Alkis Kalavasis

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31ranked-venue papers
10as first author
30since 2021 · last 2026
0000-0002-1479-5268ORCID · corroborated

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Artificial intelligence and machine learning · 22 · 8 first-author · 21 since 2021Theory of computation · 9 · 2 first-author · 9 since 2021
YearPublicationVenuePosition
2026 DDPM Score Matching and Distribution Learning (Extended Abstract)
abstract
Score estimation is the backbone of score-based generative models (SGMs), and particularly denoising diffusion probabilistic models (DDPMs). A fundamental theoretical result in this area is that, given access to accurate score estimates, SGMs can efficiently generate from any realistic data distribution (Chen, Chewi, Li, Li, Salim, and Zhang, ICLR’23; Lee, Lu, and Tan, ALT’23). This can be viewed as a result on distribution learning, where the learned distribution is implicit as the law of the output of a sampler. However, it is unclear how score estimation relates to more classical forms of distribution learning, such as parameter estimation and density estimation. We present a framework reducing the other two forms of distribution learning to score estimation, which has various implications in statistical and computational learning theory: parameter estimation, where denoising score matching in DDPMs is asymptotically efficient; density estimation, where estimated scores can be lifted to a $(\epsilon,\delta)$-PAC density estimator and yield minimax rates over Hölder classes and a quasi-polynomial PAC density estimation algorithm for Gaussian location mixtures; and lower bounds for score estimation, where PAC density estimation yields computational lower bounds for score estimation of general distribution families and cryptographic lower bounds for score estimation of general Gaussian mixture models.
Sinho Chewi, Alkis Kalavasis, Anay Mehrotra, Omar Montasser
COLT2
2026 Can SGD Select Good Fishermen? Local Convergence under Self-Selection Biases (Extended Abstract)
abstract
We revisit the problem of estimating $k$ linear regressors in $d$ dimensions from samples affected by self-selection bias under the maximum selection rule. Our main result is an algorithm with sample complexity $O(d)\cdot \operatorname{poly}(k,1/\varepsilon)$ and running time $\operatorname{poly}(d,k,1/\varepsilon)+(k\log k)^{O(k)}$ for recovering the regressors up to joint squared error $\varepsilon^2$. The key ingredient is the first local-convergence algorithm for the maximum self-selection model. Our approach reduces self-selection to estimation from coarse observations, where the learner observes only the cell of a partition containing the latent sample. The self-selection reduction induces a structured non-convex partition. We prove that this partition preserves enough information locally and that the resulting negative log-likelihood is locally convex around the true parameters. These two geometric properties allow projected stochastic gradient descent, initialized from an existing warm start, to obtain the stated end-to-end guarantee.
Alkis Kalavasis, Anay Mehrotra, Felix Zhou 0002
COLT1
2026 On the Learning Curves of Revenue Maximization
abstract
Learning curves are a fundamental primitive in supervised learning, describing how an algorithm’s performance improves with more data and providing a quantitative measure of its generalization ability. Formally, a learning curve plots the decay of an algorithm’s error for a fixed underlying distribution as a function of the number of training samples. Prior work on revenue-maximizing learning algorithms, starting with the seminal work of Cole and Roughgarden, adopts a distribution-free perspective (which parallels the PAC learning framework in learning theory). This approach evaluates performance against the hardest possible sequence of valuation distributions, one for each sample size, effectively defining the upper envelope of learning curves over all possible distributions, thus leading to error bounds that do not capture the shape of the learning curves.
Steve Hanneke, Alkis Kalavasis, Shay Moran, Grigoris Velegkas
STOC2
2026 Learning Mixture Models via Efficient High-Dimensional Sparse Fourier Transforms
abstract
In this work, we give a poly(d,k) time and sample algorithm for efficiently learning the parameters (i.e., the means and the mixture weights) of a mixture of k spherical distributions in d dimensions. Unlike all previous methods, our techniques apply to heavy-tailed distributions and include examples that do not even have finite covariances. Our method succeeds whenever the component distributions have a characteristic function with sufficiently heavy tails. Examples of such distributions include the Laplace distribution and uniform over [−1, 1] but crucially exclude Gaussians.
