Computing functions of random variables via reproducing kernel Hilbert space representations
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Abstract
We describe a method to perform functional operations on probability distributions of random variables. The method uses reproducing kernel Hilbert space representations of probability distributions, and it is applicable to all operations which can be applied to points drawn from the respective distributions. We refer to our approach as kernel probabilistic programming. We illustrate it on synthetic data and show how it can be used for nonparametric structural equation models, with an application to causal inference.
Keywords
Kernel methods Probabilistic programming Causal inference1 Introduction
Data types, derived structures, and associated operations play a crucial role for programming languages and the computations we carry out using them. Choosing a data type, such as Integer, Float, or String, determines the possible values, as well as the operations that an object permits. Operations typically return their results in the form of data types. Composite or derived data types may be constructed from simpler ones, along with specialized operations applicable to them.

our approach will, in most cases, provide approximate results only; however, we include a statistical analysis of the convergence properties of our approximations, and

for each data type involved (as either input or output), we require a positive definite kernel capturing a notion of similarity between two objects of that type. Some of our results require, moreover, that the kernels be characteristic in the sense that they lead to injective mappings into associated Hilbert spaces.
The remainder of the present article is organized as follows. After describing the necessary preliminaries, we provide an exposition of our approach (Sect. 3). Section 4 analyzes an application to the problem of cause–effect inference using structural equation models. We conclude with a limited set of experimental results.
2 Kernel Maps
2.1 Positive definite kernels
The concept of representing probability distributions in a reproducing kernel Hilbert space (RKHS) has recently attracted attention in statistical inference and machine learning (Berlinet and Agnan 2004; Smola et al. 2007). One of the advantages of this approach is that it allows us to apply RKHS methods to probability distributions, often with strong theoretical guarantees (Sriperumbudur et al. 2008, 2010). It has been applied successfully in many domains such as graphical models (Song et al. 2010, 2011), twosample testing (Gretton et al. 2012), domain adaptation (Huang et al. 2007; Gretton et al. 2009; Muandet et al. 2013), and supervised learning on probability distributions (Muandet et al. 2012; Szabó et al. 2014). We begin by briefly reviewing these methods, starting with some prerequisites.
2.2 Kernel maps for points
2.3 Kernel maps for sets and distributions
What information about a sample \(\mathbf{X}\) does the kernel map \(\mu [\mathbf{X}]\) [see (10)] contain?
\(k(x,x') = \langle x,x'\rangle \)  Mean of \(\mathbf{X}\) 
\(k(x,x') = (\langle x,x'\rangle +1)^n\)  Moments of \(\mathbf{X}\) up to order \(n\in {\mathbb {N}}\) 
\(k(x,x')\) strictly p.d.  All of \(\mathbf{X}\) (i.e., \(\mu \) injective) 
\(k(x,x') = \langle x,x'\rangle \)  Expectation of p 
\(k(x,x') = \big (\langle x,x'\rangle +1\big )^n\)  Moments of p up to order \(n\in {\mathbb {N}}\) 
\(k(x,x')\) characteristic/universal  All of p (i.e., \(\mu \) injective) 
One can show that \(\mu [\hat{p}_m]\) converges at rate \(m^{1/2}\) [cf. Smola et al. (2007), (Song 2008, Theorem 27), and LopezPaz et al. (2015)]:
Theorem 1
This shows that the map is not a oneway road: we can map our objects of interest into the RKHS, perform linear algebra and geometry on them (Schölkopf and Smola 2002), and at the end answer questions of interest. In the next section, we shall take this a step further, and discuss how to implement rather general operations in the RKHS.
Before doing so, we mention two additional applications of kernel maps. We can map conditional distributions and perform Bayesian updates (Fukumizu et al. 2008, 2013; Zhang et al. 2011), and we can connect kernel maps to Fourier optics, leading to a physical realization as Fraunhofer diffraction (Harmeling et al. 2013).
3 Computing functions of independent random variables
3.1 Introduction and earlier work
A random variable (RV) is a measurable function mapping possible outcomes of an underlying random experiment to a set \({\mathcal {Z}}\) (often, \({\mathcal {Z}}\subset {\mathbb {R}}^d\), but our approach will be more general). The probability measure of the random experiment induces the distribution of the random variable. We will below not deal with the underlying probability space explicitly, and instead directly start from random variables X, Y with distributions \(p_X,p_Y\) and values in \({\fancyscript{X}},{\fancyscript{Y}}\). Suppose we have access to (data from) \(p_X\) and \(p_Y\), and we want to compute the distribution of the random variable f(X, Y), where f is a measurable function defined on \({\fancyscript{X}}\times {\fancyscript{Y}}\).
