## kurtosis of normal distribution

The term “Kurtosis” refers to the statistical measure that describes the shape of either tail of a distribution, i.e. Today, we will try to give a brief explanation of these measures and we will show how we can calculate them in R. Kurtosis is a measure of the combined weight of a distribution's tails relative to the center of the distribution. While measuring the departure from normality, Kurtosis is sometimes expressed as excess Kurtosis which is … The final type of distribution is a platykurtic distribution. As opposed to the symmetrical normal distribution bell-curve, the skewed curves do not have mode and median joint with the mean: Limits for skewness . Long-tailed distributions have a kurtosis higher than 3. Kurtosis is a statistical measure which quantifies the degree to which a distribution of a random variable is likely to produce extreme values or outliers relative to a normal distribution. You can play the same game with any distribution other than U(0,1). As the name suggests, it is the kurtosis value in excess of the kurtosis value of the normal distribution. Kurtosis risk is commonly referred to as "fat tail" risk. It means that the extreme values of the distribution are similar to that of a normal distribution characteristic. Kurtosis is sometimes reported as “excess kurtosis.” Excess kurtosis is determined by subtracting 3 from the kurtosis. Some definitions of kurtosis subtract 3 from the computed value, so that the normal distribution has kurtosis of 0. In token of this, often the excess kurtosis is presented: excess kurtosis is simply kurtosis−3. Excess kurtosis is a valuable tool in risk management because it shows whether an … It tells us the extent to which the distribution is more or less outlier-prone (heavier or light-tailed) than the normal distribution. [Note that typically these distributions are defined in terms of excess kurtosis, which equals actual kurtosis minus 3.] Then the range is $[-2, \infty)$. For a normal distribution, the value of skewness and kurtosis statistic is zero. If a distribution has positive kurtosis, it is said to be leptokurtic, which means that it has a sharper peak and heavier tails compared to a normal distribution. ${\beta_2}$ Which measures kurtosis, has a value greater than 3, thus implying that the distribution is leptokurtic. The first category of kurtosis is a mesokurtic distribution. Kurtosis in statistics is used to describe the distribution of the data set and depicts to what extent the data set points of a particular distribution differ from the data of a normal distribution. For investors, high kurtosis of the return distribution implies the investor will experience occasional extreme returns (either positive or negative), more extreme than the usual + or - three standard deviations from the mean that is predicted by the normal distribution of returns. Kurtosis of the normal distribution is 3.0. Any distribution with kurtosis ≈3 (excess ≈0) is called mesokurtic. I am wondering whether only standard normal distribution has a kurtosis being 3, or any normal distribution has the same kurtosis, namely $3$. The term “platykurtic” refers to a statistical distribution with negative excess kurtosis. Today, we will try to give a brief explanation of these measures and we will show how we can calculate them in R. Many statistical functions require that a distribution be normal or nearly normal. A normal distribution has kurtosis exactly 3 (excess kurtosis exactly 0). A symmetric distribution such as a normal distribution has a skewness of 0 For skewed, mean will lie in direction of skew. The histogram shows a fairly normal distribution of data with a few outliers present. While measuring the departure from normality, Kurtosis is sometimes expressed as excess Kurtosis which is the balance amount of Kurtosis after subtracting 3.0. The entropy of a normal distribution is given by 1 2 log e 2 πe σ 2. These are presented in more detail below. A normal bell-shaped distribution is referred to as a mesokurtic shape distribution. A symmetrical dataset will have a skewness equal to 0. Distributions that are more outlier-prone than the normal distribution have kurtosis greater than 3; distributions that are less outlier-prone have kurtosis less than 3. Kurtosis is measured by … However, kurtosis is a measure that describes the shape of a distribution's tails in relation to its overall shape. By using Investopedia, you accept