# deviance.poisson.md

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| ---------------- | --------------: |
| deviance.poisson | R Documentation |

### Poisson Deviance

#### Description

A generic S3 function to compute the *poisson deviance* score for a\
regression model. This function dispatches to S3 methods in`deviance.poisson()` and performs no input validation. If you supply NA\
values or vectors of unequal length (e.g. `length(x) != length(y)`), the\
underlying `C++` code may trigger undefined behavior and crash your `R`\
session.

**Defensive measures**

Because `deviance.poisson()` operates on raw pointers, pointer-level\
faults (e.g. from NA or mismatched length) occur before any `R`-level\
error handling. Wrapping calls in `try()` or `tryCatch()` will *not*\
prevent `R`-session crashes.

To guard against this, wrap `deviance.poisson()` in a "safe" validator\
that checks for NA values and matching length, for example:

{% code overflow="wrap" lineNumbers="true" %}

```r
safe_deviance.poisson <- function(x, y, ...) {
  stopifnot(
    !anyNA(x), !anyNA(y),
    length(x) == length(y)
  )
  deviance.poisson(x, y, ...)
}
```

{% endcode %}

Apply the same pattern to any custom metric functions to ensure input\
sanity before calling the underlying `C++` code.

#### Usage

```r
## Generic S3 method
## for Poisson Deviance
deviance.poisson(...)

## Generic S3 method
## for weighted Poisson Deviance
weighted.deviance.poisson(...)
```

#### Arguments

| `...` | <p>Arguments passed on to <code>deviance.poisson.numeric</code>,<code>weighted.deviance.poisson.numeric</code></p><p><code>actual,predicted</code></p><p>A pair of \<double> vectors of length <code>n</code>.</p><p><code>w</code></p><p>A \<double> vector of sample weights.</p> |
| ----- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |

#### Value

A \<double> value

#### References

James, Gareth, et al. An introduction to statistical learning. Vol. 112.\
No. 1. New York: springer, 2013.

Hastie, Trevor. "The elements of statistical learning: data mining,\
inference, and prediction." (2009).

Virtanen, Pauli, et al. "SciPy 1.0: fundamental algorithms for\
scientific computing in Python." Nature methods 17.3 (2020): 261-272.

#### Examples

```r
## Generate actual
## and predicted values
actual_values    <- c(1.3, 0.4, 1.2, 1.4, 1.9, 1.0, 1.2)
predicted_values <- c(0.7, 0.5, 1.1, 1.2, 1.8, 1.1, 0.2)

## Evaluate performance
SLmetrics::deviance.poisson(
   actual    = actual_values, 
   predicted = predicted_values
)
```

```

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