Alkis Kalavasis, Pravesh Kothari, Shuchen Li, Manolis Zampetakis
STOC1
2025 What Makes Treatment Effects Identifiable? Characterizations and Estimators Beyond Unconfoundedness (Extended Abstract)
abstract
Most of the widely used estimators of the \emph{average treatment effect} (ATE) in causal inference rely on the assumptions of \emph{unconfoundedness} and \emph{overlap}. Unconfoundedness requires that the observed covariates account for all correlations between the outcome and treatment. Overlap requires the existence of randomness in treatment decisions for all individuals. Nevertheless, many types of studies frequently violate unconfoundedness or overlap, for instance, observational studies with deterministic treatment decisions - popularly known as Regression Discontinuity designs - violate overlap. In this paper, we initiate the study of general conditions that enable the \emph{identification} of the average treatment effect, extending beyond unconfoundedness and overlap. In particular, following the paradigm of {statistical} learning theory, we provide an interpretable condition that is sufficient and nearly necessary for the identification of ATE. Moreover, this condition characterizes the identification of the \emph{average treatment effect on the treated} (ATT) and can be used to characterize other treatment effects as well. To illustrate the utility of our condition, we present several well-studied scenarios where our condition is satisfied and, hence, we prove that ATE can be identified in regimes that prior works could not capture. For example, under mild assumptions on the data distributions, this holds for the models proposed by Tan (2006) and Rosenbaum (2002), and the Regression Discontinuity design model introduced by Thistlethwaite and Campbell (1960). For each of these scenarios, we also show that, under natural additional assumptions, ATE can be estimated from finite samples. We believe these findings open new avenues for bridging learning-theoretic insights and causal inference methodologies, particularly in observational studies with complex treatment mechanisms.
Yang Cai 0001, Alkis Kalavasis, Katerina Mamali, Anay Mehrotra, Manolis Zampetakis
COLT2
2025 Does Generation Require Memorization? Creative Diffusion Models using Ambient Diffusion
abstract
There is strong empirical evidence that the stateof-the-art diffusion modeling paradigm leads to models that memorize the training set, especially when the training set is small. Prior methods to mitigate the memorization problem often lead to decrease in image quality. Is it possible to obtain strong and creative generative models, i.e., models that achieve high generation quality and low memorization? Despite the current pessimistic landscape of results, we make significant progress in pushing the trade-off between fidelity and memorization. We first provide theoretical evidence that memorization in diffusion models is only necessary for denoising problems at low noise scales (usually used in generating high-frequency details). Using this theoretical insight, we propose a simple, principled method to train the diffusion models using noisy data at large noise scales. We show that our method significantly reduces memorization without decreasing the image quality, for both text-conditional and unconditional models and for a variety of data availability settings.
Kulin Shah, Alkis Kalavasis, Adam R. Klivans, Giannis Daras
ICML2
2025 Computational Lower Bounds for No-Regret Learning in Normal-Form Games
Ioannis Anagnostides, Alkis Kalavasis, Tuomas Sandholm
STOC2
2025 On the Limits of Language Generation: Trade-Offs between Hallucination and Mode-Collapse
Alkis Kalavasis, Anay Mehrotra, Grigoris Velegkas
STOC1
2025 Label Ranking Through Nonparametric Regression
abstract
Abstract Label Ranking (LR) corresponds to the problem of learning a hypothesis that maps features to rankings over a finite set of labels. We adopt a nonparametric regression approach to LR and obtain theoretical performance guarantees for this fundamental practical problem. We introduce a generative model for Label Ranking, in noiseless and noisy nonparametric regression settings. In the noiseless setting, we focus on the computational aspects of the LR problem with full rankings and provide guarantees for time-efficient learning algorithms using decision trees and random forests in the high-dimensional regime. In the noisy setting, we consider the more general cases of LR with incomplete rankings from a statistical viewpoint and obtain sample complexity bounds using the One-Versus-One approach of multiclass classification. Lastly, we complement our theoretical contributions with experiments, aiming to understand how the input regression noise affects the observed output.
