Age at onset of dementia was determined for a sample of adults between the ages of 60 and 75. For 15 subjects, the results were ΣX = 1008, and Σ(X-M)2 = 140.4. Use this information to answer the following:
a. What is the mean and SD for this data?
b. Based on the data you have and the Normal Curve Tables, what percentage of people might start to show signs of dementia at or before age 62?
c. If we consider the normal range of onset in this population to be +/-1 z-score from the mean, what two ages correspond to this?
d. A neuropsychologist is interested only in studying the most deviant portion of this population, that is, those individuals who fall within the top 10% and the bottom 10% of the distribution. She must determine the ages that mark these boundaries. What are these ages?