# High dimensional two-sample testing

# Introduction

The main use of density ratio estimation is informative distribution comparison. Informative, because it allows to evaluate how two distributions differ at every region of the space of the data. Consider that we have samples from two, potentially different, distributions, \(p_\text{nu}(x)\) and \(p_\text{de}(x)\). Then, the density ratio between the two distributions is defined as \[ r(x) = \frac{p_\text{nu}(x)}{p_\text{de}(x)}, \] and can take any value between \(0\) and \(\infty\), according to whether the numerator or denominator distribution is larger at location \(x\), which is defined over the multivariate space of the data. Differences between two distributions can be summarized using divergence measures (such as the Pearson or Kullback-Leibler divergence), and these divergences measures can again be used to test the null hypothesis that two distributions are equal. In this vignette, we show how to use density ratio estimation and the `densityratio` package to perform two-sample testing.

# Two-sample testing with density ratios

Consider that we have samples from two \(m\)-dimensional distributions, \(p_\text{nu}(x)\) and \(p_\text{de}(x)\), and we want to test the null hypothesis that the samples come from the same distribution (that is, \(p_\text{nu}(x) = p_\text{de}(x)\)). If we are interested solely in differences in the means of the distributions, we could potentially use a (multivariate) \(t\)-test (e.g., Hotelling’s \(t\)-squared), but this might not be feasible if the variance-covariance matrix is not invertible. However, if we are interested in more general differences between distributions, we would have to settle for a non-parametric test, such as a multivariate extension of the Kolmogorov-Smirnov test. Alternatively, we can use divergence based tests, which are based on some divergence measure (see, e.g., Sugiyama et al., 2011), which can have higher power than the Kolmogorov-Smirnov test (see, e.g., Volker et al., 2023). The `densityratio` package implements multiple divergence based tests (all implemented in `summary()`), dependent on the estimation method: `ulsif()`, `spectral()`, `kmm()`, and `lhss()` use the Pearson divergence, `kliep()` uses the Kullback-Leibler divergence. In this vignette, we use the test implemented in the `spectral()` density ratio estimation method, which is particularly tailored towards high-dimensional data.

The Pearson divergence is defined as \[ \begin{aligned} PE(p_\text{nu}, p_\text{de}) &= \frac 1 2 \int \left( \frac{p_\text{nu}(x)}{p_\text{de}(x)} - 1 \right)^2 p_\text{de}(x) dx \\ &= \frac 1 2 \int r(x) p_\text{nu}(x) dx - \int r(x) p_\text{de} dx + \frac 1 2, \end{aligned} \] and can be interpreted as the expected squared difference of the density ratio from unity, over the denominator distribution. If the two distributions are (almost) equal, the squared deviation will be small, and thus the Pearson divergence will be small. When there are large differences between the two distributions, the squared deviation will be large, and thus the Pearson divergence will be large. Since we do not know the numerator and denominator densities, nor the density ratio, we have to estimate the Pearson divergence from the samples. The density ratio can be estimated using the `spectral()` method (see the Get Started vignette and Izbicki et al., 2014). Subsequently, can estimate the Pearson divergence empirically, by averaging the density ratios over the numerator and denominator samples. That is, we estimate the Pearson divergence as \[ \hat{PE}(p_\text{nu}, p_\text{de}) = \frac{1}{2n_\text{nu}} \sum_{i = 1}^{n_\text{nu}} \hat{r}(x_i^{(\text{nu})}) - \frac{1}{n_\text{de}} \sum_{j = 1}^{n_\text{de}} \hat{r}(x_j^{(\text{de})}) + \frac{1}{2}, \] where \(\hat{r}(x)\) denotes the estimated density ratio at location \(x\), and \(n_\text{nu}\) and \(n_\text{de}\) are the number of samples from the numerator and denominator distributions, respectively.

Finally, we can compare the estimated Pearson divergence to a reference distribution. However, to the best of my knowledge, there is no known reference distribution for the Pearson divergence, and since it is a non-negative quantity, a normal approximation might not be feasible. Therefore, we use a permutation test to obtain a null distribution, as proposed by Sugiyama et al. (2011). That is, we randomly re-allocate the samples from the numerator and denominator distributions, estimate the density ratio function, and compute the Pearson divergence repeatedly, such that we obtain a reference distribution of Pearson divergences under the null hypothesis. Subsequently, we evaluate the probability that the obtained Pearson divergence is larger than the Pearson divergences from the null distribution, which gives rise to a \(p\)-value. This approach achieves nominal type I error control rates, as shown by Volker (2025).

# Empirical example

To illustrate the divergence-based test, we use the `colon` dataset (included in the `densityratio` package), which contains the expression levels of 2000 genes in 22 colon tumor tissues, and 40 non-tumor tissues (Alon et al., 1999). Our goal is to evaluate whether the expression levels of the genes are different for the two groups (over all genes simultaneously).

    library(densityratio)

    numerator <- subset(colon, class == "tumor", select = -class)
    denominator <- subset(colon, class == "normal", select = -class)

    dr <- spectral(numerator, denominator)

    summary(dr, test = TRUE, parallel = TRUE)
    #>
    #> Call:
    #> spectral(df_numerator = numerator, df_denominator = denominator)
    #>
    #> Kernel Information:
    #>   Kernel type: Gaussian with L2 norm distances
    #>   Number of kernels: 40
    #>
    #> Optimal sigma: 110.8282
    #> Optimal subspace: 35
    #> Optimal kernel weights (cv): num [1:35] 1.05247 0.56802 0.00296 0.15847 -0.24053 ...
    #>
    #> Pearson divergence between P(nu) and P(de): 1.972
    #> Pr(P(nu)=P(de)) < .001
    #> Bonferroni-corrected for testing with r(x) = P(nu)/P(de) AND r*(x) = P(de)/P(nu).

The `summary()` function computes the Pearson divergence, and performs a permutation test to evaluate the null hypothesis that the two distributions are equal. In this case, the probability that the samples come from the same distribution is very small, and thus the gene expression levels are different between the two groups. Evaluating which genes are most important for this difference is not straightforward which such high-dimensional data, and would require alternative methods (perhaps dimension reduction before conducting density ratio estimation, or a lasso-type analysis).

# References

Alon, U., Barkai, N., Notterman, D. A., Gish, K., Ybarra, S., Mack, D., & Levine, A. J. (1999). Broad patterns of gene expression revealed by clustering of tumor and normal colon tissues probed by oligonucleotide arrays. Proceedings of the National Academy of Sciences, 96(12), 6745-6750. https://doi.org/10.1073/pnas.96.12.6745

Izbicki, R., Lee, A., & Schafer, C. (2014). High-dimensional density ratio estimation with extensions to approximate likelihood computation. Proceedings of Machine Learning Research, 33, 420-429. https://proceedings.mlr.press/v33/izbicki14.html

Sugiyama, M., Suzuki, T., Itoh, Y., Kanamori, T., & Kimura, M. (2011). Least-squares two-sample test. Neural Networks, 24, 735-751. http://dx.doi.org/10.1016/j.neunet.2011.04.003

Volker, T. B. (2025). Divergence-based testing using density ratio estimation techniques. https://gist.github.com/thomvolker/58197e535ec458752bccbb5b611046ce

Volker, T. B., de Wolf, P.-P., & Van Kesteren, E.-J. (2023). Assessing the utility of synthetic data: A density ratio perspective. UNECE Expert Meeting on Statistical Data Confidentiality. https://doi.org/10.5281/zenodo.8315054