Question:
The Wechsler Adult Intelligence Scale – Revised (WAISR) follow a Normal model with mean 100 and a standard deviation 15.
 What percentage of adults taking this test will score below 80?
 Describe the highest 10% of the scores of adults taking this test

Solution Notes:
First off, you should draw and label a Normal Distribution graph with all of
the information you have, so you can visually see if your answers will make sense.
 Use the normalcdf functions with a low of 0 and high of 80,
plug in the mean of 100 and standard deviation equal to 15.
normalcdf(0,80,100,15) = 0.09121
Does 9.1% make sense here?
The mean is 100, so one standard deviation away from the mean
would represent a range from 85115. Two standard deviations would
represent a range from 70130. 80 is between 1 and 2 standard deviations
away from the mean. Plotting this information on a graph should
help you to explain that this is a reasonable answer.
 Draw another Normal Distribution  look at the area that you
would shade in to represent the highest 10%, or the score S that
would reprent the 090% range.
The function that you would use to find the answer is invnorm
with parameters .9 (to find the 90% lowest  or 10% highest score),
the mean=100 and standard deviation=15.
invnorm(.9, 100, 15) = 119.22
Check your answer with normalcd  do 90% of the people taking the test score below 119.22?
normalcf(0,119.22, 100, 15) = .89997  so 119.22 looks like a good answer.