Dimitris Fotakis 0001, Alkis Kalavasis, Eleni Psaroudaki
Theory Comput. Syst.2
2024 On Sampling from Ising Models with Spectral Constraints
abstract
We consider the problem of sampling from the Ising model when the underlying interaction matrix has eigenvalues lying within an interval of length $γ$. Recent work in this setting has shown various algorithmic results that apply roughly when $γ< 1$, notably with nearly-linear running times based on the classical Glauber dynamics. However, the optimality of the range of $γ$ was not clear since previous inapproximability results developed for the antiferromagnetic case (where the matrix has entries $\leq 0$) apply only for $γ>2$. To this end, Kunisky (SODA'24) recently provided evidence that the problem becomes hard already when $γ>1$ based on the low-degree hardness for an inference problem on random matrices. Based on this, he conjectured that sampling from the Ising model in the same range of $γ$ is NP-hard. Here we confirm this conjecture, complementing in particular the known algorithmic results by showing NP-hardness results for approximately counting and sampling when $γ>1$, with strong inapproximability guarantees; we also obtain a more refined hardness result for matrices where only a constant number of entries per row are allowed to be non-zero. The main observation in our reductions is that, for $γ>1$, Glauber dynamics mixes slowly when the interactions are all positive (ferromagnetic) for the complete and random regular graphs, due to a bimodality in the underlying distribution. While ferromagnetic interactions typically preclude NP-hardness results, here we work around this by introducing in an appropriate way mild antiferromagnetism, keeping the spectrum roughly within the same range. This allows us to exploit the bimodality of the aforementioned graphs and show the target NP-hardness by adapting suitably previous inapproximability techniques developed for antiferromagnetic systems.
Andreas Galanis, Alkis Kalavasis, Anthimos Vardis Kandiros
APPROX/RANDOM2
2024 Universal Rates for Regression: Separations between Cut-Off and Absolute Loss
abstract
In this work we initiate the study of regression in the universal rates framework of Bousquet et al. Unlike the traditional uniform learning setting, we are interested in obtaining learning guarantees that hold for all fixed data-generating distributions, but do not hold uniformly across them. We focus on the realizable setting and we consider two different well-studied loss functions: the cut-off loss at scale $\gamma > 0$, which asks for predictions that are $\gamma$-close to the correct one, and the absolute loss, which measures how far away the prediction is from the correct one. Our results show that the landscape of the achievable rates in the two cases is completely different. First we give a trichotomic characterization of the optimal learning rates under the cut-off loss: each class is learnable either at an exponential rate, a (nearly) linear rate or requires arbitrarily slow rates. Moving to the absolute loss, we show that the achievable learning rates are significantly more involved by illustrating that an infinite number of different optimal learning rates is achievable. This is the first time that such a rich landscape of rates is obtained in the universal rates literature.
Idan Attias, Steve Hanneke, Alkis Kalavasis, Amin Karbasi, Grigoris Velegkas
COLT3
2024 Smaller Confidence Intervals From IPW Estimators via Data-Dependent Coarsening (Extended Abstract)
abstract
Inverse propensity-score weighted (IPW) estimators are prevalent in causal inference for estimating average treatment effects in observational studies. Under unconfoundedness, given accurate propensity scores and $n$ samples, the size of confidence intervals of IPW estimators scales down with $n$, and, several of their variants improve the rate of scaling. However, neither IPW estimators nor their variants are robust to inaccuracies: even if a single covariate has an $\epsilon>0$ additive error in the propensity score, the size of confidence intervals of these estimators can increase arbitrarily. Moreover, even without errors, the rate with which the confidence intervals of these estimators go to zero with $n$ can be arbitrarily slow in the presence of extreme propensity scores (those close to 0 or 1). We introduce a family of Coarse IPW (CIPW) estimators that captures existing IPW estimators and their variants. Each CIPW estimator is an IPW estimator on a coarsened covariate space, where certain covariates are merged. Under mild assumptions, e.g., Lipschitzness in expected outcomes and sparsity of extreme propensity scores, we give an efficient algorithm to find a robust estimator: given $\epsilon$-inaccurate propensity scores and $n$ samples, its confidence interval size scales with $\epsilon+(1/\sqrt{n})$. In contrast, under the same assumptions, existing estimators’ confidence interval sizes are $\Omega(1)$ irrespective of $\epsilon$ and $n$. Crucially, our estimator is data-dependent and we show that no data-independent CIPW estimator can be robust to inaccuracies.
