Archive for dead leaves

Familial inference

Posted in Statistics, University life with tags , , , , , , , , , on October 3, 2023 by xi'an

An ISBA-BNP webinar on Wednesday, 4 October, at 17:00 UTC by my friend Steve McEachern:

Familial inference: Tests for hypotheses on a family of centers

Many scientific disciplines face a replicability crisis. While these crises have many drivers, we focus on one. Statistical hypotheses are translations of scientific hypotheses into statements about one or more distributions. The most basic tests focus on the centers of the distributions. Such tests implicitly assume a specific center, e.g., the mean or the median. Yet, scientific hypotheses do not always specify a particular center. This ambiguity leaves a gap between scientific theory and statistical practice that can lead to rejection of a true null. The gap is compounded when we consider deficiencies in the formal statistical model. Rather than testing a single center, we propose testing a family of plausible centers, such as those induced by the Huber loss function (the Huber family). Each center in the family generates a point null hypothesis and the resulting family of hypotheses constitutes a familial null hypothesis. A Bayesian nonparametric procedure is devised to test the familial null. Implementation for the Huber family is facilitated by a novel pathwise optimization routine. Along the way, we visit the question of what it means to be the center of a distribution. The favorable properties of the new test are demonstrated theoretically and in case studies.
This is joint work with Ryan Thompson (University of New South Wales), Catherine Forbes (Monash University), and Mario Peruggia (The Ohio State University).

leaves & masks [cover]

Posted in Books, Kids, pictures, Travel with tags , , , , , , , on December 13, 2022 by xi'an

Le Monde puzzle [#947]

Posted in Books, Kids, pictures, R, Statistics with tags , , , , , , , on February 2, 2016 by xi'an

Another boardgame in Le Monde mathematical puzzle :

Given an 8×8 chequerboard,  consider placing 2×2 tiles over this chequerboard until (a) the entire surface is covered and (b) removing a single 2×2 tile exposes some of the original chequerboard. What is the maximal number of 2×2 tiles one can set according to this scheme? And for a 10×10 chequerboard?

This puzzle reminded me of Wilfrid Kendall’s introduction to perfect sampling  with leaves seen through a ceiling window falling from above, until sky was no longer visible. The representation was used by Wilfrid to explain that coupling from the past did not need to go all the way back to infinity:

Defining a random coverage of the chequerboard by those 2×2 tiles amounts to selecting a random permutation þ of 1:(n-1)² and finding the subvector of þ producing a full coverage

[sourcecode language=”r” gutter=”false”]
grid=matrix(0,n,n)
path=sample(1:(n-1)^2) #random permutation
path=path+((path-1)%/%(n-1)) #account for border shift
i=1
neigh=c(0,1,n,n+1)
while (min(grid)==0){ #all entries covered
grid[path[i]+neigh]=grid[path[i]+neigh]+1
i=i+1
}
i=i-1
[/sourcecode]

Then removing superfluous tiles:

[sourcecode language=”r” gutter=”false”]
for (k in sample(1:i)){
krid=grid
krid[path[k]+neigh]=krid[path[k]+neigh]-1
if (min(krid)>0){ #off used below
off[k]=FALSE; grid=krid} #useless tile
}
[/sourcecode]

And checking the remaining ones are truly essential:

[sourcecode language=”r” gutter=”false”]
mingrid=0
for (k in (1:i)[off]){
krid=grid
krid[path[k]+neigh]=krid[path[k]+neigh]-1
mingrid=max(mingrid,min(krid))
}
sky=(mingrid>0) #rejection of the grid
[/sourcecode]

leads to the maximum number of tiles to be [at least] M=16,25,37,54 for n=6,8,10,12…