It estimates the Willmott's index of agreement (d) for a continuous predicted-observed dataset.

## Arguments

- data
(Optional) argument to call an existing data frame containing the data.

- obs
Vector with observed values (numeric).

- pred
Vector with predicted values (numeric).

- tidy
Logical operator (TRUE/FALSE) to decide the type of return. TRUE returns a data.frame, FALSE returns a list; Default : FALSE.

- na.rm
Logic argument to remove rows with missing values (NA). Default is na.rm = TRUE.

## Value

an object of class `numeric`

within a `list`

(if tidy = FALSE) or within a
`data frame`

(if tidy = TRUE).

## Details

The d index it is a normalized, dimensionless metric that tests general agreement. It measures both accuracy and precision using squared residuals. It is bounded between 0 and 1. The disadvantage is that d is an asymmetric index, that is, dependent to what is orientation of predicted and observed values. For the formula and more details, see online-documentation

## References

Willmott (1981).
On the validation of models.
*Phys. Geogr. 2, 184–194.* doi:10.1080/02723646.1981.10642213