June 10, 2025
Supported by the National Science Foundation under Grant No. SES-1921523

Points for Discussion

  • Nice technical derivation of a default prior for the smoothness parameter.
  • Identifiability (or lack thereof) of the smoothness and range parameters.
    This should get worse as the design becomes more sparse (i.e., “roughness” and “wiggliness” begin to look very similar)
  • Why do we care about the precise number of derivatives?
  • How should we normalize the measurements to make the information values comparable?
  • Role of the measurement error and possible need for a nugget.
    The error variance is the product of the GP variance and a scaling factor.
    Is this choice dictated by modeling considerations of computational convenience?
  • Role of available covariates.
    Do they really need to be measured on a regular grid? What if they are not?
  • Trade off between modeling the mean response and the correlation structure.
    This could also lead to issues with identifiability, necessitating restrictions on the smoothness of the mean function relative to the smoothness of the GP covariance.
  • Computer models output vs. real data
  • Compare the predictive performance to that of a Bayesian method where the range parameter has a full posterior and the smoothness parameter has a plug-in estimate (or is fixed a priori)
  • Need for data analysis.

Swiss Rainfall data

  • Description of the data given in Han and De Oliveira (2024), Bayesian Analysis.
  • “The data consists of daily rainfall totals that felt (sic) in Switzerland on May 8, 1986, measured in 1/10 of a millimeter, and collected over 467 stations with the coordinates of the sampling locations measured in kilometers.”
  • “An exploratory analysis reveals no apparent spatial trend.”
  • The elevation at each location is also available.
  • Issues with the analysis:
  • Does the square root transformation eliminate the need for modeling the mean structure?
  • Is an isotropic covariance structure appropriate?

Rainfall (left) and Altitude (right) 2D

Rainfall 2D

Altitude 2D

Tschubby - Own work- Wikipedia

Adam Peterson - Own work - Wikipedia

Rain vs. Altitude

sqrt(Rain) vs. Altitude

Rainfall 3D by Altitude

Altitude 3D by Rainfall

Moral (in Spain and Switzerland alike!)