You might want to look at Measures of Central Tendency before learning about standard deviation. If you are already familiar with the mean of a data set, let's go!
Standard deviation is a measure of dispersion, and it is used in statistics to see how spread out values are from the mean in a data set.
Standard deviation formula
The formula for standard deviation is:
\[ \sigma = \sqrt{\dfrac{\sum(x_i-\mu)^2}{N}}\]
Where:
\(\sigma\) is the standard deviation
\(\sum\) is the sum
\(x_i\) is an individual number in the data set
\( \mu\) is the mean of the data set
\(N\) is the total number of values in the data set
So, in words, the standard deviation is the square root of the sum of how far each data point is from the mean squared, divided by the total number of data points.
The variance of a set of data is equal to the standard deviation squared, \(\sigma^2\).
Standard deviation graph
The concept of standard deviation is pretty useful because it helps us predict how many of the values in a data set will be at a certain distance from the mean. When carrying out a standard deviation, we assume that the values in our data set follow a normal distribution. This means that they are distributed around the mean in a bell-shaped curve, as below.
Standard deviation graph. Image: M W Toews, CC BY-2.5 i
The \(x\)-axis represents the standard deviations around the mean, which in this case is \(0\). The \(y\)-axis shows the probability density, which means how many of the values in the data set fall between the standard deviations of the mean. This graph, therefore, tells us that \(68.2\%\) of the points in a normally-distributed data set fall between \(-1\) standard deviation and \(+1\) standard deviation of the mean, \(\mu\).
How do you calculate standard deviation?
In this section, we will look at an example of how to calculate the standard deviation of a sample data set. Let's say you measured the height of your classmates in cm and recorded the results. Here's your data:
165, 187, 172, 166, 178, 175, 185, 163, 176, 183, 186, 179
From this data we can already determine \(N\), the number of data points. In this case, \(N = 12\). Now we need to calculate the mean, \(\mu\). To do that we simply add all the values together and divide by the total number of data points, \(N\).
\[ \begin{align} \mu &= \frac{165 + 187+172+166+178+175+185+163+176+183+186+179}{12} \\ &= 176.25. \end{align} \]
Now we have to find
\[ \sum(x_i-\mu)^2.\]
For this we can construct a table:
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