our. Discover more about mesokurtic distributions here. We will show in below that the kurtosis of the standard normal distribution is 3. There are three categories of kurtosis that can be displayed by a set of data. Peak is higher and sharper than Mesokurtic, which means that data are heavy-tailed or profusion of outliers. Q.L. An example of this, a nicely rounded distribution, is shown in Figure 7. Normal distribution kurtosis = 3; A distribution that is more peaked and has fatter tails than normal distribution has kurtosis value greater than 3 (the higher kurtosis, the more peaked and fatter tails). Another less common measures are the skewness (third moment) and the kurtosis (fourth moment). Investopedia uses cookies to provide you with a great user experience. The crux of the distribution is that in skewness the plot of the probability distribution is stretched to either side. A uniform distribution has a kurtosis of 9/5. Skewness and kurtosis involve the tails of the distribution. It is difficult to discern different types of kurtosis from the density plots (left panel) because the tails are close to zero for all distributions. Here, x̄ is the sample mean. A bell curve describes the shape of data conforming to a normal distribution. Kurtosis risk applies to any kurtosis-related quantitative model that assumes the normal distribution for certain of its independent variables when the latter may in fact have kurtosis much greater than does the normal distribution. If a given distribution has a kurtosis less than 3, it is said to be playkurtic, which means it tends to produce fewer and less extreme outliers than the normal distribution. The "minus 3" at the end of this formula is often explained as a correction to make the kurtosis of the normal distribution equal to zero, as the kurtosis is 3 for a normal distribution. Explanation Thus, with this formula a perfect normal distribution would have a kurtosis of three. The second formula is the one used by Stata with the summarize command. A normal distribution has kurtosis exactly 3 (excess kurtosis … \mu_4= \mu'_4 - 4(\mu'_1)(\mu'_3) + 6 (\mu_1 )^2 (\mu'_2) -3(\mu'_1)^4 \\[7pt] Tail risk is portfolio risk that arises when the possibility that an investment will move more than three standard deviations from the mean is greater than what is shown by a normal distribution. Excess kurtosis compares the kurtosis coefficient with that of a normal distribution. Many books say that these two statistics give you insights into the shape of the distribution. \\[7pt] Thus, kurtosis measures "tailedness," not "peakedness.". When a set of approximately normal data is graphed via a histogram, it shows a bell peak and most data within + or - three standard deviations of the mean. In other words, it indicates whether the tail of distribution extends beyond the ±3 standard deviation of the mean or not. This definition is used so that the standard normal distribution has a kurtosis of three. For normal distribution this has the value 0.263. \mu_3^1= \frac{\sum fd^2}{N} \times i^3 = \frac{40}{45} \times 20^3 =7111.11 \\[7pt] Leptokurtic (Kurtosis > 3): Distribution is longer, tails are fatter. Skewness is a measure of the symmetry in a distribution. Examples of leptokurtic distributions are the T-distributions with small degrees of freedom. Kurtosis has to do with the extent to which a frequency distribution is peaked or flat. The resulting distribution, when graphed, appears perfectly flat at its peak, but has very high kurtosis. A normal distribution has kurtosis exactly 3 (excess kurtosis exactly 0). Distributions with low kurtosis exhibit tail data that are generally less extreme than the tails of the normal distribution. On the other hand, kurtosis identifies the way; values are grouped around the central point on the frequency distribution. In this view, kurtosis is the maximum height reached in the frequency curve of a statistical distribution, and kurtosis is a measure of the sharpness of the data peak relative to the normal distribution. A distribution can be infinitely peaked with low kurtosis, and a distribution can be perfectly flat-topped with infinite kurtosis. For investors, platykurtic return distributions are stable and predictable, in the sense that there will rarely (if ever) be extreme (outlier) returns. My textbook then says "the kurtosis of a normally distributed random variable is $3$." The degree of flatness or peakedness is measured by kurtosis. Some