Lebo, a research psychologist, suspects that the addition of

certain food supplements to the diet of elderly people will reduce the decline

in cognitive functioning that comes about because of aging. She decides to test

this using a neuropsychological test that measure perceptual speed (the NPS

test). It is known that the distribution of scores on this test is

approximately normal and that a mean of µ=50 and a standard deviation of σ=15

was found in the population of persons older than 65.

To investigate her hypothesis, Lebo obtains a random sample

of n=100 persons older than 65. Each member of this sample is given a daily

dose of supplements over a period of six months. At the end of this time, each

person is tested on the NPS test and a mean of x¯ = 58.4 and standard deviation

of s = 16.8 is found. She plans to test the hypothesis at α = 0.05.

The

effective difference between the performance of the elderly people tested by

Lebo and elderly people in general is - - - - -.