Standard deviation is a number used to tell how measurements for a group are spread out from the mean value. In other words, it's a measure of how spread out the data is.

Its symbol is **"σ"**. Also, we can call it **"Sigma"**(The greek alphabet).

The following formulas are used to calculate the standard deviation.

**Population Standard Deviation Formula:**

**Sample Standard Deviation Formula:**

There is a small but very important difference between population and sample standard deviation formula. That is **"N"** and **"N-1"** at denominator. Therefore, when we calculate for sample standard deviation, we need to divide by **"N-1"** (Instead of **"N"**).

Here **N** is for population size and **N-1** is for sample size. Whereas other components are same in both equations.

Also, **"μ"** and **"x̄"** (x-bar) are the same things. That is **mean**. x̄ is the sample mean and μ is the population mean.