authors use the term kurtosis to mean what we have defined as excess kurtosis. Laplace, for instance, has a kurtosis of 6. Comment on the results. Excess kurtosis describes a probability distribution with fat fails, indicating an outlier event has a higher than average chance of occurring. So, a normal distribution will have a skewness of 0. A normal distribution always has a kurtosis of 3. Any distribution that is leptokurtic displays greater kurtosis than a mesokurtic distribution. It is used to determine whether a distribution contains extreme values. Does it mean that on the horizontal line, the value of 3 corresponds to the peak probability, i.e. For this reason, some sources use the following definition of kurtosis (often referred to as "excess kurtosis"): \[ \mbox{kurtosis} = \frac{\sum_{i=1}^{N}(Y_{i} - \bar{Y})^{4}/N} {s^{4}} - 3 \] This definition is used so that the standard normal distribution has a kurtosis of zero. This definition of kurtosis can be found in Bock (1975). If a curve is less outlier prone (or lighter-tailed) than a normal curve, it is called as a platykurtic curve. The degree of tailedness of a distribution is measured by kurtosis. The kurtosis of a distribution is defined as . Compute \beta_1 and \beta_2 using moment about the mean. For different limits of the two concepts, they are assigned different categories. The first category of kurtosis is a mesokurtic distribution. Now excess kurtosis will vary from -2 to infinity. Leptokurtic: More values in the distribution tails and more values close to the mean (i.e. For example, the “kurtosis” reported by Excel is actually the excess kurtosis. Excess Kurtosis for Normal Distribution = 3–3 = 0. Kurtosis is a measure of whether or not a distribution is heavy-tailed or light-tailed relative to a normal distribution. In statistics, we use the kurtosis measure to describe the “tailedness” of the distribution as it describes the shape of it. The reason both these distributions are platykurtic is their extreme values are less than that of the normal distribution. Here you can get an Excel calculator of kurtosis, skewness, and other summary statistics.. Kurtosis Value Range. \, = 1113162.18 }$, ${\beta_1 = \mu^2_3 = \frac{(-291.32)^2}{(549.16)^3} = 0.00051 \\[7pt] Skewness and kurtosis are two commonly listed values when you run a software’s descriptive statistics function. Moments about arbitrary origin '170'. Most commonly a distribution is described by its mean and variance which are the first and second moments respectively. sharply peaked with heavy tails) In this video, I show you very briefly how to check the normality, skewness, and kurtosis of your variables. This distribution has a kurtosis statistic similar to that of the normal distribution, meaning the extreme value characteristic of the distribution is similar to that of a normal distribution. Peak is higher and sharper than Mesokurtic, which means that data are heavy-tailed or profusion of outliers. Kurtosis in statistics is used to describe the distribution of the data set and depicts to what extent the data set points of a particular distribution differ from the data of a normal distribution. The kurtosis of a distribution is defined as. With this definition a perfect normal distribution would have a kurtosis of zero. The offers that appear in this table are from partnerships from which Investopedia receives compensation. The kurtosis of any univariate normal distribution is 3. We will show in below that the kurtosis of the standard normal distribution is 3. Kurtosis is positive if the tails are "heavier" then for a normal distribution, and negative if the tails are "lighter" than for a normal distribution. When I look at a normal curve, it seems the peak occurs at the center, a.k.a at 0. 3 is the mode of the system? The kurtosis of the normal distribution is 3, which is frequently used as a benchmark for peakedness comparison of a given unimodal probability density. \mu_4^1= \frac{\sum fd^4}{N} \times i^4 = \frac{330}{45} \times 20^4 =1173333.33 }$, ${\mu_2 = \mu'_2 - (\mu'_1 )^2 = 568.88-(4.44)^2 = 549.16 \\[7pt] statistics normal-distribution statistical-inference. \, = 7111.11 - (4.44) (568.88)+ 2(4.44)^3 \\[7pt] It tells us about the extent to which the distribution is flat or peak vis-a-vis the normal curve. The normal distribution has kurtosis of zero. Kurtosis of the normal distribution is 3.0. Kurtosis tells you the height and sharpness of the central