UNIT
MEASURES OF SKEWNESS SKEWNESS AND KURTOSIS
Structure
Objectives Introduction Concept of Skewness 6.2.1 Karl Pearson's Measure of Skewness 6.2.2 Bow ley's Measure of Skewness 6.2.3 Kelly 's Measure of Skewness Moments Concept and M easure of Kurtosi Let Us Sum U Key W ord Some Useful Books Answ ers or Hints to Che ck Your Progress Exercises
.0
OBJECTIVES
After going through this Unit, you w ill be able ta distinguish between a sym metrical and a skew ed distribution; distribution; compute various coefficients to measure the extent of skewness in a distribution; distinguish distinguish between platykurhc, mesokurtic and leptokurtic leptokurtic distributions; and comp ute the coefficient coefficient of kurtosis. kurtosis.
6.
INTRODUCTION
In this Unit you w ill learn various various technique techniquess to distingu sh between various shapes of a frequency distribution. This is the final Unit with regard regard to the summ arisation of univariate data. This Unit will make you familiar familiar with the concept of skew ness and kurtosis. The need to study these concepts arises fiom the fact that the measures of central tendency and dispersion fail to describe a distribution completely. completely. It It is p ossible to have fkq uency ue ncy distributions which d iffer widely in their nature and composition and yet may have same central tendency and dispersion. dispersion. Thus, there is need to sup plemen t the measu res of central tendency and dispersion. Consequentl Consequently, y, in in t s nit, we shall discuss discuss two such measures, measures, viz, measures of skewness and kurtosis.
6.
CONCEPT OF SKEWNESS
The skew ness of a distribution is defined defined as the lack of symmetry. a symm etrical etrical distribution, the Mean, Median and Mode are equal to each other and the
Su~n~narisntion Cnivariate Data
part is mirror image of the other (Fig. 6.1). If some observations, of ver high (low) magnitude, are added to such a distribution, its right (left) tail gets elongated.
Symmetrical Distribution
Fig. 6.1
Positively Skewed Distribution
Negatively Skewed Distribution
Fig. 6.2
These observations are also known as extreme observations. The presence of extreme observations on the right hand side of a distribution makes it positively skewed and the three averages, viz., mean, median and mode, will no longer be equal. We shall in fact have Mean Median Mode when a distribution is positively skewed. On the other hand, the presence of extreme observations to the left hand side of a distribution make it negatively skewed and the relationship between mean, median and mode is: Mean Median Mode. In Fig. 6.2 we depict the shapes of positively skewed and negatively skewed distributions. The direction and extent of skewness can be measured in various ways. We shall discuss four measures.@skewness in this Unit.
2 .1 K a l
' sMeasure
Skewness
In Fig.
6.2 you noticed that the mean, median and mode are not equal in a skewed distribution. The Karl Pearson's measure of skewness is based upon the divergence of mean from in a skewed distribution. Since Mean Mode in a symmetrical distribution, (Mean Mode) can be taken as an absolute measure of skewn ess. The absolute measure of skewness for a distribution depends upon the unit of measurement. For example, if the mean 2.45 qet re and mode 2.14 metre, then absolute measure of skewness will be 2.14 metre 0.31 metre. For the same distribution, if we change 2.45 ae tr
centimetre 2 14 centimetre 3 centimetre. In order to avoid such a problem Karl Pearson takes a relative measure of skewn ess. relative m easure, independent of the units of m easurement, is defined as th Ka rl Pearso Coeficient Skewness iven by Mean
Mode
s.d.
The sign of
gives the direction and its m agnitude gives the extent of skew ness.
0, the distribution is positively skewed, and if
it is negatively skewed.
So far we have seen that s strategically dependent upon m ode. mode is not defined for a distribution we cannot find But em pirical relation between m ean, median and mode states that, for a m oderately symm etrical distribution, we h ave Mean Mode (Mean Median) Hence Karl Pearson's coefficient of skewness is defined in t
of median as
Example 6.1: Com pute the Karl Pearson's coefficient of skewness fiom the following data: Table 6.1 Height (in inches)
Number of Persons
58
10
59
18
60
30
61
42
62 63
28
64
16
65
Table for the co mp utation of m ean and s.d. Height
61
No
of
persons 10 18
60
30
61
42
62
35
63
28
64
16
65
Total
187
fi
fu2
30
90
-3 30
72 30
35 11 48
44
32
128
75
611
Measures
Skewness
nd
Kurtosis
Summarisation of Univariate Data
Mean
75 187
61
61.4
o find mode, we note that height is a continuous variable. It is assumed that the height has been measured under the approximation that a measurement on height that is, e.g., greater than 58 but less than 58.5 is taken as 58 inches while a measurement greater than or equal to 58.5 but less than 59 is taken as 59 inches. Thus the given data can be written as
Height (in inches) 57.5
58.5
58.5
59.5
59.5
60.5
60.5
61.5
61.5
62.5
62.5
63.5
63.5
64.5
64.5
65.5
By inspection, the modal class is 60.5
Mode
60.5
12
12+7
No. of persons
61.5.
