set_prior {revdbayes}

Construction of prior distributions for extreme value model parameters

Description

Constructs a prior distribution for use as the argument prior in rpost and rpost_rcpp. The user can either specify their own prior function, returning the log of the prior density, (using an R function or an external pointer to a compiled C++ function) and arguments for hyperparameters or choose from a list of in-built model-specific priors. Note that the arguments model = "gev", model = "pp" and model =="os" are equivalent because a prior is specified is the GEV parameterisation in each of these cases. Note also that for model = "pp" the prior GEV parameterisation relates to the value of noy subsequently supplied to rpost or rpost_rcpp. The argument model is used for consistency with rpost.

Usage

set_prior(
  prior = c("norm", "loglognorm", "mdi", "flat", "flatflat", "jeffreys", "beta", "prob",
    "quant"),
  model = c("gev", "gp", "pp", "os"),
  ...
)

Arguments

prior

Either

• An R function, or a pointer to a user-supplied compiled C++ function, that returns the value of the log of the prior density (see Examples), or

• A character string giving the name of the prior. See Details for a list of priors available for each model.

model

A character string. If prior is a character string then model gives the extreme value model to be used. Using either model = "gev", model = "pp" or model = "os" will result in the same (GEV) parameterisation. If prior is a function then the value of model is stored so that in the subsequent call to rpost, consistency of the prior and extreme value model parameterisations can be checked.

...

Further arguments to be passed to the user-supplied or in-built prior function. For details of the latter see Details and/or the relevant underlying function: gp_norm, gp_mdi, gp_flat, gp_flatflat, gp_jeffreys, gp_beta, gev_norm, gev_loglognorm, gev_mdi, gev_flat, gev_flatflat, gev_beta, gev_prob, gev_quant. All these priors have the arguments min_xi (prior lower bound on \xi) and max_xi (prior upper bound on \xi).

Details

Of the in-built named priors available in revdbayes only those specified using prior = "prob" (gev_prob), prior = "quant" (gev_quant) prior = "norm" (gev_norm) or prior = "loglognorm" (gev_loglognorm) are proper. If model = "gev" these priors are equivalent to priors available in the evdbayes package, namely prior.prob, prior.quant, prior.norm and prior.loglognorm.

The other in-built priors are improper, that is, the integral of the prior function over its support is not finite. Such priors do not necessarily result in a proper posterior distribution. Northrop and Attalides (2016) consider the issue of posterior propriety in Bayesian extreme value analyses. In most of improper priors below the prior for the scale parameter \sigma is taken to be 1/\sigma, i.e. a flat prior for \log \sigma. Here we denote the scale parameter of the GP distribution by \sigma, whereas we use \sigma_u in the revdbayes vignette.

For all in-built priors the arguments min_xi and max_xi may be supplied by the user. The prior density is set to zero for any value of the shape parameter \xi that is outside (min_xi, max_xi). This will override the default values of min_xi and max_xi in the named priors detailed above.

Extreme value priors. It is typical to use either prior = "prob" (gev_prob) or prior = "quant" (gev_quant) to set an informative prior and one of the other prior (or a user-supplied function) otherwise. The names of the in-built extreme value priors set using prior and details of hyperparameters are:

• "prob". A prior for GEV parameters (\mu, \sigma, \xi) based on Crowder (1992). See gev_prob for details. See also Northrop et al. (2017) and Stephenson (2016).

• "quant". A prior for GEV parameters (\mu, \sigma, \xi) based on Coles and Tawn (1996). See gev_quant for details.

• "norm".

  For model = "gp": (\log \sigma, \xi), is bivariate normal with mean mean (a numeric vector of length 2) and covariance matrix cov (a symmetric positive definite 2 by 2 matrix).

  For model = "gev": (\mu, \log \sigma, \xi), is trivariate normal with mean mean (a numeric vector of length 3) and covariance matrix cov (a symmetric positive definite 3 by 3 matrix).

• "loglognorm". For model = "gev" only: (\log \mu, \log \sigma, \xi), is trivariate normal with mean mean (a numeric vector of length 3) and covariance matrix cov (a symmetric positive definite 3 by 3 matrix).

• "mdi".

  For model = "gp": (an extended version of) the maximal data information (MDI) prior, that is,

  \pi(\sigma, \xi) = \sigma^{-1} \exp[-a(\xi + 1)], {\rm ~for~} \sigma > 0, \xi \geq -1, a \geq 0.

  The value of a is set using the argument a. The default value is a = 1, which gives the MDI prior.

  For model = "gev": (an extended version of) the maximal data information (MDI) prior, that is,

  \pi(\mu, \sigma, \xi) = \sigma^{-1} \exp[-a(\xi + 1)], {\rm ~for~} \sigma > 0, \xi \geq -1, a \geq 0.

  The value of a is set using the argument a. The default value is a = \gamma, where \gamma = 0.57721 is Euler's constant, which gives the MDI prior.

  For each of these cases \xi must be is bounded below a priori for the posterior to be proper (Northrop and Attalides, 2016). An argument for the bound \xi \geq -1 is that for \xi < -1 the GP (GEV) likelihood is unbounded above as -\sigma/\xi (\mu - \sigma/\xi)) approaches the sample maximum. In maximum likelihood estimation of GP parameters (Grimshaw, 1993) and GEV parameters a local maximum of the likelihood is sought on the region \sigma > 0, \xi \geq -1.

• "flat".

  For model = "gp": a flat prior for \xi (and for \log \sigma):

  \pi(\sigma, \xi) = \sigma^{-1}, {\rm ~for~} \sigma > 0.

  For model = "gev": a flat prior for \xi (and for \mu and \log \sigma):

  \pi(\mu, \sigma, \xi) = \sigma^{-1}, {\rm ~for~} \sigma > 0.