Alkis Kalavasis, Anay Mehrotra, Manolis Zampetakis
COLT1
2024 Replicable Learning of Large-Margin Halfspaces
abstract
We provide an efficient replicable algorithm for the problem of learning large-margin halfspaces. Our results improve upon the algorithms provided by Impagliazzo, Lei, Pitassi, and Sorrell (STOC, 2022). We design the first dimension-independent replicable algorithm for this task which runs in polynomial time, is proper, and has strictly improved sample complexity compared to the one achieved by Impagliazzo et al. (STOC, 2022) with respect to all the relevant parameters. Moreover, our algorithm has sample complexity that is optimal with respect to the accuracy parameter $\epsilon$. Departing from the requirement of polynomial time algorithms, using the DP-to-Replicability reduction of Bun et al. (STOC 2023), we show how to obtain a replicable algorithm for large-margin halfspaces with improved sample complexity with respect to the margin parameter $\tau$, but running time doubly exponential in $1/\tau^2$ and worse sample complexity dependence on $\epsilon$ than our previous algorithm. We then design an improved algorithm with better sample complexity than both of our previous algorithms and running time exponential in $1/\tau^{2}.$
Alkis Kalavasis, Amin Karbasi, Kasper Green Larsen, Grigoris Velegkas, Felix Zhou 0002
ICML1
2024 On the Complexity of Computing Sparse Equilibria and Lower Bounds for No-Regret Learning in Games
Ioannis Anagnostides, Alkis Kalavasis, Tuomas Sandholm, Manolis Zampetakis
ITCS2
2024 Injecting Undetectable Backdoors in Obfuscated Neural Networks and Language Models
abstract
As ML models become increasingly complex and integral to high-stakes domains such as finance and healthcare, they also become more susceptible to sophisticated adversarial attacks. We investigate the threat posed by undetectable backdoors, as defined in Goldwasser et al. [2022], in models developed by insidious external expert firms. When such backdoors exist, they allow the designer of the model to sell information on how to slightly perturb their input to change the outcome of the model. We develop a general strategy to plant backdoors to obfuscated neural networks, that satisfy the security properties of the celebrated notion of indistinguishability obfuscation. Applying obfuscation before releasing neural networks is a strategy that is well motivated to protect sensitive information of the external expert firm. Our method to plant backdoors ensures that even if the weights and architecture of the obfuscated model are accessible, the existence of the backdoor is still undetectable. Finally, we introduce the notion of undetectable backdoors to language models and extend our neural network backdoor attacks to such models based on the existence of steganographic functions.
Alkis Kalavasis, Amin Karbasi, Argyris Oikonomou, Katerina Sotiraki, Grigoris Velegkas, Manolis Zampetakis
NeurIPS1
2024 On the Computational Landscape of Replicable Learning
abstract
We study computational aspects of algorithmic replicability, a notion of stability introduced by Impagliazzo, Lei, Pitassi, and Sorrell [STOC, 2022]. Motivated by a recent line of work that established strong statistical connections between replicability and other notions of learnability such as online learning, private learning, and SQ learning, we aim to understand better the computational connections between replicability and these learning paradigms. Our first result shows that there is a concept class that is efficiently replicably PAC learnable, but, under standard cryptographic assumptions, no efficient online learner exists for this class. Subsequently, we design an efficient replicable learner for PAC learning parities when the marginal distribution is far from uniform, making progress on a question posed by Impagliazzo et al. [STOC, 2022]. To obtain this result, we design a replicable lifting framework inspired by Blanc, Lange, Malik, and Tan [STOC, 2023], that transforms in a black-box manner efficient replicable PAC learners under the uniform marginal distribution over the Boolean hypercube to replicable PAC learners under any marginal distribution, with sample and time complexity that depends on a certain measure of the complexity of the distribution. Finally, we show that any pure DP learner can be transformed in a black-box manner to a replicable learner, with time complexity polynomial in the confidence and accuracy parameters, but exponential in the representation dimension of the underlying hypothesis class.
