Computes scores for the assessment of sharpness of a fitted model for time series of counts.

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`object` |
an object of class |

`individual` |
logical. If |

`cutoff` |
positive integer. Summation over the infinite sample space {0,1,2,...} of a distribution is cut off at this value. This affects the quadratic, spherical and ranked probability score. |

`response` |
integer vector. Vector of observed values |

`pred` |
numeric vector. Vector of predicted values |

`distr` |
character giving the conditional distribution. Currently implemented are the Poisson ( |

`distrcoefs` |
numeric vector of additional coefficients specifying the conditional distribution. For |

`...` |
further arguments are currently ignored. Only for compatibility with generic function. |

The scoring rules are penalties that should be minimised for a better forecast, so a smaller scoring value means better sharpness.
Different competing forecast models can be ranked via these scoring rules.
They are computed as follows:
For each score *s* and time *t* the value *s(P[t],Y[t])* is computed, where *P[t]* is the predictive
c.d.f. and *Y[t]* is the observation at time *t*. To obtain the overall score for one model the average of the score of all observations
*(1/n) ∑ s(P[t],Y[t])*
is calculated.

For all *t ≥q 1*, let *p[y]=P(Y[t]=y | F[t-1])* be the density function of the predictive distribution at *y* and
*||p||^2= ∑ p[y]^2* be a quadratic sum over the whole sample space *y=0,1,2,...* of the predictive distribution.
*μ_P[t]* and *σ_P[t]* are the mean and the standard deviation of the predictive distribution, respectively.

Then the scores are defined as follows:

Logarithmic score: *logs(P[t],Y[t])= -log p[y] *

Quadratic or Brier score: *qs(P[t],Y[t])= -2p[y] + ||p||^2*

Spherical score: *sphs(P[t],Y[t])= -p[y] / ||p||*

Ranked probability score: *rps(P[t],Y[t])=∑ (P[t](x) - 1(Y[t]≤ x))^2* (sum over the whole sample space *x=0,1,2,...*)

Dawid-Sebastiani score: *dss(P[t],Y[t]) = ( (Y[t]-μ_P[t]) / (σ_P[t]) )^2 + 2 log σ_P[t]*

Normalized squared error score: *nses(P[t],Y[t])= ( (Y[t]-μ_P[t]) \ (σ_P[t]) )^2*

Squared error score: *ses(P[t],Y[t])=(Y[t]-μ_P[t])^2*

For more information on scoring rules see the references listed below.

Returns a named vector of the mean scores (if argument `individual=FALSE`

, the default) or a data frame of the individual scores for each observation (if argument `individual=TRUE`

). The scoring rules are named as follows:

`logarithmic` |
Logarithmic score |

`quadratic` |
Quadratic or Brier score |

`spherical` |
Spherical score |

`rankprob` |
Ranked probability score |

`dawseb` |
Dawid-Sebastiani score |

`normsq` |
Normalized squared error score |

`sqerror` |
Squared error score |

Philipp Probst and Tobias Liboschik

Christou, V. and Fokianos, K. (2013) On count time series prediction. *Journal of Statistical Computation and Simulation* (published online), http://dx.doi.org/10.1080/00949655.2013.823612.

Czado, C., Gneiting, T. and Held, L. (2009) Predictive model assessment for count data. *Biometrics* **65**, 1254–1261, http://dx.doi.org/10.1111/j.1541-0420.2009.01191.x.

Gneiting, T., Balabdaoui, F. and Raftery, A.E. (2007) Probabilistic forecasts, calibration and sharpness. *Journal of the Royal Statistical Society: Series B (Statistical Methodology)* **69**, 243–268, http://dx.doi.org/10.1111/j.1467-9868.2007.00587.x.

`tsglm`

for fitting a GLM for time series of counts.

`pit`

and `marcal`

for other predictive model assessment tools.

`permutationTest`

in package `surveillance`

for the Monte Carlo permutation test for paired individual scores by Paul and Held (2011, *Statistics in Medicine* **30**, 1118–1136, http://dx.doi.org/10.1002/sim.4177).

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