# Understand the equations of quantile regression forest (Meinshausen)?

I am trying to implement a quantile regression forest (https://www.jmlr.org/papers/volume7/meinshausen06a/meinshausen06a.pdf).

But, I have some difficulties to understand how the quantiles are computed. I will try to summarize the part of interest in order to then explain exactly what I don't understand.

Let be $$n$$ independent observations $$(X_i, Y_i)$$. A tree $$T$$ parametrized with a realization $$\theta$$ of a random variable $$\Theta$$ is denoted by $$T(\theta)$$.

• Grow $$k$$ trees $$T(\theta_t)$$, $$t = 1, . . . , k$$, as in random forests. However, for every leaf of every tree, take note of all observations in this leaf, not just their average.
• For a given $$X = x$$, drop $$x$$ down all trees. Compute the weight $$\omega_i(x, \theta_t)$$ of observation $$i \in \{1, . . . , n\}$$ for every tree as in (4). Compute weight $$\omega_i(x)$$ for every observation $$i \in \{1, . . . , n\}$$ as an average over $$\omega_i(x, \theta_t)$$, $$t = 1, . . . , k$$, as in (5).
• Compute the estimate of the distribution function as in (6) for all $$y \in \mathbb{R}$$.

Where the equations (4), (5), (6) are given below.

$$\omega_i(x, \theta_t) = \frac{ 1 \{ X_i \in R(x, \theta_t) \} }{\text{#} \{ j : X_j \in R(x, \theta_t) \} } \ \ \ (4)$$

$$\omega_i(x) = k^{-1} \sum_{t=1}^k \omega_i(x, \theta_t) \ \ \ \ (5)$$

$$\hat{F}(y|X=x) = \sum_{i=1}^n \omega_i (x) 1\{Y_i \leq y\} \ \ \ (6)$$

Where $$R(x, \theta_t)$$ denotes the rectangular area corresponding to the unique leaf of the tree $$T(\theta_t)$$ that $$x$$ belongs to.

I can compute (4) and (5) but I don't understand how to compute (6) and then estimate quantiles. I would also add that I don't know where all observations in leaves (first step of the algorithm) are used.

Can someone give some elements to understand this algorithm ? Any help would be appreciated.

• Hopefully someone here can/will answer you, but you can also email the author of the paper with this question. I'm sure he'd be happy to answer you. If you ask for it, he may even be able to give you source code. Sep 18 '20 at 5:12

It's a good idea to remember what you're trying to predict and that is: $$\mathbb{E}[Y | X=x]$$. The simplest estimate is a single tree:

$$\hat{\mu}(x) = \sum_{i \leq n} w_i(x, \theta) Y_i$$

with:

• $$w_i(x, \theta)$$ the single tree node weights (equation 4 in the paper)
• $$Y_i$$ your observations

As we all know this is not a great estimate (high variance among others) so he defines a better estimator (random forest) as:

$$\hat{\mu}(x) = \sum_{i \leq n} w_i(x) Y_i$$

where the $$w_i(x)$$ are averages over the multiple trees (equation 5). So what does that mean? Your best estimator for $$\mathbb{E}[Y | X=x]$$ is $$\hat{\mu}(x)$$. What do you do if you want an estimation of a function $$\phi$$ of $$Y$$? You just transform your observations of $$Y$$ and use the same formula, i.e.:

$$\hat{\mu_{\phi}}(x) = \sum_{i \leq n} w_i(x) \phi(Y_i)$$

which would then be an estimator of $$\mathbb{E}[\phi(Y)|X=x]$$. If you define $$\phi(Y) = \mathbb{1}_{Y \leq y}$$ then you get that:

$$\hat{\mu_{y}}(x) = \sum_{i \leq n} w_i(x) \mathbb{1}_{Y_i \leq y}$$

is an estimator of $$\mathbb{E}[\mathbb{1}_{Y \leq y}|X=x] = F(y|X=x)$$.

I avoided a lot of low-level details here but to be clear this relates to the delta method i.e. convergence of a function of estimator. This kind of convergence is not trivial to prove (but I guess that's why they wrote a paper about it)