Kurtosis tells you how “peaked” your graph is, or how high the graph is around the mean. It’s also the fourth moment in statistics. A positive value means that you have too little data in your tails. A negative value means that you have too much data in your tail. This heaviness or lightness in the tails means that your data looks more peaked (or less peaked).
What does it Mean?
Kurtosis is measured against the standard normal distribution. The standard normal distribution has a kurtosis of 3, so if your values are close to that then your graph is nearly normal. These nearly normal distributions are called mesokurtic.
Mesokurtic distributions are more technically defined as having a kurtosis of zero, although the distribution doesn’t have to be exactly zero in order for it to be classified as mesokurtic. The most common mesokurtic distributions are:
- The normal distribution.
- Any distribution with a Gaussian shape and zero probability at other places on the real line.
- The binomial distribution is mesokurtic for some values (i.e. for p = 1/2±√(1/12).
In addition to mesokurtic, the two other types of kurtosis are:
- Platykurtic distributions, which have negative kurtosis. An example of a very platykurtic distribution is a uniform distribution, which has as much data in each tail as it does in the peak.
- Leptokurtic distributions, which have positive kurtosis. The most leptokurtic distribution is students’s t distribution which has the bulk of the data in the peak.
What is Excess kurtosis?
Excess kurtosis is just kurt – 3. For example, the excess for the normal distribution is 0 – 3 = -3.
- Negative excess means there is less of a peak (more data in the tails).
- Positive excess means there is more of a peak (less data in the tails).
These graphs (from Wikipedia) should help you see the difference between negative, zero, and positive excess.
|Uniform: excess = −1.2.||Normal: excess = 0.||Logistic: excess = 1.2|
How to Calculate by hand or with technology.
Kurtosis is the fourth moment, so to calculate it by hand, use the following formula:
The 4th moment = (x14 + x24 + x34 + . . . + xn4)/n.
For Minitab and SPSS, you can find the option in the “Descriptive Statistics” tab.
Kurtosis in Excel 2013
Watch the video or read the steps below:
There are two options in Excel for finding kurtosis: the KURT Function and the Data Analysis Toolpak (How to load the Data Analysis Toolpak).
Kurtosis Excel 2013: KURT function
Step 1: Type your data into columns in an Excel worksheet.
Step 2: Click a blank cell.
Step 3: Type “=KURT(A1:A99)” where A1:99 is the cell locations for your data.
Kurtosis Excel 2013: Data Analysis
Step 1: Click the “Data” tab and then click “Data Analysis.”
Step 2: Click “Descriptive Statistics” and then click “OK.”
Step 3: Click the Input Range box and then type the location for your data. For example, if you typed your data into cells A1 to A10, type “A1:A10” into that box
Step 4: Click the radio button for Rows or Columns, depending on how your data is laid out.
Step 5: Click the “Labels in first row” box if your data has column headers.
Step 6: Click the “Descriptive Statistics” check box.
Step 7: Select a location for your output. For example, click the “New Worksheet” radio button.
Step 8: Click “OK.”
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