Thus, we have
61.13
Hence, the Karl Pearson's coeficient of skewness
61.4
61.13
0.153.
Thus the distribution is positively skewed. 6.2.2
Bowley's Measure of Skewness
This measure is based on quartiles. For symmetrical distribution, it is seen that can be taken Q, and Q3are equidistant ftom median. Thus (Q3 Md) (M as an absolute measure of skewness. relative measure of skewness, known as Bowley's coefficient (SQ),s given by
The Bowley's coefficient for the data on heights given in Table 6.1 is computed below. Height (in inches)
No. of persons V)
Cumulative Frequency
57.5
58.5
10
10-
58.5
59.5
18
2
59.5
60.5
60.5
61.5
42
100
61.5
62.5
35
135
62.5
63.5
2
163
63.5
64.5
16
179
64.5
65.5
5
187
Computation of Q, Since
46.75, the first quartile class is 59.5
la,
59.5,
Computation o Since
28, fa,= 30 and
60.5,
1.
(Q,)
93.5, the median class is 60.5
Im
60.5. Thus
58, fm
42 and
61.5. Thus 1.
Computation o Since
140.25, the third quartile class is 62.5 la
62.5,
135,
Hence, Bowley's coefficient 6.2.3
28 and
63.5. Thus
1.
62.688 61.345 60.125 6 2 6 8 8 60.125
0.048
Kelly's Measure of Skewness
Bowley's measure of skewness is based on the middle 50% of the observations because it leaves 25% of the observaticins on each extreme of the distribution. As an improvement over Bowley's measure, Kelly has suggested a measure based on P, nd, o that only 10% of the observations on each extreme are ignored.
Measures
of Skewness
Kurtosis
Summarisation o Univariate Data
Kelly's coefficient of skewness, denoted by
is given by
Note that (median). The value of , for the data given in Table 6.1, can be computed as given below. Compu tation of
Since
oN
100
lo
18.7.
ls7
100
10th percentile lies in the class 58.
59.5. Thus
Computation of
Since
100 lpw
168.3,
100 63.5,
and
163,
63.5
90th percentile lies in the class 63.5
163
64.5. Thus
1.
63.831.
Hence, Kelly's coefficient It may be noted here that although the coefficient S,, So and comparable, however, in the absence of skewness, each of them will
are not equal to
zero.
Check Your
Progress
1) Compute the Karl Pearson's coefficient of skewness from the following data Daily Expenditure (Rs.) No. of families
0-20
20-40
40-60
60-80
80-100
13
25
27
19
16
2)
Measures of Skewness and Kurtosis
The following figures relate to the size of capital of 285 companies Capital
Ks.
acs.)
5
11-15
16 20
21-25
38
48
o. of companies
26 30 31-3
ibtal 28
Compute the Bowley's and Kelly's coefficients of skewness and interpret the results.
3) The following measures were computed for Mean
50, coefficient of Variation
frequency distribution
35% and
Karl Pearson's Coefficient of Skewness
0.25.
Compute Standard Deviation, Mode and Median of the distribution.
6.3
MOMENTS
The rth moment about mean of a distribution, denoted by p,, is given by
0, 1, 2, 3, 4,
where
=1
.........
Thus, rth moment about mean is the mean of the rth power of deviations of observations from their arithmetic mean. In particular, if
0, we have
if
1, we have
if
if
,
e
3, we have
= - ~ =1 h ( x i -x)O
x)=o,
=1
- - ~ f ; ( x i =l
?h=1
(x
=a2,
and so on.
Summarisation of Univariate Data
In addition to the above, we can defin raw moments as moments about any
arbitrary mean. et
denote an arbitrary mean, then
When
ut
moment about
is defined as
0, we get various moments about origin.
Moment Measure of Skewness The m oment m easure of skewness is based on the property that, for a symmetrical distribution, all odd ordered central mom ents are equal to zero. We note that p, 0, for every distribution, therefore, the low est order moment that can provide an absolute measure o f skew ness 'is p3 Further, a coefficient of skewness, independent of the units of m easurement, is given by CI = a3=-
where
p, an
y, are defined as the first beta an first
gamma coefficients respectively. P, is measure of kurtosis as you will com e t
know in the next Section.
Very often, the skewness is measured in terms of
CL F12
here the sign of
skewness is determined by the sign of p,
Example 6.2: Compute the Moment coefficient of skewness (P,) from the following data. Marks Obtained Frequency
0-10
10-20
20-30
30-40
22
6
40-50
50-60
60-70
16
Table for the computations of mean, s.d. and p, Class Intervals
Frequency
Mid-
values
X10 18
10 10
20
20
30
30
40
24
40
50
16
50 60
54
162
48
96
22
22
45
16
16
16
60
55
24
48
96
70
65
24
72
Total
Since Xf
10
the mean o f the distribution is 35.
