31 Measures Of Central Tendency Page

3.1 Measures of Central Tendency (Page 1 of 16)

3.1 Measures of Central Tendency
Mean, Median and Mode
a.

mean, x =

Example 1

b.

! x = sum of the entries n number of entries

Find the mean of

26, 18, 12, 31, 42

The median is the middle value of an ordered set of data. If there is an even number of data values, then the median is the mean of the two middle values.

Example 2

Find the median of

25, 30, 37, 21, 38

Example 3

Find the median of

3, 7, 9, 4, 8, 2, 6, 5

c.

The mode is the most frequently occurring data value.

Example 4
a.
Find the mode of
8, 6, 5, 6, 4, 3, 5, 8, 7, 7, 5, 6, 2, 0, 5, 7, 6, 6, 7, 8

b.

When is the mode commonly used as an “average?”

3.1 Measures of Central Tendency (Page 2 of 16)

Guided Exercise
The unit load of
40 randomly selected students from a college is:

1
17
12
15
18

12
17
12
14

14
16
15
16

17
15
16
17

13
14
12
15

16
12
18
19

18
12
20
12

20
13
19
13

13
17
12
13

12
14
15
15

a. Organize the data from smallest to largest. 11

12

13

14

15

16

17

b. Find the median, mean and mode.

c. If the state is going to fund the college according to the
“average” credit load, which
“average” do you think the college will report? Why?

Sort and 1-Variable Statistics
i. Enter the data into a list, say L1 : STAT / 1: Edit ii. Compute 1-variable statistics on the data list
STAT / CALC / 1: 1-Var Stats L1 iii. Find the mode by sorting the list in ascending order
STAT / 2: SortA(L1

18

19

20

3.1 Measures of Central Tendency (Page 3 of 16)

Guided Exercise 4
Rowdy Rho Fraternity is in danger of losing campus approval if they do not raise the mean GPA of the entire group to at least 2.2.
This terms GPA’s are
1.8 2.0 2.0 2.0 2.0 1.9 1.8
2.3 2.5 2.3 1.9 2.2 2.0 2.3
a.

What is the mean GPA? Is RR Fraternity going to lose its campus approval?

b. Rod claims he made a 2.0 because he was sick for 6 weeks. He believes he would have made a 3.9 if he had been well. Would
Rod have saved the fraternity with a 3.9 GPA?

c.

Suppose the college had required the fraternities to have a median GPA of 2.2. Would Rod have saved the fraternity if he had earned a 3.9 GPA?

d. What can you say about the effect of and exceptionally high
(or low) value on the mean and the median?

3.1 Measures of Central Tendency (Page 4 of 16)

Resistant Measures and Trimmed Mean
1. A resistant measure is one that is less influenced by extreme data values. The mean is less resistant than the median (i.e. the mean is more influenced by extreme data values).
2. A measure of central tendency that is more resistant than the mean is the trimmed mean. A 5% trimmed mean is computed by trimming 5% of the lowest data values and 5% of the highest data values before computing the mean. Thus, extreme values do not have as much influence.
Example 5
The class sizes of 20 randomly chosen
Introductory Algebra classes in California are shown. a. Compute the mean, median and mode.
Mean:
Median:
Mode:
b. Compute a 5% trimmed mean.

14
23
35
42

20
25
35
50

20
30
35
50

20
30
40
80

20
30
40
80

3.2 Measures of Variation (Page 5 of 16)

Weighted Average (Mean)
A weighted average, or weighted mean, is used to average a list of numbers when the numbers are assigned varying importance, or weight. ! xw
Weighted Average =
!w
where w is the weight (or frequency) of data value x.
Example 13
Suppose Jim earned the following grades in Biology.
Compute Jim’s average for
Biology.

Assignment Grade Weight
Exam 1
79
20%
Exam 2
65
20%
Final Exam
84
30%
Lab
81
15%
Term Paper
85
15%

Weighted Average on the TI-83/84
Enter the data and the corresponding weights into two lists and run
1-Variable Statistics:
STAT / CALC / 1: 1-Var Stats Ldata, Lfrequency or
STAT / CALC / 1: 1-Var Stats Ldata, Lweight

3.2 Measures of Variation (Page 6 of 16)

3.2 Measures of Variance (or Dispersion)
Example 6
Compute the mean and the median for the following two sets of data Data Set
Set A: 28 30 32 34 36
Set B: 10 20 32 44 54

Mean

Median

Measures of central tendency try to get a measure of all the data into a single number, the central value (mean, median, mode), without regard for how spread-out (consistent) the data is. To more completely describe sets of data we need a numerical measure of how spread-out data is - these measures are called measures of dispersion (or variance).
Measures of Dispersion or Variance gauge how spread-out (or consistent) the data is.
Sample Measures of Variance (Dispersion)
1. Range = high value - low value
2. Sample Standard Deviation = s =

" (x ! x )

2

n !1

where n = sample size, x = sample mean
3. Sample Variance = s 2
Example 7
Compute the range, sample standard and sample variance deviation for the data in example 6.