For instance, if our operation is addition \(f(X,Y)=X+Y\), and the distributions \(p_X\) and \(p_Y\) have densities, we can compute the density of the distribution of f(X, Y) by convolving those densities. If the distributions of X and Y belong to some parametric class, such as a class of distributions with Gaussian density functions, and if the arithmetic expression is elementary, then closedform solutions for certain favorable combinations exist. At the other end of the spectrum, we can resort to numerical integration or sampling to approximate f(X, Y).
Arithmetic operations on random variables are abundant in science and engineering. Measurements in realworld systems are subject to uncertainty, and thus subsequent arithmetic operations on these measurements are operations on random variables. An example due to Springer (1979) is signal amplification. Consider a set of n amplifiers connected together in a serial fashion. If the amplification of the ith amplifier is denoted by \(X_i\), then the total amplification, denoted by Y, is \(Y=X_1\cdot X_2\cdots X_n\), i.e., a product of n random variables.
A wellestablished framework for arithmetic operation on independent random variables (iRVs) relies on integral transform methods (Springer 1979). The above example of addition already suggests that Fourier transforms may help, and indeed, people have used transforms such as the ones due to Fourier and Mellin to derive the distribution function of either the sum, difference, product, or quotient of iRVs (Epstein 1948; Springer and Thompson 1966; Prasad 1970; Springer 1979). Williamson (1989) proposes an approximation using Laguerre polynomials and a notion of envelopes bounding the cumulative distribution function. This framework also allows for the treatment of dependent random variables, but the bounds can become very loose after repeated operations. Milios (2009) approximates the probability distributions of the input random variables as mixture models (using uniform and Gaussian distributions), and apply the computations to all mixture components.
Jaroszewicz and Korzen (2012) consider a numerical approach to implement arithmetic operations on iRVs, representing the distributions using piecewise Chebyshev approximations. This lends itself well to the use of approximation methods that perform well as long as the functions are wellbehaved. Finally, Monte Carlo approaches can be used as well and are popular in scientific applications [see e.g., Ferson (1996)].
The goal of the present paper is to develop a derived data type representing a distribution over another data type and to generalize the available computational operations to this data type, at least approximately. This would allow us to conveniently handle error propagation as in the example discussed earlier. It would also help us perform inference involving conditional distributions of such variables given observed data. The latter is the main topic of a subject area that has recently begun to attract attention, probabilistic programming (Gordon et al. 2014). A variety of probabilistic programming languages has been proposed (Wood et al. 2014; Paige and Wood 2014; Cassel 2014). To emphasize the central role that kernel maps play in our approach, we refer to it as kernel probabilistic programming (KPP).
3.2 Computing functions of independent random variables using kernel maps
The key idea of KPP is to provide a consistent estimator of the kernel map of an expression involving operations on random variables. This is done by applying the expression to the sample points and showing that the resulting kernel expansion has the desired property. Operations involving more than one RV will increase the size of the expansion, but we can resort to existing RKHS approximation methods to keep the complexity of the resulting expansion limited, which is advisable in particular if we want to use it as a basis for further operations. The benefits of KPP are threefold. First, we do not make parametric assumptions on the distributions associated with the random variables. Second, our approach applies not only to realvalued random variables, but also to multivariate random variables, structured data, functional data, and other domains, as long as positive definite kernels can be defined on the data. Finally, it does not require explicit density estimation as an intermediate step, which is difficult in high dimensions.
Theorem 2
As an aside, note that (23) is an RKHSvalued twosample Ustatistic.
3.3 Approximating expansions
To keep computational cost limited, we need to use approximations when performing multistep operations. If for instance, the outcome of the first step takes the form (23), then we already have \(m\,\times \, n\) terms, and subsequent steps would further increase the number of terms, thus quickly becoming computationally prohibitive.
We can do so by using the methods described in Chap. 18 of Schölkopf and Smola (2002). They fall in two categories. In reduced set selection methods, we provide a set of expansion points [e.g., all points \(f(x_i,y_j)\) in (23)], and the approximation method sparsifies the vector of expansion coefficients. This can be for instance done by solving eigenvalue problems or linear programs. Reduced set construction methods, on the other hand, construct new expansion points. In the simplest case, they proceed by sequentially finding approximate preimages of RKHS elements. They tend to be computationally more demanding and suffer from local minima; however, they can lead to sparser expansions.
3.4 Computing functions of RKHS approximations
One of the advantages of our approach is that (23) and (28) apply for general data types. In other words, \({\fancyscript{X}},{\fancyscript{Y}},{\fancyscript{Z}}\) need not be vector spaces—they may be arbitrary nonempty sets, as long as positive definite kernels can be defined on them.