260
21
The second moment is equal to the variance (oZ) and its positive square root is equal to standard deviation (a ). 260
=-~100=260, 100
and
Since the sign of p3 s positive and p, is small, the distribution is slightly positively skewed. If the mean of a distribution is not convenient figure like 35, as in the above example, the computation of various central moments may become a cumbersom task. Alternatively, we can first compute raw moments and then convert them into central moments by using the equations obtained below. Conversion o Raw Moments. into Centra l Moments We can write
-Eh[(xi
-A)-piIr
(Since
=GZh(Xi-~)=z-~)
Expanding the term within brackets by binomial theorem, we get
From the above, we can write C I p ~ - l p i r ~ 2 p : - 2 p i 2 - C 3p ;
particular, taking
2,
p;-2~lp;2+2~2p;p~2
p 1 +""'.
4, etc., we get (since
3 l c a s u r e s of S k e w n e s s Kurtosis
S u m m a r i s a t i o n of Univariate Data
Example 6.3: Compute the first four moments about mean from the following data. ClassIntervals Frequency V)
0- 10
1 0 -2 0
2 0 -3 0
3
Table for computations of raw moments (Take
Class Intervals
10
25).
X-25
Mid-Value
u
fu2
fu3
fu4
1
16
10
20
15
20
30
25
30
40
Total
3 0 -4 0
0
0
3
- 3
0
0
2 10
-3-
From the above table, we can write
lo
and
I(,
Moments about Mean By definition,
Check Your Progress 1) Calculate the first four moments about mean for the following distribution. Also calculate 9, and comment upon the nature of skewness.
Marks Frequency
20 8
20
40 28
40
60 35
60
80 17
80
100 12
Measures of Skewness and Kurtosis
2) The first three moment of a distribution about the value 3 of a variable are 2,10 and 30respectively. Obtain p2 p3and hence Comment upon the nature of skewness.
6.4 CONCEPT AND MEASURE OF KURTOSIS Kurtosis is another measure of the shape of a distribution. Whereas skewness measures the lack of symmetry of the frequency curve of a distribution, kurtosis is a measure of the relative peakedness of its fi-equency curve. Various frequency divided three The three shapes are termed as Leptokurtic, Mesokurtic and Platykurhc as shown in Fig. 6.3.
Leptokurtic
Mesokurtic Platykurtic
Fig.
Summarisation U n i v a r i n te D a t a
measure of kurtosis is given by
coefficient given by Karl Pearson.
The value of p2 for a mesokurtic curve. When P2 3, the curvt: is more peaked than the mesokurtic curve and is tenned as leptokurtic. Similarly, when p2 the curve is less peaked than the mesokurtic curve and is called as platykurtic curve. Example 6.4: The first four central moments of a distribution are 0,2.5,0.7 an 18.75. Examine the skewness and kurtosis of the distribution.
To examine skewness, we compute
Since p3
and
is small, the distribution is moderately positively skewed.
Kurtosis is given by the coefficient
P4
---i-
18.75
3.0
(q2
Hence the curve is mesokurtic. Check
Your Progress
Compute the first four central moments h two beta coefficients.
2)
he following data. Also find the
Value
5
10
15
20
25
30
Frequency
8
15
20
32
23
17
35
The first four moments of distribution are 1,4, 10 and 46 respectively. Compute the moment coefficients of skewness and kurtosis and comment upon the nature of the distribution.
Measures
.5 LET US SUM UP
Kurtosis
In this Unit you have learned about the measures of skewness and kurtosis. These two concepts are used to get an idea about the shape of the fiequency curve of a distribution. Skewness a measure of the lack of symmetrywhereas kurtosis is a measure of the relative peakedness of the top of a fiequency curve.
KEY WORDS
6.6
Skewness: Departure from symmetry is skewness. Moment of Order r: It is defined as the arithmetic mean of the rt power of deviations of observations.
Coefficient of Kurtosis: It is a measure of the relative peakedness of the top of a frequency curve.
6.7 SOM E USEPUL BOOKS Elhance, D. N. and Allahabad.
lhance, 1988, Fundam entals of Statistics Kitab Mahal,
Nagar, A. L. and R. K. Dass, 1983, Basic Statistics, Oxford University Press, DeIhi
Mansfield, E., 1991, Statistics for Busin ess and Economics: Method s and Applications, W.W. Norton and Co. Yule, U. and M. Kendall, 1991, An Introduction to the T Universal Books, Delhi.
6.8
v of Statistics,
ANSW ERS OR HINTS TO CHECK YOUR PROGRESS EXER CISES
Check Your Progress 1) 0.237 2) 3)
0.12,
0.243
17.5, 54.38, 51.46
Check Your Progress 1) 0,499.64, 2579.57, 5891 11.61, 0.053, skewness is positive. 2)
of Skewness
5, 6, -14,0.907, since p3 s negative the distribution is negatively skewed.
Check Your Progress 1)
0,59.99,
50.18, 835 6.64,0.012 (negatively skewed), 2.32 (platykurtic)
2)
0,3. Thus thedistribution is symmetrical and mesokurtic. Such a distribution is also known as a Normal Distribution
