### 20.1.1 Mean

Recall that the mean of a list x1,…,xn is simply their numeric average (x1+⋯+xn)/n. The mean command finds the mean of a list.

- mean takes one mandatory argument and one optional argument:
  - L, a list or matrix of numbers.
  - W, a list or matrix of weights, the same size as L.
- mean(L ⟨,W⟩) returns the mean of the list or a list with the means of the columns of the matrix.

#### Examples

mean([1,2,3,4])

5
2

since (1+2+3+4)/4=5/2.

mean([[1,2,3],[5,6,7]])

| ⎡ ⎣ | 3,4,5 | ⎤ ⎦

since (1+5)/2=3, (2+6)/2=4 and (3+7)/2=5.

mean([2,4,6,8],[2,2,3,3])

27
5

since (2· 2+4· 2+6· 3+8· 3)/(2+2+3+3)= 27/5.

mean([[1,2],[3,4]],[[1,2],[2,1]])

| ⎡ ⎢ ⎢ ⎣ |
7
3
| , |
8
3
| ⎤ ⎥ ⎥ ⎦

since (1· 1+3· 2)/(1+2)=7/3 and (2· 2+4· 1)/(2 + 1)=8/3.