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REVIEW 3 major objections 4 minor 23 references

Epistemic Uncertainty Sampling

T0 review · 3 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash

Pith's one-line read Active learners should query instances with high epistemic uncertainty—ignorance about the model—not high aleatoric uncertainty from noise.

desk verdict A competent and honest application of a known epistemic/aleatoric split to uncertainty sampling, with a real limitation: the key measure can vanish under model misspecification. read the letter →

arxiv 1909.00218 v1 pith:JS7GDTOE submitted 2019-08-31 cs.LG stat.ML

classification cs.LGstat.ML
keywords activelearninguncertaintysamplingepistemicaleatoriclikelihood-basedplausibilityParzenwindowclassifierdecisiontreelogisticregression
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

Active learning is about spending labels where they will teach the most. This paper argues that standard uncertainty sampling conflates two different things: epistemic uncertainty, which comes from not knowing the right model and can be reduced by more data, and aleatoric uncertainty, which comes from inherent randomness in the labels and cannot. The paper's conjecture is that, in uncertainty sampling, an instance's usefulness is better reflected by its epistemic component, and it instantiates the query rule $x^* = \arg\max_{x\in U} u_e(x)$ using likelihood-based plausibility scores. Experiments with Parzen-window classifiers, decision trees, and logistic regression on eight datasets support the conjecture, with epistemic uncertainty sampling performing best and aleatoric-only sampling worst. If the conjecture holds, active learners can become more label-efficient simply by asking whether the uncertainty they see is reducible before querying.

What carries the argument

The machinery is the normalized likelihood $\pi_H(h)=L(h)/L(h_{ml})$ of a model $h$ given the training data, combined with the two degrees of support $\pi(1\mid x)=\sup_{h\in H}\min[\pi_H(h), 2h(x)-1]$ and $\pi(0\mid x)=\sup_{h\in H}\min[\pi_H(h), 1-2h(x)]$. These support degrees turn training data into a plausibility ranking over hypotheses and then split total uncertainty into $u_e$ and $u_a$. Everything else is computation: for Parzen windows and decision-tree leaves, constant hypotheses per region reduce the supports to one-dimensional scalar maximizations solved with Brent's method; for logistic regression, the supports are found by maximizing a strictly concave regularized log-likelihood over parallel hyperplanes, with the level $\alpha$ discretized and solved by Newton-CG or sequential least squares.

What would settle it

Run the comparison on a synthetic binary problem where one region has clean labels and another has labels flipped with probability $p$, for several values of $p$. If instances with high $u_e$ are not the ones whose addition reduces test error fastest, or if the accuracy advantage of the $u_e$ rule fails to grow with $p$, the central conjecture is false.

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Extended reading notes

Core claim

The paper's central claim is that in uncertainty sampling the utility of an unlabeled instance is better captured by its epistemic uncertainty than by its aleatoric uncertainty, so the learner should query $x^* = \arg\max_{x\in U} u_e(x)$ instead of maximizing predictive entropy. With $u_e(x)=\min[\pi(1\mid x),\pi(0\mid x)]$ and $u_a(x)=1-\max[\pi(1\mid x),\pi(0\mid x)]$, the first is high when plausible models disagree strongly about which class is more likely, while the second is high when every plausible model puts the class probability near one half. The paper derives these degrees of support from normalized likelihoods, instantiates the query rule for local learners and logistic regression, and presents experiments in which epistemic uncertainty sampling achieves the best accuracy and aleatoric sampling the worst, with standard entropy and random sampling in between. The paper interprets the smaller gains under logistic regression as evidence that the method's advantage is tied to how long the hypothesis space retains genuine model uncertainty.

Load-bearing premise

The method assumes that the numbers it computes from the training data truly separate 'uncertainty that more data can fix' from 'uncertainty that is just noise', rather than merely reflecting the arbitrary choice of model family.

