Name: ___________________________________
STT1600 LAB 3: Descriptive Statistics & the Empirical Rule
Purpose: In this lab, we will use Excel to generate summary statistics of a data set consisting of a sample
of birth weights of newborn babies and compute some descriptive statistics for this data set. We will
also examine the Empirical Rule using this data set. Fill in the answers to the questions on both pages
of this lab and turn this in.
For your lab report, fill in the blanks and answer questions on this sheet and turn it in.
1. Open the excel data set “babywts” which contains the birth weights of n=1200 newborn babies (in
pounds).
2. Click your mouse at the top of column A and then click the “data” tab in the ribbon. Click on “Sort”
and choose “Smallest to Largest”. Notice that the birth weights are now sorted from smallest to
largest.
3. Compute the mean birth weight by clicking in an empty cell and typing “=average(A1:A1200)”.
4. Compute the standard deviation of birth weights by clicking in an empty cell and typing:
“=STDEV(A1:A1200)”.
a. What is the mean weight (round to two digits)? _________________
b. What is the standard deviation of the weights (round to 3 digits)? __________________
c. Are the mean and standard deviations you computed STATISTICS or PARAMETERS? (circle
one)
STATISTICS
PARAMETERS
5. Empirical Rule: In this step, we will compute an interval of weights: the mean ± 2 standard
deviations. To get the lower end of this interval, click in an empty cell in your excel spreadsheet and
type “=AVERAGE(A1:A1200) - 2*STDEV(A1:A1200)”. Click in an empty cell next to this cell and type
the same thing except change the “-“ to “+”. You have just computed the mean ± two standard
deviations.
a. Write down the interval of weights you just computed ____________________________.
b. According to the empirical rule, approximately what percentage of birth weights should fall
in this range (if the distribution is bell-shaped)? ________________________
c. Look through the 1200 sorted birth weights in Column A and compute the actual percentage
of observations that fall in this range and record the value? (Note: it may be easier to count
how many of the 1200 weights do NOT lie in this interval)________________________
d. Does the actual percentage you computed in part (c) match closely with the Empirical Rule
value? (circle one) YES
NO
e. Repeat steps (a)-(d) for the interval of the mean plus/minus 3 standard deviations and fill in
the blanks:
The interval: __________________
Percentage expected in this interval by Empirical Rule: _________
Actual Percentage of birth weights in this interval: __________________
6. In this step of the lab, we will use excel to compute median and also various percentiles of the birth
weight data set using the “PERCENTILE” function in excel.
a. Compute the median birth weight by clicking in an empty cell and typing
“=MEDIAN(A1:A1200)”.
What is the median birth weight? _________________
b. Compute the 50th percentile of the birth weights by clicking in an empty cell and typing
“=PERCENTILE(A1:A1200, 0.50)”. Note that the 0.50 tell excel to find the 50th percentile.
What is the 50th percentile? ___________________
What is the relation between the 50th percentile and the median from part (a)?
_________________________________________________________
c. Repeat part (b) for the 20th percentile of the birth weights.
What is the 20th percentile? _________________
d. Repeat part (b) for the 95th percentile. What is the 95th percentile? ___________________
e. In what row can you find the 95th percentile of the baby weights? (Hint: compute 95% of
1200) ___________________________
f. Suppose a parent has a baby whose weight is at the 95th percentile (the value you found in
part (d)), write a sentence explaining to the parent how their baby’s weight compares to the
weights of other babies.
___________________________________________________________________________