The `Mean`

function returns the average of the values of the
argument variables which have nonmissing values. Examples:

```
avg=Mean(x, y, z);
avg=Mean(of x1-x10);
avg=Mean(of x y z);
```

The first and last example assign `avg`

the average of the variables
`x`

, `y`

, and `z`

which have nonmissing values. Note the
differences in syntax. The last example returns the average of the variables among ```
x1, ..., x10
```

which have nonmissing values.