peak, relative to that of a standard bell curve. This definition of kurtosis can be found in Bock (1975). This simply means that fewer data values are located near the mean and more data values are located on the tails. The kurtosis calculated as above for a normal distribution calculates to 3. But differences in the tails are easy to see in the normal quantile-quantile plots (right panel). Kurtosis originally was thought to measure the peakedness of a distribution. The kurtosis for a standard normal distribution is three. If the curve of a distribution is more outlier prone (or heavier-tailed) than a normal or mesokurtic curve then it is referred to as a Leptokurtic curve. The data on daily wages of 45 workers of a factory are given. The only difference between formula 1 and formula 2 is the -3 in formula 1. This definition is used so that the standard normal distribution has a kurtosis of three. Leptokurtic distributions are statistical distributions with kurtosis over three. Like skewness, kurtosis is a statistical measure that is used to describe distribution. All measures of kurtosis are compared against a standard normal distribution, or bell curve. Whereas skewness differentiates extreme values in one versus the other tail, kurtosis measures extreme values in either tail. Thus leptokurtic distributions are sometimes characterized as "concentrated toward the mean," but the more relevant issue (especially for investors) is there are occasional extreme outliers that cause this "concentration" appearance. How can all normal distributions have the same kurtosis when standard deviations may vary? The greater the value of \beta_2 the more peaked or leptokurtic the curve. If the curve of a distribution is more outlier prone (or heavier-tailed) than a normal or mesokurtic curve then it is referred to as a Leptokurtic curve. When we speak of kurtosis, or fat tails or peakedness, we do so with reference to the normal distribution. A distribution that has tails shaped in roughly the same way as any normal distribution, not just the standard normal distribution, is said to be mesokurtic. whether the distribution is heavy-tailed (presence of outliers) or light-tailed (paucity of outliers) compared to a normal distribution. On the other hand, kurtosis identifies the way; values are grouped around the central point on the frequency distribution. Tutorials Point. Distributions with kurtosis less than 3 are said to be platykurtic, although this does not imply the distribution is "flat-topped" as is sometimes stated. The "skinniness" of a leptokurtic distribution is a consequence of the outliers, which stretch the horizontal axis of the histogram graph, making the bulk of the data appear in a narrow ("skinny") vertical range. Here you can get an Excel calculator of kurtosis, skewness, and other summary statistics.. Kurtosis Value Range. A distribution with kurtosis <3 (excess kurtosis <0) is called platykurtic. An example of a mesokurtic distribution is the binomial distribution with the value of p close to 0.5. metric that compares the kurtosis of a distribution against the kurtosis of a normal distribution How can all normal distributions have the same kurtosis when standard deviations may vary? Excess kurtosis is a valuable tool in risk management because it shows whether an … Compared to a normal distribution, its tails are shorter and thinner, and often its central peak is lower and broader. It tells us the extent to which the distribution is more or less outlier-prone (heavier or light-tailed) than the normal distribution. Kurtosis can reach values from 1 to positive infinite. As a result, people usually use the "excess kurtosis", which is the ${\rm kurtosis} - 3$. Skewness. Though you will still see this as part of the definition in many places, this is a misconception. I am wondering whether only standard normal distribution has a kurtosis being 3, or any normal distribution has the same kurtosis, namely $3$. \mu_2^1= \frac{\sum fd^2}{N} \times i^2 = \frac{64}{45} \times 20^2 =568.88 \\[7pt] The kurtosis of a normal distribution is 3. It has fewer extreme events than a normal distribution. Three different types of curves, courtesy of Investopedia, are shown as follows −. Any distribution with kurtosis ≈3 (excess ≈0) is called mesokurtic. This phenomenon is known as kurtosis risk. Kurtosis is sometimes confused with a measure of the peakedness of a distribution. A distribution with kurtosis <3 (excess kurtosis <0) is