# Mean, variance and standard deviation of discrete random variable

This online calculator calculates mean, variance and standard deviation of random variable entered in the form of value-probability table

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This calculator can help you to calculate basic discrete random variable metrics: **mean** or **expected value**, **variance**, and **standard deviation**.

**Mean** or **expected value** of discrete random variable is defined as

**Variance** of random variable is defined as

An alternative way to compute the variance is

The positive square root of the variance is called the **standard deviation** .

As you can see, these metrics have quite simple formulas. Sometimes you need to find them to solve probability theory problems. For discrete random variable, the trick, of course, is to find correct value-probability pairs, then it is just simple math of additions and multiplications. So, this calculator can take care of simple math for you, once you enter value-probability pairs into the table. You can find example of usage below the calculator.

### Example

Problem: A set of 10 microwave ovens includes 3 that are defective. If 5 of the ovens are chosen at random for shipment to a hotel, how many defective ovens can they expect?

How to use the calculator:

- Select the current data in the table (if any) by clicking on top checkbox and delete it by clicking on "bin" icon on table header.
- Add value-probability pairs (you need to determine them, but it is the essence of the problem). Note that the quickest way to do it is to "import" data. Click on "import" icon on table header and enter the following values

`0;0.0833`

`1;0.4167`

`2;0.4167`

`3;0.0833`

After that you will get the mean of 1.5. Of course 1.5 defective ovens does not make any physical sense. Instead, it should be interpreted as an average value if repeated shipments will be made under these conditions.

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