3.4.1 Convergence analysis in an idealized setting
We analyze the convergence of (28) under the assumption that the expansion points are actually samples \(x_1,\dots ,x_m\) from X and \(y_1,\dots ,y_n\) from Y, which is for instance the case if the expansions (27) are the result of reduced set selection methods (cf. Sect. 3.3). Moreover, we assume that the expansion coefficients \(\alpha _1,\dots ,\alpha _m\) and \(\beta _1,\dots ,\beta _n\) are constants, i.e., independent of the samples.
The following proposition gives a sufficient condition for the approximations in (27) to converge. Note that below, the coefficients \(\alpha _1,\dots ,\alpha _m\) depend on the sample size m, but for simplicity we refrain from writing them as \(\alpha _{1,m},\dots ,\alpha _{m,m}\); and likewise, for \(\beta _1,\dots ,\beta _n\). We make this remark to ensure that readers are not puzzled by the below statement that \(\sum _{i=1}^m \alpha _i^2 \rightarrow 0\) as \(m\rightarrow \infty \).
Proposition 1
Proof
The next result shows that if our approximations (27) converge in the sense of Proposition 1, then the estimator (28) (with expansion coefficients summing to 1) is consistent.
Theorem 3
Proof
Note that in the simplest case, where \(\alpha _i=1/m\) and \(\beta _j=1/n\), we have \(\sum _i\alpha _i^2=1/m\) and \(\sum _j\beta _j^2 = 1/n\), which proves Theorem 2. It is also easy to see from the proof that we do not strictly need \(\sum _i\alpha _i=\sum _j\beta _j=1\)—for the estimator to be consistent, it suffices if the sums converge to 1. For a sufficient condition for this convergence, see Kanagawa and Fukumizu (2014).
3.4.2 More general expansion sets
To conclude our discussion of the estimator (28), we turn to the case where the expansions (27) are computed by reduced set construction, i.e., they are not necessarily expressed in terms of samples from X and Y. This is more difficult, and we do not provide a formal result, but just a qualitative discussion.
We assume that (27) are good approximations of the kernel means of two unknown random variables X and Y; we also assume that f and the kernel mean map along with its inverse are continuous. We have no samples from X and Y, but we can turn (27) into sample estimates based on artificial samples \(\mathbf{X}\) and \(\mathbf{Y}\), for which we can then appeal to our estimator from Theorem 2.
To this end, denote by \(\mathbf{X}' =( x'_1,\ldots ,x'_{m'})\) and \(\mathbf{Y}'=(y'_1,\ldots ,y'_{n'})\) the expansion points in (27). We construct a sample \(\mathbf{X}=( x_1,x_2,\dots )\) whose kernel mean is close to \(\sum _{i=1}^{m'}\alpha _i\varPhi _x(x'_i)\) as follows: for each i, the point \(x'_i\) appears in \(\mathbf{X}\) with multiplicity \(\lfloor m\cdot \alpha _i \rfloor \), i.e., the largest integer not exceeding \(m\cdot \alpha _i\). This leads to a sample of size at most m. Note, moreover, that the multiplicity of \(x'_i\), divided by m, differs from \(\alpha _i\) by at most 1 / m, so effectively we have quantized the \(\alpha _i\) coefficients to this accuracy.
3.4.3 Larger sets of random variables
As above, we note that (i) g need not be realvalued, it can take values in some set \({\fancyscript{Z}}\) for which we have a (possibly characteristic) positive definite kernel; (ii) we can extend this to general kernel expansions like (28); and (iii) if we use Gaussian kernels with width tending to 0, we can think of the above as a sampling method.
4 Dependent RVs and structural equation models
For dependent RVs, the proposed estimators are not applicable. One way to handle dependent RVs is to appeal to the fact that any joint distribution of random variables can be written as a structural equation model with independent noises. This leads to an interesting application of our method to the field of causal inference.
We consider a model \(X_i = f_i ( \text {PA}_i , U_i )\), for \(i=1,\dots ,p\), with jointly independent noise terms \(U_1,\dots ,U_p\). Such models arise for instance in causal inference (Pearl 2009). Each random variable \(X_i\) is computed as a function \(f_i\) of its noise term \(U_i\) and its parents \(\text {PA}_i\) in an underlying directed acyclic graph (DAG). Every graphical model w.r.t. a DAG can be expressed as such a structural equation model with suitable functions and noise terms [e.g., Peters et al. (2014)].
In practice, we would apply the following procedure: we are given a sample \((x_1,y_1),\dots ,(x_m,y_m)\). We estimate the function f as well as the residual noise terms \(u_1,\dots ,u_m\) by regression, and likewise for the backward function g (Peters et al. 2014). Strictly speaking, we need to use separate subsamples to estimate function and noise terms, respectively, see Kpotufe et al. (2014).
Below, we show that \(\Delta _{emp}\) converges to 0 for additive noise models (36). For the incorrect model (37), however, \(\Delta _{emp}^{bw}\) will in the generic case not converge to zero. We can thus use the comparison of both values for deciding causal direction.
