**Calculate the standard deviation helps you to analyze the result of your AVERAGE calculation.**

## When you need to use the standard deviation?

Let's say you have a classroom with 10 students

- For the first exam, all the students have the same result 5
- For the second exam, 5 students have the mark 0 and 5 students have the mark 10

The average of the 2 exams is the same but the analyze is totally different.

In this situation, **the standard deviation calculation will help you to have a better analyze of your averages**.

## Calculation of the standard deviation

To calculate the standard deviation, you don't need to know the formula. With Excel, you just have to call the **STDEV **function and you will return the result in a cell.

In this example, you simply write =STDEV(range) to return the standard deviation.

=STDEV(B2:B11) => 0

=STDEV(C2:C11) => 5.27

**What does these results mean?**

**0**means that all values in of the series is equal to the average. There is no gap (or deviation) between the average and the values of the series.- On the other hand, for the second series,
**the result is very far from 0**and even exceeds the value of the average.

In other words, **the standard deviation represents the dispersion of the data around the **average.

The more the standard deviation is close to 0, the more the data is centered on the average.

## Several formulas in Excel, why?

As you have certainly noticed, there are several functions in Excel to calculate **the standard deviation.**

In Excel 2010, Microsoft engineers have asked to signifiant statisticians to improve the speed of the functions and also their accuracy for large number of data.

- The STDEV.P calculates standard deviation based on the entire population (N). This function replace the former STDEVP function.
- The STDEV.S calculates standard deviation based on a sample (N-1). This function replace the former STDEV function.

The difference between the 2 calculation modes concerns the sample and therefore the divisor. If you calculate the standard deviation with the entire population, the divisor will be equal to N (with N, number of elements). When you calculate for a sample, the divisor is N-1.

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