Skewness Kurtosis - Statistics for Data Analyst

Secrets of Skewness and Measures of Skewness (2021)

Oct 12, 2024Mar 10, 2021 by Muhammad Imdad Ullah

If the curve is symmetrical, a deviation below the mean exactly equals the corresponding deviation above the mean. This is called symmetry. Here, we will discuss Skewness and Measures of Skewness.

Skewness is the degree of asymmetry or departure from the symmetry of a distribution. Positive Skewness means when the tail on the right side of the distribution is longer or fatter. The mean and median will be greater than the mode. Negative Skewness is when the tail of the left side of the distribution is longer or fatter than the tail on the right side.

Measures of Skewness

Karl Pearson Measures of Relative Skewness

In a symmetrical distribution, the mean, median, and mode coincide. In skewed distributions, these values are pulled apart; the mean tends to be on the same side of the mode as the longer tail. Thus, a measure of the asymmetry is supplied by the difference ($mean – mode$). This can be made dimensionless by dividing by a measure of dispersion (such as SD).

The Karl Pearson measure of relative skewness is
$$\text{SK} = \frac{\text{Mean}-\text{mode}}{SD} =\frac{\overline{x}-\text{mode}}{s}$$
The value of skewness may be either positive or negative.

The empirical formula for skewness (called the second coefficient of skewness) is

$$\text{SK} = \frac{3(\text{mean}-\text{median})}{SD}=\frac{3(\tilde{X}-\text{median})}{s}$$

Bowley Measures of Skewness

In a symmetrical distribution, the quartiles are equidistant from the median ($Q_2-Q_1 = Q_3-Q_2$). If the distribution is not symmetrical, the quartiles will not be equidistant from the median (unless the entire asymmetry is located in the extreme quarters of the data). The Bowley suggested measure of skewness is

$$\text{Quartile Coefficient of SK} = \frac{Q_(2-Q_2)-(Q_2-Q_1)}{Q_3-Q_1}=\frac{Q_2-2Q_2+Q_1}{Q_3-Q_1}$$

This measure is always zero when the quartiles are equidistant from the median and is positive when the upper quartile is farther from the median than the lower quartile. This measure of skewness varies between $+1$ and $-1$.

Moment Coefficient of Skewness

In any symmetrical curve, the sum of odd powers of deviations from the mean will be equal to zero. That is, $m_3=m_5=m_7=\cdots=0$. However, it is not true for asymmetrical distributions. For this reason, a measure of skewness is devised based on $m_3$. That is

\begin{align}
\text{Moment of Coefficient of SK}&= a_3=\frac{m_3}{s^3}=\frac{m_3}{\sqrt{m_2^3}}\\
&=b_1=\frac{m_3^2}{m_2^3}
\end{align}

For perfectly symmetrical curves (normal curves), $a_3$ and $b_1$ are zero.

FAQs about SKewness

What is skewness?
If a curve is symmetrical then what is the behavior of deviation below and above the mean?
What is Bowley’s Measure of Skewness?
What is Karl Person’s Measure of Relative Skewness?
What is the moment coefficient of skewness?
What is the positive and negative skewness?

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Skewness in Statistics A Measure of Asymmetry (2017)

Sep 21, 2024Jun 20, 2017 by Muhammad Imdad Ullah

The article is about Skewness in Statistics, a measure of asymmetry. Skewed and skew are widely used terminologies that refer to something that is out of order or distorted on one side. Similarly, when referring to the shape of frequency distributions or probability distributions, the term skewness also refers to the asymmetry of that distribution. A distribution with an asymmetric tail extending out to the right is referred to as “positively skewed” or “skewed to the right”. In contrast, a distribution with an asymmetric tail extending out to the left is “negatively skewed” or “skewed to the left”.

Skewness in Statistics

It ranges from minus infinity ($-\infty$) to positive infinity ($+\infty$). In simple words, skewness (asymmetry) is a measure of symmetry, or in other words, skewness is a lack of symmetry.