3.2 Measures of Variation (Page 7 of 16)

Example 8
Two hybrids of roses were developed for extra large blossoms.
The diameter of the blossoms (in inches) are given as follows:
Hybrid A:
Hybrid B:

2
5

3
5

4
5

5
6

6
6

8
6

10
7

10
8

Find the range, sample standard deviation, and sample variance of each hybrid of rose (remember the units).

Population Notation, Measures and Formulas
a.
Population size is denoted N
!x
b.
Population mean, µ (read “mew”): µ =
N
c.

Population standard deviation = ! =

d.

Population variance = ! 2

# (x " µ )

2

N

Example 9
Eight endangered geese in a zoo measured the following weights
(in pounds):
12.7 15.2 19.4 8.2 16.4 10.8 14.6 23.5
Find the mean, standard deviation and variance of the population
(remember the units).

3.2 Measures of Variation (Page 8 of 16)

Fact Standard deviations can be compared only when the units are the same and/or the populations are similar.
Coefficient of Variation
The Coefficient of Variation is a unit-less measure of variance and expresses the standard deviation as a percent of the mean. s Sample Coefficient of Variation:
CV = ! 100% x CV =

Population Coefficient of Variation:

!
" 100% x Example 10
Compute the coefficient of variations for the data of the hybrid roses in example 8.
Rose

x

s

Hybrid A

6

3.071

Hybrid B

6

1.069

CV

Example 11
In the stock market the “volatility” or “activity level” of a stock is often measured by the CV. The following is data for 7/89:
DJIA
IBM
Disney
mean closing value
2254.03 113.58
101.3
standard deviation
61.39
1.22
4.51
Coefficient of variation
2.40%
1.07%
4.45%
Which one of the three is most volatile? Why?

3.2 Measures of Variation (Page 9 of 16)

Chebyshevʼs Theorem
For any set of data and for any constant k > 1, the percent of the data values that must lie within k standard deviations on either side of the mean is at least
"
1%
1!
$# k 2 '& ( 100%
That is,
1. Start at the mean.
2. The percent of the data within ±k standard deviations of the mean is (1! 1/ k 2 ) " 100% .
Example 12
(a) Compute the minimum proportion of data falling within k = 2 standard deviations of the mean.

(b) Summarize part (a) in words.

(c) Repeat parts (a)-(b) for k = 3, 4, 5, and 10.

Chebyshevʼs Theorem gives the Minimum Percentage of Data that lie within k Standard Deviations of the Mean k "
1%
1!
$# k 2 '& ( 100%

2

3

4

5

10

75%

88.9%

93.8%

96%

99%

3.2 Measures of Variation (Page 10 of 16)

Guided Exercise #8
A newspaper periodically runs an ad in its own advertising section offering a free month’s subscription. In this way, management can get an idea of how many people read the classifieds. Over a period of two years the mean number of responses was x = 525 with a sample standard deviation of s = 30.
a. What is the smallest percentage of data we expect to fall within 2 standard deviations of the mean (i.e. between 465 and
585).

b. Determine the interval from A to B about the mean in which
88.9% of the data fall.

c.

What is the smallest percent of respondents to the ad that falls within 2.5 standard deviation of the mean?

d. What is the interval from A to B from part c. Explain its meaning in this application.

3.3 Mean and Standard Deviation of Grouped Data (Page 11 of 16)

3.2 Mean and Standard Deviation of Grouped Data
Mean & Standard Deviation of Grouped Data
1. Make a frequency distribution table [from the histogram if necessary]. a. Compute the class midpoint for each class; this is the “best guess” of each data value in the class. Place the class midpoints in list L1.
b. Place the corresponding frequency of each class in list L2.
2. Compute 1-variable statistics on L1 and L2
STAT / CALC / 1: 1-Var Stats Ldata, Lfrequency or
STAT / CALC / 1: 1-Var Stats Ldata, Lweight
Example 15
The BLM did a study of the water table near Cluster, WY, in the month of June. A random samples of 20 wells showed the distance to the ground to water level in feet is: distance (feet) 12-14 15-17 18-20 21-23 24-26
1
3
8
2
6
number of wells
Estimate the mean and standard deviation of the well depth data.

