`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. |

... |
Optional, unquoted names of variables 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. You may also
use functions like `:` or tidyselect's `select_helpers()` . |

## 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, e17age, c160age, c12hour)
#> # A tibble: 1 x 3
#> e17age c160age c12hour
#> <dbl> <dbl> <dbl>
#> 1 9.30 15.2 47.9
```