Skewness by Karl Pearson

Karl Pearson (1857-1936) first suggested measuring skewness by standardizing the difference between the mean and the mode, such that, $\frac{\mu-mode}{\text{standard deviation}}$. Since population modes are not well estimated from sample modes, therefore Stuart and Ord, 1994 suggested that one can estimate the difference between the mean and the mode as being three times the difference between the mean and the median. Therefore, the estimate of skewness will be $$\frac{3(M-median)}{\text{standard deviation}}$$. Many of the statisticians use this measure but after eliminating the ‘3’, that is, $$\frac{M-Median}{\text{standard deviation}}$$. This statistic ranges from $-1$ to $+1$. According to Hildebrand, 1986, absolute values above 0.2 indicate great skewness.

Fisher’s Skewness

Skewness has also been defined concerning the third moment about the mean, that is $\gamma_1=\frac{\sum(X-\mu)^3}{n\sigma^3}$, which is simply the expected value of the distribution of cubed $Z$ scores, measured in this way is also sometimes referred to as “Fisher’s skewness”. When the deviations from the mean are greater in one direction than in the other direction, this statistic will deviate from zero in the direction of the larger deviations.

From sample data, Fisher’s skewness is most often estimated by: $$g_1=\frac{n\sum z^3}{(n-1)(n-2)}$$. For large sample sizes ($n > 150$), $g_1$ may be distributed approximately normally, with a standard error of approximately $\sqrt{\frac{6}{n}}$. While one could use this sampling distribution to construct confidence intervals for or tests of hypotheses about $\gamma_1$, there is rarely any value in doing so.

Bowleys’ Coefficient of Skewness

Arthur Lyon Bowley (1869-19570, has also proposed a measure of asymmetry based on the median and the two quartiles. In a symmetrical distribution, the two quartiles are equidistant from the median but in an asymmetrical distribution, this will not be the case. The Bowley’s coefficient of skewness is $$\frac{q_1+q_3-2\text{median}}{Q_3-Q_1}$$. Its value lies between 0 and $\pm1$.

The most commonly used measures of Asymmetry (those discussed here) may produce some surprising results, such as a negative value when the shape of the distribution appears skewed to the right.

Impact of Lack of Symmetry

Researchers from the behavioral and business sciences need to measure the lack of symmetry when it appears in their data. A great amount of asymmetry may motivate the researcher to investigate the existence of outliers. When making decisions about which measure of the location to report and which inferential statistic to employ, one should take into consideration the estimated skewness of the population. Normal distributions have zero skewness. Of course, a distribution can be perfectly symmetric but may be far away from the normal distribution. Transformations of variables under study are commonly employed to reduce (positive) asymmetry. These transformations may include square root, log, and reciprocal of a variable.

In summary, by understanding and recognizing how skewness affects the data, one can choose appropriate analysis methods, gain more insights from the data, and make better decisions based on the findings.

FAQs About Skewness

What statistical measure is used to find the asymmetry in the data?
Define the term Skewness.
What is the difference between symmetry and asymmetry concept?
Describe negative and positive skewness.
What is the difference between left-skewed and right-skewed data?
What is a lack of symmetry?
Discuss the measure proposed by Karl Pearson.
Discuss the measure proposed by Bowley’s Coefficient of Skewness.
For what distribution, the skewness is zero?
What is the impact of transforming a variable?

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Skewness Formula, Introduction, Interpretation (2012)

Sep 21, 2024Jul 11, 2012 by Muhammad Imdad Ullah

Skewness is the degree of asymmetry or departure from the symmetry of the distribution of a real-valued random variable.

Positive Skewed
If the frequency curve of distribution has a longer tail to the right of the central maximum than to the left, the distribution is said to be skewed to the right or to have positively skewed. In a positively skewed distribution, the mean is greater than the median and the median is greater than the mode i.e. $$Mean > Median > Mode$$

Negative Skewed
If the frequency curve has a longer tail to the left of the central maximum than to the right, the distribution is said to be skewed to the left or to be negatively skewed. In a negatively skewed distribution, the mode is greater than the median and the median is greater than the mean i.e. $$Mode > Median > Mean$$

In a symmetrical distribution, the mean, median, and mode coincide. In a skewed distribution, these values are pulled apart.

Pearson’s Coefficient of Skewness Formula

Karl Pearson, (1857-1936) introduced a coefficient to measure the degree of skewness of distribution or curve, which is denoted by $S_k$ and defined by

\begin{eqnarray*}
S_k &=& \frac{Mean – Mode}{Standard Deviation}\\
S_k &=& \frac{3(Mean – Median)}{Standard Deviation}\\
\end{eqnarray*}
Usually, this coefficient varies between –3 (for negative) to +3 (for positive) and the sign indicates the direction of skewness.