3.3 Mean and Standard Deviation of Grouped Data (Page 12 of 16)

Exercise 7, page 132
Estimate the mean and standard deviation and coefficient of variation from the histogram in figure 3-4 on page 133.
Class Best Guess Frequency x or Weight

3.4 Percentiles and Box-and-Whisker Plots (Page 13 of 16)

3.3 Percentiles and Box-and-Whisker Plots
What does a median score of 55 mean?
Percentiles
A percentile ranking gives a rank relative to all other data values.
For whole numbers P, the Pth percentile of a distribution is a value such that P% of the data fall at or below that value. Thus, the median value is the same as the 50th percentile value.
Guided Exercise 10
Suppose you challenge freshman composition by taking an exam.
a. If your score was in the 89th percentile, what percentage of scores was at or below your score?

b. If the scores ranged from 0 to 100 and your raw score was 95, does that mean that your score is at the 95th percentile?

Quartiles, Interquartile Range, 5-Number Summary
Quartiles are percentiles that divide the data into fourths. The first quartile Q1 is the 25th percentile, the second quartile Q2 is the median, and the third quartile Q3 is the 75th percentile. The interquartile range is Q3 – Q1; it is a measure of how spread-out the middle 50% of the data is. The 5-number summary is the lowest value, Q1, median, Q3, and the highest value.
Inner Quartile Range = Q3 – Q1
25%
Lowest

25%
Q1

25%

Q2
Median
50th percentile

25%
Q3

Highest

3.4 Percentiles and Box-and-Whisker Plots (Page 14 of 16)

Computing Quartiles
1. Rank the data from smallest to largest. STAT/ 2:SortA(
2. Find the median, Q2.
3. The first quartile Q1 is the median of the lower half of the data.
4. The third quartile Q3 is the median of the upper half of the data. Guided Exercise #11
The calorie-count for 22 ice-cream bars are:
342 377 319 353 295 234 294 286 377 182 310
439 111 201 182 197 209 147 190 151 131 151
a.

Sort the list and find the median.
111 131 147 151 151 182 182 190 197 201 209
234 286 294 295 310 319 342 353 377 377 439

b. Find Q1 and explain its meaning in this application.

c.

Find Q3 and explain its meaning in this application.

d. Find the interquartile range and explain its meaning in this application. e.

Redo parts a-d on your calculators
STAT / 2: SortA(
STAT / CALC / 1: 1-Var Stats

3.4 Percentiles and Box-and-Whisker Plots (Page 15 of 16)

Example 17
The following are the average cost ($) for certain camera models surveyed. Rank the data and compute Q1, the median, and Q3, and the interquartile range.
280 300 310 360 370 400 410 430 470 560 600
640 650 800 800 830

Box-and-Whisker Plots
1. Enter the data into a list and run 1Box-and Whisker Plot variable statistics to find the 5-number
Highest
summary: lowest value, Q1, median, Q3,
Q3
highest value
Median
2. Draw an axis (horizontal or vertical) and scale it to include the lowest and highest
Q1
values.
3. To the right of (or above) the axis draw a
Lowest
box around the interquartile range (from
Q1 to Q3) and a line inside the box at the median. 4. Draw whiskers from Q1 to the lowest value, and from Q3 to the highest value.
Example 18
The following are the average cost ($) for certain camera models surveyed. Draw a box-and-whisker plot for the data. Scale the axis appropriately. 280 300 310 360 370 400 410 430 470 560 600
640 650 800 800 830

3.4 Percentiles and Box-and-Whisker Plots (Page 16 of 16)

Example 19
The annual salaries (in $1000) of 16 liberal arts majors at Renata College follow. Compute the five number summary and construct a box-andwhisker plot. (Title / Scale)

13.7 17.9 18.3 19.2 20.5
22.0 23.6 23.8 24.1 24.6
26.1 26.8 27.0 28.5 29.5
33.5

Exercise 12
Compare the three box-and-whisker graphs.
a. Does the percentage change appear to be skewed right or left? Explain.
b. Which stock has the most spread?
c.

Which stock is the most volatile?

d. Which stock had more weekly declines than increases?
e.

Which stock had more weekly increases than declines?

f.

If you were a conservative investor, which stock would you buy?