`gmd()`

computes Gini's mean difference for a numeric vector
or for all numeric vectors in a data frame.

## Arguments

- x
A vector or data frame.

- select
Optional, names of variables as character vector that should be
selected for further processing. Required, if `x`

is a data frame (and no vector)
and only selected variables from `x`

should be processed.

## Value

For numeric vectors, Gini's mean difference. For non-numeric vectors
or vectors of length < 2, returns `NA`

.

## Note

Gini's mean difference is defined as the mean absolute difference between
any two distinct elements of a vector. Missing values from `x`

are silently
removed.

## References

David HA. Gini's mean difference rediscovered. Biometrika 1968(55): 573-575

## Examples

```
data(efc)
gmd(efc$e17age)
#> [1] 9.297005
gmd(efc, c("e17age", "c160age", "c12hour"))
#> variable gmd
#> 1 e17age 9.297005
#> 2 c160age 15.185966
#> 3 c12hour 47.874877
```