Most people are right handed due to the propensity of the left hemisphere of the brain to control sequential movement. Similarly, the fact that some tasks are performed better with the left hand is likely due to the superiority of the right hemisphere of the brain in processing the necessary information. Does such cerebral specialization in spatial processing occur in adults with Down syndrome?
A 2 X 2 factorial experiment was conducted to answer this question (American Journal on Mental Retardation, May 1995). A sample of adults with Down syndrome was compared with a control group of individuals of a similar age, but not affected by the condition. Thus, one factor was Group at two levels (Down syndrome and control), and the second factor was Handedness (left or right) of the subject. All the subjects performed a task that typically yields a left-hand advantage. The response variable was €œlaterality index,€ measured on a -100 - to 100-point scale. (A large positive index indicates a right-hand advantage, a large negative index a left-hand advantage.)
a. Identify the treatments in this experiment.
b. Construct a graph that would support a finding of no interaction between the two factors.
c. Construct a graph that would support a finding of interaction between the two factors.
d. The F-test for factor interaction yielded an observed significance level of p6 .05. Interpret this result.
e. Multiple comparisons of all pairs of treatment means yielded the rankings shown in the table below. Interpret the results.
f. The experiment wise error rate for part e was .05. Interpret this value.

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Mean laterality index: Group/Handed 4 .5 Down Control Control Down/ Left Righ f Right