Thesepotential alternative hypotheses cannot explain why there was little differencein the two groups’ memory performance for random melodies. Theinteraction effect makes such simple correlational explanations much lessplausible.There are several steps that the investigator must take in carrying out thegeneral procedure for drawing causal inferences based on the natural groupsdesign.Step 1: Develop a Theory The first step is to develop a theory explaining why adifference should occur in the performance of groups that have been differentiatedon the basis of an individual differences variable. For example, Halpernand Bower theorized that musicians and nonmusicians differed in musical performancebecause of the way that these groups cognitively organize (“chunk”)melodies.Step 2: Identify a Relevant Variable to Manipulate The second step is to select anindependent variable that can be manipulated and that is presumed to influencethe likelihood that this theoretical process will occur. Halpern and Bowersuggested that type of musical structure was a variable associated with ease ofchunking.