Theorem 4
Proof
Equation (40) follows from Theorem 2 since \(\Big \Vert \frac{1}{m}\sum _{i=1}^m \varPhi (y_i)  \mu [Y]\Big \Vert \rightarrow 0\) and \( \Big \Vert \frac{1}{m^2}\sum _{i,j=1}^{m^2} \varPhi (f(x_i)+u_j)  \mu [Y]\Big \Vert \rightarrow 0 \) (all convergences in this proof are in probability).
As an aside, note that Theorem 4 would not hold if in (39) we were to estimate \(\mu \big [g(Y)+V\big ]\) by \(\frac{1}{m} \sum _{i=1}^m \varPhi \big (g(y_i)+v_i\big )\) instead of \(\frac{1}{m^2} \sum _{i,j=1}^m \varPhi \big (g(y_i)+v_j\big )\).
5 Experiments
5.1 Synthetic data
We consider basic arithmetic expressions that involve multiplication \(X \,\times \, Y\), division X / Y, and exponentiation \(X^Y\) on two independent scalar RVs X and Y. Letting \(p_X = {\mathcal {N}}(3,0.5)\) and \(p_Y = {\mathcal {N}}(4,0.5)\), we draw i.i.d. samples \({\mathbf {X}}=\{x_1,\ldots ,x_m\}\) and \({\mathbf {Y}}=\{y_1,\ldots ,y_m\}\) from \(p_X\) and \(p_Y\).
In the experiment, we are interested in the convergence (in RKHS norm) of our estimators to \(\mu \big [f(X,Y)\big ]\). Since we do not have access to the latter, we use an independent sample to construct a proxy \(\hat{\mu }\big [f(X,Y)\big ] = (1/\ell ^2)\sum _{i,j=1}^{\ell }\varPhi _z\big (f(x_i,y_j)\big )\). We found that \(\ell =100\) led to a sufficiently good approximation.
 1.
The samplebased estimator (23).
 2.
The estimator (28) based on approximations of the kernel means, taking the form \(\hat{\mu }[X] := \sum _{i=1}^{m'}\alpha _i\varPhi _x(x_i)\) and \(\hat{\mu }[Y] := \sum _{j=1}^{m'}\beta _j\varPhi _y(y_j)\) of \(\mu [X]\) and \(\mu [Y]\), respectively. We used the simplest possible reduced set selection method: we randomly subsampled subsets of size \(m'\approx 0.4\cdot m\) from \({\mathbf {X}}\) and \({\mathbf {Y}}\), and optimized the coefficients \(\{\alpha _1,\ldots ,\alpha _{m'}\}\) and \(\{\beta _1,\ldots ,\beta _{m'}\}\) to best approximate the original kernel means (based on \({\mathbf {X}}\) and \({\mathbf {Y}}\)) in the RKHS norm (Schölkopf and Smola 2002, Sect. 18.3).
 3.
Analogously to the case of one variable (17), we may also look at the estimator \(\hat{\mu }_3[\mathbf{X},\mathbf{Y}] := (1/m)\sum _{i=1}^m\varPhi _z\big (f(x_i,y_i)\big )\), which sums only over m mutually independent terms, i.e., a small fraction of all terms of (23).
5.2 Causal discovery via functions of kernel means
We also apply our KPP approach to bivariate causal inference problem (cf. Sect. 4). That is, given a pair of realvalued random variables X and Y with joint distribution \(p_{XY}\), we are interested in identifying whether X causes Y (denote as \(X\rightarrow Y\)) or Y causes X (denote as \(Y\rightarrow X\)) based on observational data. We assume an additive noise model \(E = f(C) \,+\, U\) with \(C\perp \perp U\) where C, E, and U denote cause, effect, and residual (or “unexplained”) variable, respectively. Below we present a preliminary result on the CauseEffectPairs benchmark data set (Peters et al. 2014).
Figure 2b depicts the accuracy versus the decision rate for the 81 pairs in the CauseEffectPairs benchmark collection. The method achieves an accuracy of 80 %, which is significantly better than random guessing, when forced to infer the causal direction of all 81 pairs.
6 Conclusions
We have developed a kernelbased approach to compute functional operations on random variables taking values in arbitrary domains. We have proposed estimators for RKHS representations of those quantities, evaluated the approach on synthetic data, and showed how it can be used for cause–effect inference. While the results are encouraging, the material presented in this article only describes the main ideas, and much remains to be done. We believe that there is significant potential for a unified perspective on probabilistic programming based on the described methods, and hope that some of the open problems will be addressed in future work.
Footnotes
Notes
Acknowledgments
Thanks to Dominik Janzing, Le Song, and Ilya Tolstikhin for dicussions and comments.
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