Alkis Kalavasis, Amin Karbasi, Grigoris Velegkas, Felix Zhou 0002
NeurIPS1
2024 Learning Hard-Constrained Models with One Sample
abstract
We consider the problem of estimating the parameters of a Markov Random Field with hard-constraints using a single sample. As our main running examples, we use the k-SAT and the proper coloring models, as well as general H-coloring models; for all of these we obtain both positive and negative results. In contrast to the soft-constrained case, we show in particular that single-sample estimation is not always possible, and that the existence of an estimator is related to the existence of non-satisfiable instances.
Andreas Galanis, Alkis Kalavasis, Anthimos Vardis Kandiros
SODA2
2023 Replicable Bandits
Hossein Esfandiari, Alkis Kalavasis, Amin Karbasi, Andreas Krause 0001, Vahab S. Mirrokni, Grigoris Velegkas
ICLR2
2023 Statistical Indistinguishability of Learning Algorithms
abstract
When two different parties use the same learning rule on their own data, how can we test whether the distributions of the two outcomes are similar? In this paper, we study the similarity of outcomes of learning rules through the lens of the Total Variation (TV) distance of distributions. We say that a learning rule is TV indistinguishable if the expected TV distance between the posterior distributions of its outputs, executed on two training data sets drawn independently from the same distribution, is small. We first investigate the learnability of hypothesis classes using TV indistinguishable learners. Our main results are information-theoretic equivalences between TV indistinguishability and existing algorithmic stability notions such as replicability and approximate differential privacy. Then, we provide statistical amplification and boosting algorithms for TV indistinguishable learners.
Alkis Kalavasis, Amin Karbasi, Shay Moran, Grigoris Velegkas
ICML1
2023 Optimal Learners for Realizable Regression: PAC Learning and Online Learning
abstract
In this work, we aim to characterize the statistical complexity of realizable regression both in the PAC learning setting and the online learning setting. Previous work had established the sufficiency of finiteness of the fat shattering dimension for PAC learnability and the necessity of finiteness of the scaled Natarajan dimension, but little progress had been made towards a more complete characterization since the work of Simon 1997 (SICOMP '97). To this end, we first introduce a minimax instance optimal learner for realizable regression and propose a novel dimension that both qualitatively and quantitatively characterizes which classes of real-valued predictors are learnable. We then identify a combinatorial dimension related to the graph dimension that characterizes ERM learnability in the realizable setting. Finally, we establish a necessary condition for learnability based on a combinatorial dimension related to the DS dimension, and conjecture that it may also be sufficient in this context. Additionally, in the context of online learning we provide a dimension that characterizes the minimax instance optimal cumulative loss up to a constant factor and design an optimal online learner for realizable regression, thus resolving an open question raised by Daskalakis and Golowich in STOC '22.
Idan Attias, Steve Hanneke, Alkis Kalavasis, Amin Karbasi, Grigoris Velegkas
NeurIPS3
2023 Optimizing Solution-Samplers for Combinatorial Problems: The Landscape of Policy-Gradient Method
abstract
Deep Neural Networks and Reinforcement Learning methods have empirically shown great promise in tackling challenging combinatorial problems. In those methods a deep neural network is used as a solution generator which is then trained by gradient-based methods (e.g., policy gradient) to successively obtain better solution distributions. In this work we introduce a novel theoretical framework for analyzing the effectiveness of such methods. We ask whether there exist generative models that (i) are expressive enough to generate approximately optimal solutions; (ii) have a tractable, i.e, polynomial in the size of the input, number of parameters; (iii) their optimization landscape is benign in the sense that it does not contain sub-optimal stationary points. Our main contribution is a positive answer to this question. Our result holds for a broad class of combinatorial problems including Max- and Min-Cut, Max-$k$-CSP, Maximum-Weight-Bipartite-Matching, and the Traveling Salesman Problem. As a byproduct of our analysis we introduce a novel regularization process over vanilla gradient descent and provide theoretical and experimental evidence that it helps address vanishing-gradient issues and escape bad stationary points.
