seascape models

Universal inference in R

When we perform a statistical test we’d like to have confidence what the Type I error rate is what we say it is (rate of false positive findings). Often scientists choose a rate (AKA ‘alpha’) of 0.05 for rejecting the null hypothesis.

We’d like some guarantees that our statistical test will actually have a type I rate of 5% over many, many repeated trials. For many statistical tests theory does in fact guarantee a 5% error rate over many many repeated trials.

The paper Universal inference from Wasserman et al. published in PNAS proposes a new test that guarantees a given Type I error rate for some types of statistical tests that previously didn’t have an appropriate test.

Here I’ll attempt to code their method in R.

I say ‘attempt’ because I’m not at all sure I interpreted their paper correctly. So please don’t take this blog as a ‘how to’ for the method. Instead, treat it as an exploration of type I and type II errors and how we can check them.

Feel free to email me (chris.brown@griffith.edu.au) if you have suggestions on this blog.

The method is based on likelihood ratio tests, so at the very least we’ll get to learn about LRTs in this blog.

If you are seriously interested in using universal inference, I suggest you consult a statistician.

Testing the null that a mean equals zero

Let’s first test if a some data drawn from a normal distribution are consistent with a null that the mean = 0.

We’ll define the data:

mu <- 0.4
n <- 100
set.seed(42)
y <- rnorm(n, mean = mu)

Now, do a likelihood ratio test the usual way:

(test_stat <- 2*(sum(dnorm(y, mean(y), sd(y), log = TRUE)) - sum(dnorm(y, 0, sd(y), log = TRUE))))

## [1] 17.25054

1 - pchisq(test_stat, 1)

## [1] 3.276042e-05

We just compared the likelihoods of the data given a mean of zero versus the maximum likelihood estimate of the mean (which is just the mean of the data). Then we take that ratio and find its quantile on the chisq, that’s our (very small) p-value = 3.27E-5.

If we were using linear models, we could do the same thing like this:

m1 <- lm(y~1) #intercept only, ie a mean
m0 <- lm(y~0) #no intercept, so mean = -
anova(m0, m1, test = "Chisq")

## Analysis of Variance Table
##
## Model 1: y ~ 0
## Model 2: y ~ 1
##   Res.Df    RSS Df Sum of Sq  Pr(>Chi)    
## 1    100 126.06                           
## 2     99 107.36  1    18.707 3.276e-05 ***
## ---
## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

Split LRT

Wasserman et al. propose a ‘split LRT’ where we split the data in half. This has some pleasing similarities to out of sample validation. As I understand it, this is how we’d do the test:

First split the data at random:

i <- sample(1:n, n/2, replace = FALSE)
y0 <- y[i]
y1 <- y[-i]

Now calculate the MLEs for both splits:

mu0 <- mean(y0)
sd0 <- sd(y0)

mu1 <- mean(y1)
sd1 <- sd(y1)

Then our split test statistic is (note we are taking likelihood of the y0 split split using the y1 split’s mean):

split_test_stat0 <- sum(dnorm(y0, mu1, sd1, log = TRUE)) -
                         sum(dnorm(y0, 0, sd1, log = TRUE))

And we can ask if its significant like this:

exp(split_test_stat0) >= 1/0.05

## [1] TRUE

TRUE, so reject the null, which is the same result as our Chi square test above.

They also propose a cross-fit test, that is just the average of the two split tests:

split_test_stat1 <- sum(dnorm(y1, mu0, sd0, log = TRUE)) -
  sum(dnorm(y1, 0, sd0, log = TRUE))
split_test_stat <- (split_test_stat1 + split_test_stat0)/2
exp(split_test_stat) >= 1/0.05

## [1] TRUE

Power

What about the test’s power? Well it seems a shortcoming is that the split LRT can have lower power (higher type II rate, or chance of missing real differences) than some other tests. So for a simple test like that above we are better of doing the test the regular way.

Let’s check its power for our simple test vs a chisq. I’ll write a function to do this, then iterate it.

splitLRT <- function(seed, n, mu){
  set.seed(seed)
  y <- rnorm(n, mean = mu)
  i <- sample(1:n, n/2, replace = FALSE)

  y0 <- y[i]
  y1 <- y[-i]

  mu0 <- mean(y0)
  sd0 <- sd(y0)

  mu1 <- mean(y1)
  sd1 <- sd(y1)

  #split test stat
  split_test_stat0 <- sum(dnorm(y0, mu1, sd1, log = TRUE)) -
                         sum(dnorm(y0, 0, sd1, log = TRUE))
  split_test_stat1 <- sum(dnorm(y1, mu0, sd0, log = TRUE)) -
  sum(dnorm(y1, 0, sd0, log = TRUE))
  split_test_stat <- (split_test_stat1 + split_test_stat0)/2


  #regular Chisq LRT
  test_stat <- 2*(sum(dnorm(y, mean(y), sd(y), log = TRUE)) - sum(dnorm(y, 0, sd(y), log = TRUE)))
  chisqtest <- 1 - pchisq(test_stat, 1)

  #output results as a dataframe
  data.frame(splitLRT = exp(split_test_stat), chisq = chisqtest)
}

Now let’s use our function:

xout <- lapply(1:1000, splitLRT, n = 50, mu = 0.5)
dfout <- do.call("rbind", xout)
sum(dfout$splitLRT >= (1/0.05))/1000

## [1] 0.415

sum(dfout$chisq <= 0.05)/1000

## [1] 0.932

So the split test only rejects the null 41.5% of the time, whereas the chisq rejects it 93% of the time. In other words the split test comes at the cost of lower power, as is explained in the paper.

It would be worth trying the suggestion in the paper of using k-fold cross-validation to do the splits too, maybe that would improve the power.

For larger sample sizes, the split test method does better:

xout <- lapply(1:1000, splitLRT, n = 150, mu = 0.5)
dfout <- do.call("rbind", xout)
sum(dfout$splitLRT >= (1/0.05))/1000

## [1] 0.974

sum(dfout$chisq <= 0.05)/1000

## [1] 1

What next?

The example I give above is illustrative only, there are theoretical reasons not to use the regular Chi sq test for such a simple model.

It would be interesting to try the split LRT for more sophisticated models where tests don’t currently exist. Wasserman et al. suggest its use for a number of models that ecologists use regularly, including mixing models (where data are generated from a mix of distributions), testing latent variables (such as in the multispecies hierarchical models, like BORAL, that we love here) and for structural equation models.



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