Measure to compare true observed response with predicted response in regression tasks.

## Arguments

- truth
(

`numeric()`

)

True (observed) values. Must have the same length as`response`

.- response
(

`numeric()`

)

Predicted response values. Must have the same length as`truth`

.- na_value
(

`numeric(1)`

)

Value that should be returned if the measure is not defined for the input (as described in the note). Default is`NaN`

.- ...
(

`any`

)

Additional arguments. Currently ignored.

## Details

The Symmetric Mean Absolute Percent Error is defined as $$ \frac{2}{n} \sum_{i=1}^n \frac{\left| t_i - r_i \right|}{\left| t_i \right| + \left| r_i \right|}. $$

This measure is undefined if if any \(|t| + |r|\) is \(0\).