Constantine Caramanis, Dimitris Fotakis 0001, Alkis Kalavasis, Vasilis Kontonis, Christos Tzamos
NeurIPS3
2022 Differentially Private Regression with Unbounded Covariates
abstract
We provide computationally efficient, differentially private algorithms for the classical regression settings of Least Squares Fitting, Binary Regression and Linear Regression with unbounded covariates. Prior to our work, privacy constraints in such regression settings were studied under strong a priori bounds on covariates. We consider the case of Gaussian marginals and extend recent differentially private techniques on mean and covariance estimation (Kamath et al., 2019; Karwa and Vadhan, 2018) to the sub-gaussian regime. We provide a novel technical analysis yielding differentially private algorithms for the above classical regression settings. Through the case of Binary Regression, we capture the fundamental and widely-studied models of logistic regression and linearly-separable SVMs, learning an unbiased estimate of the true regression vector, up to a scaling factor.
Jason Milionis, Alkis Kalavasis, Dimitris Fotakis 0001, Stratis Ioannidis
AISTATS2
2022 Label Ranking through Nonparametric Regression
abstract
Label Ranking (LR) corresponds to the problem of learning a hypothesis that maps features to rankings over a finite set of labels. We adopt a nonparametric regression approach to LR and obtain theoretical performance guarantees for this fundamental practical problem. We introduce a generative model for Label Ranking, in noiseless and noisy nonparametric regression settings, and provide sample complexity bounds for learning algorithms in both cases. In the noiseless setting, we study the LR problem with full rankings and provide computationally efficient algorithms using decision trees and random forests in the high-dimensional regime. In the noisy setting, we consider the more general cases of LR with incomplete and partial rankings from a statistical viewpoint and obtain sample complexity bounds using the One-Versus-One approach of multiclass classification. Finally, we complement our theoretical contributions with experiments, aiming to understand how the input regression noise affects the observed output.
Dimitris Fotakis 0001, Alkis Kalavasis, Eleni Psaroudaki
ICML2
2022 Linear Label Ranking with Bounded Noise
abstract
Label Ranking (LR) is the supervised task of learning a sorting function that maps feature vectors $x \in \mathbb{R}^d$ to rankings $\sigma(x) \in \mathbb S_k$ over a finite set of $k$ labels. We focus on the fundamental case of learning linear sorting functions (LSFs) under Gaussian marginals: $x$ is sampled from the $d$-dimensional standard normal and the ground truth ranking $\sigma^\star(x)$ is the ordering induced by sorting the coordinates of the vector $W^\star x$, where $W^\star \in \mathbb{R}^{k \times d}$ is unknown. We consider learning LSFs in the presence of bounded noise: assuming that a noiseless example is of the form $(x, \sigma^\star(x))$, we observe $(x, \pi)$, where for any pair of elements $i \neq j$, the probability that the order of $i, j$ is different in $\pi$ than in $\sigma^\star(x)$ is at most $\eta < 1/2$. We design efficient non-proper and proper learning algorithms that learn hypotheses within normalized Kendall's Tau distance $\epsilon$ from the ground truth with $N= \widetilde{O}(d\log(k)/\epsilon)$ labeled examples and runtime $\mathrm{poly}(N, k)$. For the more challenging top-$r$ disagreement loss, we give an efficient proper learning algorithm that achieves $\epsilon$ top-$r$ disagreement with the ground truth with $N = \widetilde{O}(d k r /\epsilon)$ samples and $\mathrm{poly}(N)$ runtime.