Bowley’s Coefficient of Skewness Formula (Quartile Coefficient)

Arthur Lyon Bowley (1869-1957) proposed a measure of skewness based on the median and the two quartiles.

\[S_k=\frac{Q_1+Q_3-2Median}{Q_3 – Q_1}\]
Its values lie between 0 and ±1.

Moment Coefficient of Skewness Formula

This measure of skewness is the third moment expressed in standard units (or the moment ratio) thus given by

\[S_k=\frac{\mu_3}{\sigma^3} \]
Its values lie between -2 and +2.

If $S_k$ is greater than zero, the distribution or curve is said to be positively skewed. If $S_k$ is less than zero the distribution or curve is said to be negatively skewed. If $S_k$ is zero the distribution or curve is said to be symmetrical.

The skewness of the distribution of a real-valued random variable can easily be seen by drawing a histogram or frequency curve.

The skewness may be very extreme and in such a case these are called J-shaped distributions.

FAQs about Skewness

What is the degree of asymmetry called?
What is a departure from symmetry?
If a distribution is negatively skewed then what is the relation between mean, median, and mode?
If a distribution is positively skewed then what is the relation between mean, median, and mode?
What is the relation between mean, median, and mode for a symmetrical distribution?
What is the range of the moment coefficient of skewness?

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What is the Measure of Kurtosis (2012)

Sep 21, 2024Jul 2, 2012 by Muhammad Imdad Ullah

Introduction to Kurtosis

In statistics, a measure of kurtosis is a measure of the “tailedness” of the probability distribution of a real-valued random variable. The standard measure of kurtosis is based on a scaled version of the fourth moment of the data or population. Therefore, the measure of kurtosis is related to the tails of the distribution, not its peak.

Measure of Kurtosis

Sometimes, the Measure of Kurtosis is characterized as a measure of peakedness that is mistaken. A distribution having a relatively high peak is called leptokurtic. A distribution that is flat-topped is called platykurtic. The normal distribution which is neither very peaked nor very flat-topped is also called mesokurtic. The histogram in some cases can be used as an effective graphical technique for showing the skewness and kurtosis of the data set.

Data sets with high kurtosis tend to have a distinct peak near the mean, decline rather rapidly, and have heavy tails. Data sets with low kurtosis tend to have a flat top near the mean rather than a sharp peak.

Moment ratio and Percentile Coefficient of kurtosis are used to measure the kurtosis

Moment Coefficient of Kurtosis= $b_2 = \frac{m_4}{S^2} = \frac{m_4}{m^{2}_{2}}$

Percentile Coefficient of Kurtosis = $k=\frac{Q.D}{P_{90}-P_{10}}$
where Q.D = $\frac{1}{2}(Q_3 – Q_1)$ is the semi-interquartile range. For normal distribution, this has a value of 0.263.

Dr. Wheeler defines kurtosis as:

The kurtosis parameter is a measure of the combined weight of the tails relative to the rest of the distribution.

So, kurtosis is all about the tails of the distribution – not the peakedness or flatness.

A normal random variable has a kurtosis of 3 irrespective of its mean or standard deviation. If a random variable’s kurtosis is greater than 3, it is considered Leptokurtic. If its kurtosis is less than 3, it is considered Platykurtic.

A large value of kurtosis indicates a more serious outlier issue and hence may lead the researcher to choose alternative statistical methods.

Some Examples of Kurtosis

In finance, risk and insurance are examples of needing to focus on the tail of the distribution and not assuming normality.
Kurtosis helps in determining whether the resource used within an ecological guild is truly neutral or which it differs among species.
The accuracy of the variance as an estimate of the population $\sigma^2$ depends heavily on kurtosis.

For further reading see Moments in Statistics

FAQs about Kurtosis

Define Kurtosis.
What is the moment coefficient of Kurtosis?
What is the definition of kurtosis by Dr. Wheeler?
Give examples of kurtosis from real life.

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Secrets of Skewness and Measures of Skewness (2021)

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Measures of Skewness