standard deviation problem

[thatch]

I am having a problem with a word problem.(below) I am able to successfully use the formula on the example problems, but I don't know what I am doing wrong on this one. Please help

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 Σ2 x =67878. Use this information to answer the following:.
a. What is the mean and s for this data? (2.5 pts.)
b. What is the percentile rank for an individual who shows dementia at age 62? (2.5 pts)
c. If we consider +/-1z to be the normal range of onset, what ages correspond to this? (2.5 pts.)
d. What ages identify the most deviant 20%? (2.5 pts.)
help me [/img]

 

[tkhunny]

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 Σ2 x =67878. Use this information to answer the following:

Do you mean Σx²? Does the problem statement suggest anything about the population distribution? Personally, I note that the population itself is severely decreasing from 60-75, so it is HIGHLY suspect to assume a normal distribution. Such a tiny sample! Yikes.

a. What is the mean and s for this data? (2.5 pts.)

Mean is trivial: 1008/15 = 67.2. Never ask that again! :wink:

You should have a formula for variance, as well. (Σx² - (Σx)²/n)/(n-1) ring any bells? (67878 - 1008²/15)/14 = 10.02857. Then s = 3.16679

Let's see if that nudges you far enough to get the rest. Show your formulations and calculations.