Navruz-Özdemir quantile estimator

The Navruz-Özdemir quantile estimator suggests the following equation to estimate the $p^\textrm{th}$ quantile of sample $X$:

where $B_i = B(i; n, p)$ is probability mass function of the binomial distribution $B(n, p)$, $X_{(i)}$ are order statistics of sample $X$.

In this post, I derive these equations following the paper “A new quantile estimator with weights based on a subsampling approach” (2020) by Gözde Navruz and A. Fırat Özdemir. Also, I add some additional explanations, simplify the final equation, and provide reference implementations in C# and R.

Preparation

The first steps exactly match the preparation performed for the Sfakianakis-Verginis quantile estimator. Consider sample $X = \{ X_1, X_2, \ldots, X_n \}$ ($n \geq 3$). Let $X_{(1)}, X_{(2)}, \ldots, X_{(n)}$ be the order statistics of this sample ($X_{(k)}$ is the $k^\textrm{th}$ smallest element). Now let’s build the following intervals:

where $L$ and $U$ are lower and upper bounds for $X$ values ($L$ and $U$ could be equal to $-\infty$ and $\infty$).

We want to estimate $p^\textrm{th}$ quantile $Q_p$. Obviously, $Q_p$ should belong to one of the $S_i$ intervals (because they cover all possible $Q_p$ values). With the help of $Q_p$, we can introduce variables $\delta_i$:

Since $Q_p$ is the $p^\textrm{th}$ quantile, the probability of $(X_i \leq Q_p)$ is $p$ and the probability of $(X_i > Q_p)$ is $1-p$. Thus, $\delta_i$ belongs to the bernoulli distribution: $\delta_i \sim \textrm{Bernoulli}(p)$.

Next, consider the sum of $\delta_i$ values:

Since $\delta_i$ are independent variables from the Bernoulli distribution, their sum belongs to the binomial distribution: $N \sim \textrm{Binomial(n, p)}$. Now we can get the probability of $Q_p \in S_i$:

Let $Q'_{p,i}$ be a point estimator of $Q_p$ conditioned on the event $Q_p \in S_i$. With the help of $Q'_{p,i}$, we could introduce a quantile estimator:

The Navruz-Özdemir quantile estimator

In A new family of nonparametric quantile estimators
By Michael E Sfakianakis, Dimitris G Verginis · 2008sfakianakis2008, Michael E. Sfakianakis and Dimitris G. Verginis described three options to choose $Q'_p$:

Based on these assumptions, they got three different quantile estimators:

In A new quantile estimator with weights based on a subsampling approach
By G"ozde Navruz, A Firat "Ozdemir · 2020navruz2020, Gözde Navruz and A. Fırat Özdemir suggested another way to choose $Q'_{p,i}$:

Also, following A new family of nonparametric quantile estimators
By Michael E Sfakianakis, Dimitris G Verginis · 2008sfakianakis2008, they used the following assumptions for $Q'_{p,0}$ and $Q'_{p,n}$:

Thus, we have:

Let’s also denote $B(i; n, p)$ (the probability mass function os the binomial distribution $B(n, p)$) as $B_i$ to make the equations more readable.

Let’s start to derive the equation for the Navruz-Özdemir quantile estimator. Here is the first step:

It gives us the following expression:

By regrouping the equation members, we get the final equation:

In A new quantile estimator with weights based on a subsampling approach
By G"ozde Navruz, A Firat "Ozdemir · 2020navruz2020, the estimator has slightly different representation:

Reference implementation

If you use R, here is the function that you can use in your scripts:

noquantile <- function(x, probs) {
  n <- length(x)
  if (n <= 2)
    return(quantile(x, probs))
  x <- sort(x)
  sapply(probs, function(p) {
    B <- function(x) dbinom(x, n, p)
    (B(0) * 2 * p + B(1) * p) * x[1] +
      B(0) * (2 - 3 * p) * x[2] -
      B(0) * (1 - p) * x[3] +
      sum(sapply(1:(n-2), function(i)
        (B(i) * (1 - p) + B(i + 1) * p) * x[i + 1])) -
      B(n) * p * x[n - 2] +
      B(n) * (3 * p - 1) * x[n - 1] +
      (B(n - 1) * (1 - p) + B(n) * (2 - 2 * p)) * x[n]
  })
}

If you use C#, you can take an implementation from the latest nightly version (0.3.0-nightly.90+) of Perfolizer (you need NavruzOzdemirQuantileEstimator).

References

• [Sfakianakis2008]
  Sfakianakis, Michael E., and Dimitris G. Verginis. “A new family of nonparametric quantile estimators.” Communications in Statistics—Simulation and Computation® 37, no. 2 (2008): 337-345.
  https://doi.org/10.1080/03610910701790491
• [Navruz2020]
  Navruz, Gözde, and A. Fırat Özdemir. “A new quantile estimator with weights based on a subsampling approach.” British Journal of Mathematical and Statistical Psychology 73, no. 3 (2020): 506-521.
  https://doi.org/10.1111/bmsp.12198