x!

x! refers to a factorial of x. Meaning, that if x = 5, then x! = 5*4*3*2*1 = 120. In Python we can use the scipy library:

```
from scipy.special import factorial
factorial(5)
> 120
```

**x̄**

X bar is a reference to the **sample** **mean** (as opposed to the population mean.)

In Python you can calculate a mean quite quickly with Numpy:

```
import numpy as np
np.mean([1,8,20,35])
>> 16
```

*μ*

*μ* (pronounced “mu” or “mew”) is the Greek letter used to reference the **population mean**.

Σ

A Greek Sigma of course means the **SUM** over a set.