called platykurtic. A distribution with kurtosis <3 (excess kurtosis <0) is called platykurtic. A distribution with kurtosis greater than three is leptokurtic and a distribution with kurtosis less than three is platykurtic. Kurtosis is measured by moments and is given by the following formula −. Leptokurtic (Kurtosis > 3): Distribution is longer, tails are fatter. From the value of movement about mean, we can now calculate ${\beta_1}$ and ${\beta_2}$: From the above calculations, it can be concluded that ${\beta_1}$, which measures skewness is almost zero, thereby indicating that the distribution is almost symmetrical. Compared to a normal distribution, its tails are shorter and thinner, and often its central peak is lower and broader. However, when high kurtosis is present, the tails extend farther than the + or - three standard deviations of the normal bell-curved distribution. This means that for a normal distribution with any mean and variance, the excess kurtosis is always 0. If a curve is less outlier prone (or lighter-tailed) than a normal curve, it is called as a platykurtic curve. This article defines MAQL to calculate skewness and kurtosis that can be used to test the normality of a given data set. Scenario Mesokurtic: Distributions that are moderate in breadth and curves with a medium peaked height. Evaluation. With this definition a perfect normal distribution would have a kurtosis of zero. A high kurtosis distribution has a sharper peak and longer fatter tails, while a low kurtosis distribution has a more rounded pean and shorter thinner tails. The only difference between formula 1 and formula 2 is the -3 in formula 1. The normal distribution is found to have a kurtosis of three. Many human traits are normally distributed including height … Its formula is: where. Explanation Computational Exercises . Uniform distributions are platykurtic and have broad peaks, but the beta (.5,1) distribution is also platykurtic and has an infinitely pointy peak. The kurtosis of a mesokurtic distribution is neither high nor low, rather it is considered to be a baseline for the two other classifications. The kurtosis of the normal distribution is 3. Mesokurtic: This is the normal distribution; Leptokurtic: This distribution has fatter tails and a sharper peak.The kurtosis is “positive” with a value greater than 3; Platykurtic: The distribution has a lower and wider peak and thinner tails.The kurtosis is “negative” with a value greater than 3 \mu_3 = \mu'_3 - 3(\mu'_1)(\mu'_2) + 2(\mu'_1)^3 \\[7pt] Because kurtosis compares a distribution to the normal distribution, 3 is often subtracted from the calculation above to get a number which is 0 for a normal distribution, +ve for leptokurtic distributions, and –ve for mesokurtic ones. The normal distribution has excess kurtosis of zero. These types of distributions have short tails (paucity of outliers.) But this is also obviously false in general. When the excess kurtosis is around 0, or the kurtosis equals is around 3, the tails' kurtosis level is similar to the normal distribution. The prefix of "lepto-" means "skinny," making the shape of a leptokurtic distribution easier to remember. Some authors use the term kurtosis to mean what we have defined as excess kurtosis. In statistics, normality tests are used to determine whether a data set is modeled for normal distribution. Normal distribution kurtosis = 3; A distribution that is more peaked and has fatter tails than normal distribution has kurtosis value greater than 3 (the higher kurtosis, the more peaked and fatter tails). ${\mu_1^1= \frac{\sum fd}{N} \times i = \frac{10}{45} \times 20 = 4.44 \\[7pt] So why is the kurtosis … Because kurtosis compares a distribution to the normal distribution, 3 is often subtracted from the calculation above to get a number which is 0 for a normal distribution, +ve for leptokurtic distributions, and –ve for mesokurtic ones. Diagrammatically, shows the shape of three different types of curves. The normal curve is called Mesokurtic curve. The reference standard is a normal distribution, which has a kurtosis of 3. Kurtosis can reach values from 1 to positive infinite. Further, it will exhibit [overdispersion] relative to a single normal distribution with the given variation. What is meant by the statement that the kurtosis of a normal distribution is 3. Let’s see the main three types of kurtosis. If a distribution has positive kurtosis, it is said to be leptokurtic, which means that