Question: Cerebral specialization in Down syndrome adults. 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 to process the necessary information. Does such cerebral specialization in spatial processing occur in adults with Down syndrome? A 2 × 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 to a control group of normal individuals of a similar age. Thus, one factor was Group at two levels (Down syndrome and control) and the second factor was the Handedness (left or right) of the subject. All the subjects performed a task that typically yields a left-hand advantage. The response variable was ‘‘laterality index,'' measured on a range from -100 to 100 points. (A large positive index indicates a right-hand advantage, while a large negative index indicates 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 p

(e) Multiple comparisons of all pairs of treatment means yielded the rankings shown below. Interpret the results.

(f) The experimentwise error rate for the analysis, part e, was α = .05. Interpret this value.

Mean laterlity index:        -30                    -4                     -.5                   +.5

Group/Handed:            Down/Left        Control/Right        Control/Left          Down/Right