Dimitris Fotakis 0001, Alkis Kalavasis, Vasilis Kontonis, Christos Tzamos
NeurIPS2
2022 Perfect Sampling from Pairwise Comparisons
abstract
In this work, we study how to efficiently obtain perfect samples from a discrete distribution $\mathcal{D}$ given access only to pairwise comparisons of elements of its support. Specifically, we assume access to samples $(x, S)$, where $S$ is drawn from a distribution over sets $\mathcal{Q}$ (indicating the elements being compared), and $x$ is drawn from the conditional distribution $\mathcal{D}_S$ (indicating the winner of the comparison) and aim to output a clean sample $y$ distributed according to $\mathcal{D}$. We mainly focus on the case of pairwise comparisons where all sets $S$ have size 2. We design a Markov chain whose stationary distribution coincides with $\mathcal{D}$ and give an algorithm to obtain exact samples using the technique of Coupling from the Past. However, the sample complexity of this algorithm depends on the structure of the distribution $\mathcal{D}$ and can be even exponential in the support of $\mathcal{D}$ in many natural scenarios. Our main contribution is to provide an efficient exact sampling algorithm whose complexity does not depend on the structure of $\mathcal{D}$. To this end, we give a parametric Markov chain that mixes significantly faster given a good approximation to the stationary distribution. We can obtain such an approximation using an efficient learning from pairwise comparisons algorithm (Shah et al., JMLR 17, 2016). Our technique for speeding up sampling from a Markov chain whose stationary distribution is approximately known is simple, general and possibly of independent interest.
Dimitris Fotakis 0001, Alkis Kalavasis, Christos Tzamos
NeurIPS2
2022 Learning and Covering Sums of Independent Random Variables with Unbounded Support
abstract
We study the problem of covering and learning sums $X = X_1 + \cdots + X_n$ of independent integer-valued random variables $X_i$ (SIIRVs) with infinite support. De et al. at FOCS 2018, showed that even when the collective support of $X_i$'s is of size $4$, the maximum value of the support necessarily appears in the sample complexity of learning $X$. In this work, we address two questions: (i) Are there general families of SIIRVs with infinite support that can be learned with sample complexity independent of both $n$ and the maximal element of the support? (ii) Are there general families of SIIRVs with infinite support that admit proper sparse covers in total variation distance? As for question (i), we provide a set of simple conditions that allow the infinitely supported SIIRV to be learned with complexity $ \text{poly}(1/\epsilon)$ bypassing the aforementioned lower bound. We further address question (ii) in the general setting where each variable $X_i$ has unimodal probability mass function and is a different member of some, possibly multi-parameter, exponential family $\mathcal{E}$ that satisfies some structural properties. These properties allow $\mathcal{E}$ to contain heavy tailed and non log-concave distributions. Moreover, we show that for every $\epsilon > 0$, and every $k$-parameter family $\mathcal{E}$ that satisfies some structural assumptions, there exists an algorithm with $\widetilde{O}(k) \cdot \text{poly}(1/\epsilon)$ samples that learns a sum of $n$ arbitrary members of $\mathcal{E}$ within $\epsilon$ in TV distance. The output of the learning algorithm is also a sum of random variables within the family $\mathcal{E}$. En route, we prove that any discrete unimodal exponential family with bounded constant-degree central moments can be approximated by the family corresponding to a bounded subset of the initial (unbounded) parameter space.
Alkis Kalavasis, Konstantinos Stavropoulos, Manolis Zampetakis
NeurIPS1
2022 Multiclass Learnability Beyond the PAC Framework: Universal Rates and Partial Concept Classes
abstract
In this paper we study the problem of multiclass classification with a bounded number of different labels $k$, in the realizable setting. We extend the traditional PAC model to a) distribution-dependent learning rates, and b) learning rates under data-dependent assumptions. First, we consider the universal learning setting (Bousquet, Hanneke, Moran, van Handel and Yehudayoff, STOC'21), for which we provide a complete characterization of the achievable learning rates that holds for every fixed distribution. In particular, we show the following trichotomy: for any concept class, the optimal learning rate is either exponential, linear or arbitrarily slow. Additionally, we provide complexity measures of the underlying hypothesis class that characterize when these rates occur. Second, we consider the problem of multiclass classification with structured data (such as data lying on a low dimensional manifold or satisfying margin conditions), a setting which is captured by partial concept classes (Alon, Hanneke, Holzman and Moran, FOCS'21). Partial concepts are functions that can be undefined in certain parts of the input space. We extend the traditional PAC learnability of total concept classes to partial concept classes in the multiclass setting and investigate differences between partial and total concepts.