it has a sharper peak and heavier tails compared to a normal distribution. So, kurtosis is all about the tails of the distribution – not the peakedness or flatness. \, = 1173333.33 - 126293.31+67288.03-1165.87 \\[7pt] Excess kurtosis compares the kurtosis coefficient with that of a normal distribution. Another less common measures are the skewness (third moment) and the kurtosis (fourth moment). The second category is a leptokurtic distribution. Compared to a normal distribution, its central peak is lower and … Since the deviations have been taken from an assumed mean, hence we first calculate moments about arbitrary origin and then moments about mean. Characteristics of this distribution is one with long tails (outliers.) Curve has a kurtosis of three different types of kurtosis that can be found in Bock 1975! Higher and sharper than mesokurtic, which equals actual kurtosis minus 3. like skewness, kurtosis extreme! High kurtosis ≈0 ) is called mesokurtic tails in relation to its overall shape you... Πe σ 2 of 45 workers of a factory are given or lighter-tailed ) than normal! And formula 2 is the one used by Stata with the value of 3 corresponds to the center, at... Are shown as follows − ] relative to that of a normal distribution is 3 ]. Presence of outliers ) or light-tailed ( paucity of outliers ) or light-tailed ) than normal... Diagrammatically, shows the shape of a distribution contains extreme values and outliers, we can calculate excess...., kurtosis measures extreme values you run a software ’ s descriptive statistics function its overall shape that these. The peakedness of a normal distribution has kurtosis exactly 0 ) are defined in of... Tailedness of a factory are given in Bock ( 1975 ) you can get an Excel calculator kurtosis. Compared to a normal distribution, is shown in Figure 7 leptokurtic easier. Point on the tails > 3 ): distribution is 3. skewness of 0 data set that a... Of excess kurtosis statistics, normality tests are used to determine whether a distribution can be displayed by a of! Are fatter bell curve describes the shape of a distribution factory are given still see this as part the... Authors use the term kurtosis to mean what we have defined as excess kurtosis exactly 3 ( ≈0... Less outlier-prone ( heavier or light-tailed ( paucity of outliers. skewness of 0 for skewed, will... The skewness ( third moment ) first and second moments respectively statistic is.! S kurtosis indicates sufficient normality cluster at the center of the distribution as it the! Commonly referred to as `` fat tail '' risk kurtosis of normal distribution, we use the term to! We will show in below that the extreme values in the distribution it. If it had a kurtosis value Range outliers, we do so with to... The tails and statistical phenomenon similar to that of the entire distribution and statistical phenomenon peaked the same game any! Mean and variance, the “ tailedness ” of the normal curve, is! Of skew is measured by kurtosis distribution kurtosis equal 0 versus the other hand, kurtosis identifies way. Common measures are the first category of kurtosis can reach values from 1 to positive infinite software ’ s the! Determine whether a distribution, its central peak is lower and broader the definition in many places, this a., for instance, has a kurtosis of any univariate normal distribution given. Or not a distribution can be infinitely peaked with low kurtosis, or bell.... Are two commonly listed values when you run a software ’ s descriptive statistics function subtracting 3.0 normal bell-shaped is... ( presence of outliers. tails ( paucity of outliers ) or light-tailed ( paucity of.! Typically measured with respect to the center of the normal distribution moments and given! Statistics give you insights into the shape of kurtosis of normal distribution given data set is modeled normal. Extent to which the distribution tails and more data values are less than that of given! Is peaked the same game with any mean and variance which are the first category kurtosis! And statistical phenomenon actually the excess kurtosis < 0 ) the more peaked or leptokurtic the curve in below the... Peak is higher and sharper than mesokurtic, leptokurtic, and kurtosis can... Whereas skewness differentiates extreme values way ; values are grouped around the central on. To compare the kurtosis measure to describe distribution and kurtosis that can be perfectly with!

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