• "flatflat".

  For model = "gp": flat priors for \sigma and \xi:

  \pi(\sigma, \xi) = 1, {\rm ~for~} \sigma > 0.

  For model = "gev": flat priors for \mu, \sigma and \xi:

  \pi(\mu, \sigma, \xi) = 1, {\rm ~for~} \sigma > 0.

  Therefore, the posterior is proportional to the likelihood.

• "jeffreys". For model = "gp" only: the Jeffreys prior (Castellanos and Cabras, 2007):

  \pi(\sigma, \xi) = \sigma^{-1}(1+\xi)^{-1}(1+2\xi)^{-1/2}, {\rm ~for~} \sigma > 0, \xi > -1 / 2.

  In the GEV case the Jeffreys prior doesn't yield a proper posterior for any sample size. See Northrop and Attalides (2016) for details.

• "beta". For model = "gp": a beta-type(p, q) prior is used for xi on the interval (min_xi, max_xi):

  \pi(\sigma, \xi) = \sigma^{-1} (\xi - {\min}_{\xi}) ^ {p-1} ({\max}_{\xi} - \xi) ^ {q-1}, {\rm ~for~} {\min}_{\xi} < \xi < {\max}_{\xi}.

  For model = "gev": similarly ...

  \pi(\mu, \sigma, \xi) = \sigma^{-1} (\xi - {\min}_{\xi}) ^ {p-1} ({\max}_{\xi} - \xi) ^ {q-1}, {\rm ~for~} {\min}_{\xi} < \xi < {\max}_{\xi}.

  The argument pq is a vector containing c(p,q). The default settings for this prior are p = 6, q = 9 and min_xi = -1/2, max_xi = 1/2, which corresponds to the prior for \xi proposed in Martins and Stedinger (2000, 2001).

Value

A list with class "evprior". The first component is the input prior, i.e. either the name of the prior or a user-supplied function. The remaining components contain the numeric values of any hyperparameters in the prior.

References

Castellanos, E. M. and Cabras, S. (2007) A default Bayesian procedure for the generalized Pareto distribution. Journal of Statistical Planning and Inference 137(2), 473-483. doi:10.1016/j.jspi.2006.01.006.

Coles, S. G. and Tawn, J. A. (1996) A Bayesian analysis of extreme rainfall data. Appl. Statist., 45, 463-478.

Crowder, M. (1992) Bayesian priors based on parameter transformation using the distribution function Ann. Inst. Statist. Math., 44, 405-416. https://link.springer.com/article/10.1007/BF00050695.

Grimshaw, S. D. (1993) Computing Maximum Likelihood Estimates for the Generalized Pareto Distribution. Technometrics, 35(2), 185-191. doi:10.1080/00401706.1993.10485040.

Hosking, J. R. M. and Wallis, J. R. (1987) Parameter and Quantile Estimation for the Generalized Pareto Distribution. Technometrics, 29(3), 339-349. doi:10.2307/1269343.

Martins, E. S. and Stedinger, J. R. (2000) Generalized maximum likelihood generalized extreme value quantile estimators for hydrologic data, Water Resources Research, 36(3), 737-744. doi:10.1029/1999WR900330.

Martins, E. S. and Stedinger, J. R. (2001) Generalized maximum likelihood Pareto-Poisson estimators for partial duration series, Water Resources Research, 37(10), 2551-2557. doi:10.1029/2001WR000367.

Northrop, P.J. and Attalides, N. (2016) Posterior propriety in Bayesian extreme value analyses using reference priors Statistica Sinica, 26(2), 721–743 doi:10.5705/ss.2014.034.

Northrop, P. J., Attalides, N. and Jonathan, P. (2017) Cross-validatory extreme value threshold selection and uncertainty with application to ocean storm severity. Journal of the Royal Statistical Society Series C: Applied Statistics, 66(1), 93-120. doi:10.1111/rssc.12159

Stephenson, A. (2016) Bayesian inference for extreme value modelling. In Extreme Value Modeling and Risk Analysis: Methods and Applications (eds D. K. Dey and J. Yan), 257-280, Chapman and Hall, London. doi:10.1201/b19721.

See Also

rpost and rpost_rcpp for sampling from an extreme value posterior distribution.

create_prior_xptr for creating an external pointer to a C++ function to evaluate the log-prior density.

rprior_prob and rprior_quant for sampling from informative prior distributions for GEV parameters.

gp_norm, gp_mdi, gp_flat, gp_flatflat, gp_jeffreys, gp_beta to see the arguments for priors for GP parameters.

gev_norm, gev_loglognorm, gev_mdi, gev_flat, gev_flatflat, gev_beta, gev_prob, gev_quant to see the arguments for priors for GEV parameters.

Examples

# Normal prior for GEV parameters (mu, log(sigma), xi).
mat <- diag(c(10000, 10000, 100))
pn <- set_prior(prior = "norm", model = "gev", mean = c(0,0,0), cov = mat)
pn

# Prior for GP parameters with flat prior for xi on (-1, infinity).
fp <- set_prior(prior = "flat", model = "gp", min_xi = -1)
fp

# A user-defined prior (see the vignette for details).
u_prior_fn <- function(x, ab) {
  if (x[1] <= 0 | x[2] <= -1 | x[2] >= 1) {
    return(-Inf)
  }
  return(-log(x[1]) + (ab[1] - 1) * log(1 + x[2]) +
         (ab[2] - 1) * log(1 - x[2]))
}
up <- set_prior(prior = u_prior_fn, ab = c(2, 2), model = "gp")

# A user-defined prior using a pointer to a C++ function
ptr_gp_flat <- create_prior_xptr("gp_flat")
u_prior_ptr <- set_prior(prior = ptr_gp_flat, model = "gp")