# FACTDOUBLE

The `FACTDOUBLE` formula returns the double factorial of a number. A double factorial is the product of all positive integers that have the same parity (odd or even) as the given integer. This formula is commonly used in probability theory and combinatorics.

## Usage

Use the `FACTDOUBLE` formula with the syntax shown below, it has 1 required parameter:

Parameters:
1. value (required):
The number for which to calculate the double factorial. Must be a non-negative integer.

## Examples

Here are a few example use cases that explain how to use the `FACTDOUBLE` formula in Google Sheets.

### Calculate the number of ways to arrange objects in groups

Suppose you have 8 distinct objects and you want to arrange them into 4 groups of 2 objects each. You can use the formula `COMBIN(8,2) * COMBIN(6,2) * COMBIN(4,2) * COMBIN(2,2) / FACTDOUBLE(4)` to calculate the number of possible arrangements.

### Calculate the number of ways to choose objects with specific attributes

Suppose you have 8 objects, 3 of which are red and 5 of which are blue, and you want to choose 4 objects such that exactly 2 are red and 2 are blue. You can use the formula `COMBIN(3,2) * COMBIN(5,2) / FACTDOUBLE(2)` to calculate the number of possible choices.

### Calculate the probability of a specific outcome in a sequence of events

Suppose you have a sequence of 8 independent coin flips, and you want to calculate the probability of getting exactly 4 heads and 4 tails. You can use the formula `COMBIN(8,4) / FACTDOUBLE(4)` to calculate the probability.

## Common Mistakes

`FACTDOUBLE` not working? Here are some common mistakes people make when using the `FACTDOUBLE` Google Sheets Formula:

### Incorrect argument type

One common mistake is passing a non-integer value as the argument when using FACTDOUBLE. This will result in an error. Double-check that the value being passed is an integer.

### Using the wrong function

Another common mistake is accidentally using the FACT function instead of FACTDOUBLE, or vice versa. These functions calculate different factorials and using the wrong one will result in incorrect results.

### Not including an argument

A mistake that can be made is not including an argument for the FACTDOUBLE function. This will also result in an error.

### Using a negative value

FACTDOUBLE is only defined for positive even integers. Passing a negative value will result in an error.

### Exceeding maximum value

The maximum value for which FACTDOUBLE can be calculated is 170. Attempting to calculate the factorial of a larger number will result in an error.

The following functions are similar to `FACTDOUBLE` or are often used with it in a formula:

• `FACT`

The `FACT` formula calculates the factorial of a given number. The factorial of a number is the product of all positive integers from 1 to the given number. This formula is commonly used in probability and combinatorics.

• `COMBIN`

The `COMBIN` function calculates the number of combinations for a given number of items. A combination is a way of selecting items from a larger set where order does not matter. This function is commonly used in probability and statistics.

• `PERMUT`

The `PERMUT` function returns the number of permutations of a set of objects. A permutation is a way of arranging objects in a specific order. The function takes two arguments: `n`, the total number of objects, and `k`, the number of objects to arrange. The result is the number of possible permutations. This function is commonly used in combinatorics and probability theory.

• `GAMMALN`

The `GAMMALN` function returns the natural logarithm of the absolute value of the Gamma function, Γ(x). The Gamma function is defined as an extension of the factorial function to complex and real numbers. The function is commonly used in probability theory and statistics to compute probabilities and to model continuous distributions such as the chi-squared and F distributions.

You can learn more about the `FACTDOUBLE` Google Sheets function on Google Support.