Alkis Kalavasis, Grigoris Velegkas, Amin Karbasi
NeurIPS1
2022 Efficient Parameter Estimation of Truncated Boolean Product Distributions
Dimitris Fotakis 0001, Alkis Kalavasis, Christos Tzamos
Algorithmica2
2021 Aggregating Incomplete and Noisy Rankings
abstract
We consider the problem of learning the true ordering of a set of alternatives from largely incomplete and noisy rankings. We introduce a natural generalization of both the Mallows model, a popular model of ranking distributions, and the extensively studied model of ranking from pairwise comparisons. Our selective Mallows model outputs a noisy ranking on any given subset of alternatives, based on an underlying Mallows distribution. Assuming a sequence of subsets where each pair of alternatives appears frequently enough, we obtain strong asymptotically tight upper and lower bounds on the sample complexity of learning the underlying complete central ranking and the (identities and the) ranking of the top k alternatives from selective Mallows rankings. Moreover, building on the work of (Braverman and Mossel, 2009), we show how to efficiently compute the maximum likelihood complete ranking from selective Mallows rankings.
Dimitris Fotakis 0001, Alkis Kalavasis, Konstantinos Stavropoulos
AISTATS2
2021 Efficient Algorithms for Learning from Coarse Labels
abstract
For many learning problems one may not have access to fine grained label information; e.g., an image can be labeled as husky, dog, or even animal depending on the expertise of the annotator. In this work, we formalize these settings and study the problem of learning from such coarse data. Instead of observing the actual labels from a set $\mathcal{Z}$, we observe coarse labels corresponding to a partition of $\mathcal{Z}$ (or a mixture of partitions). Our main algorithmic result is that essentially any problem learnable from fine grained labels can also be learned efficiently when the coarse data are sufficiently informative. We obtain our result through a generic reduction for answering Statistical Queries (SQ) over fine grained labels given only coarse labels. The number of coarse labels required depends polynomially on the information distortion due to coarsening and the number of fine labels $|\mathcal{Z}|$. We also investigate the case of (infinitely many) real valued labels focusing on a central problem in censored and truncated statistics: Gaussian mean estimation from coarse data. We provide an efficient algorithm when the sets in the partition are convex and establish that the problem is NP-hard even for very simple non-convex sets.
Dimitris Fotakis 0001, Alkis Kalavasis, Vasilis Kontonis, Christos Tzamos
COLT2
2020 Efficient Parameter Estimation of Truncated Boolean Product Distributions
abstract
We study the problem of estimating the parameters of a Boolean product distribution in $d$ dimensions, when the samples are truncated by a set $S \subset \{0, 1\}^d$ accessible through a membership oracle. This is the first time that the computational and statistical complexity of learning from truncated samples is considered in a discrete setting. We introduce a natural notion of \emph{fatness} of the truncation set $S$, under which truncated samples reveal enough information about the true distribution. We show that if the truncation set is sufficiently fat, samples from the true distribution can be generated from truncated samples. A stunning consequence is that virtually any statistical task (e.g., learning in total variation distance, parameter estimation, uniformity or identity testing) that can be performed efficiently for Boolean product distributions, can also be performed from truncated samples, with a small increase in sample complexity. We generalize our approach to ranking distributions over $d$ alternatives, where we show how fatness implies efficient parameter estimation of Mallows models from truncated samples. Exploring the limits of learning discrete models from truncated samples, we identify three natural conditions that are necessary for efficient identifiability: (i) the truncation set $S$ should be rich enough; (ii) $S$ should be accessible through membership queries; and (iii) the truncation by $S$ should leave enough randomness in all directions. By carefully adapting the Stochastic Gradient Descent approach of (Daskalakis et al., FOCS 2018), we show that these conditions are also sufficient for efficient learning of truncated Boolean product distributions.
Dimitris Fotakis 0001, Alkis Kalavasis, Christos Tzamos
COLT2