Editorial extensions

If this is right

  • Replacing entropy or least-confidence with $u_e(x)$ as the utility score should give an active learner comparable or better accuracy at the same label budget, since the experiments show the epistemic rule dominating standard uncertainty sampling.
  • Aleatoric-only sampling is a cautionary baseline: querying the instances where all plausible models agree on a near-random class probability spends labels where the label noise itself is the problem.
  • The difference between epistemic and aleatoric scores is a practical diagnostic: a large gap means the learner's uncertainty is mostly reducible, while a small gap means further labels are unlikely to sharpen the model.
  • For algorithms with strong inductive bias, such as linear logistic regression, epistemic uncertainty can be exhausted quickly, so epistemic sampling converges to standard uncertainty sampling; the paper's generic construction means the rule can be attached to any hypothesis space for which the supports can be computed.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • A natural stress test not run in the paper: on a synthetic binary task with known label-flip noise in one region, the gap between $u_e$ sampling and $u_a$ sampling should widen as the flip rate rises, because the aleatoric rule will systematically drain the budget in the noisy region.
  • The logistic-regression finding suggests a general dependency: the amount of epistemic uncertainty an instance carries depends on the expressiveness of the hypothesis space, so kernel-based or deep models, which keep model uncertainty alive longer, would plausibly show larger gains from epistemic sampling than linear models do.
  • The same $u_e$ measure could serve as a stopping rule: once the pool-wide epistemic uncertainty is low, the expected benefit of further queries is small, which would let an active learner stop early and save labeling cost.
  • If the conjecture is correct, the success of entropy-based uncertainty sampling in earlier work can be reinterpreted as approximating epistemic uncertainty; explicit separation should matter most when label noise is heterogeneous across the input space.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 4 minor

Summary. The paper proposes epistemic uncertainty sampling, a query strategy for active learning in binary classification. Building on the normalized-likelihood framework of Senge et al. (2014), it defines epistemic uncertainty u_e(x) (Eq. 9) and aleatoric uncertainty u_a(x) (Eq. 10) as functions of support degrees of the two classes, and suggests querying the instance maximizing u_e (Eq. 11). Closed-form support degrees are derived for Parzen window classifiers and decision trees (Section 4.1), while logistic regression is handled by discretizing the parameter constraint alpha (Section 4.2). Experiments on eight UCI data sets compare epistemic, aleatoric, standard uncertainty, and random sampling; epistemic sampling performs best for the two local learners and comparably to standard uncertainty for logistic regression.

Significance. If the central conjecture is valid, the paper makes a useful conceptual contribution by separating reducible from irreducible uncertainty in active learning and providing a concrete, algorithm-agnostic measure. The derivations for Parzen windows and decision trees are closed-form and clear. However, the claim that u_e tracks reducible uncertainty is asserted rather than proven, and the empirical evidence lacks error bars and significance tests. The paper's own logistic-regression results show the measure can collapse under model misspecification, so the generality of the conclusion is not established. The work is nevertheless a reasonable starting point for further study.

major comments (3)
  1. [Section 3, Eqs. (9)-(10)] The central claim that u_e measures reducible uncertainty is not established. Equations (9)-(10) define u_e as the minimum of support degrees obtained from normalized likelihoods over a fixed hypothesis space H. This measures internal disagreement among plausible models, not reducibility with respect to the true data-generating process. Under model misspecification, plausible models can agree at a point even though labeling it would substantially reduce error, and u_e can vanish exactly where error reduction is most needed. The authors concede this behavior for logistic regression in Section 5.2, where u_e 'disappears quite quickly' under the linearity assumption. Since the conclusion in Section 6 states that 'the epistemic part is the better criterion' without qualification, this is a load-bearing gap. The authors should either state precise conditions on H under which arg max u_e corresponds to maximal expected error reduction, or add an experiment that directly measures whether u_e tracks error reduction under misspecification.
  2. [Section 4.2, Eq. (23)] The global optimum of the non-concave problem (19) is approximated by scanning 50 discretized alpha values and taking the best restricted maximum, without any certificate of global optimality. Because u_e is used as the query score, a poor approximation could change instance rankings and thereby affect the experimental conclusions. The authors should provide evidence that the discretization is adequate, for example by comparing against finer grids or multiple initializations, or by using a certified global optimization technique on a subset of instances.
  3. [Section 5, Figures 2-4] The experimental section reports only averaged accuracies over 10-fold cross-validation, without standard deviations, confidence intervals, or significance tests. The claimed advantage of epistemic over standard uncertainty sampling is described as 'not huge' but 'visible and consistent' (Section 5.1). Since the pool is also randomly subsampled at 10% in each iteration, the results may have substantial variability. Given that the paper's main evidence is empirical, the absence of any statistical assessment makes it difficult to evaluate whether the observed differences are reliable. Paired tests (e.g., Wilcoxon signed-rank across datasets) and error bars or per-fold spreads should be reported.
minor comments (4)
  1. [Section 4.1, after Eq. (14)] The notation is confusing: 'p' denotes both the number of positive instances and the probability ph(1|x). Renaming the count variable (e.g., to 'k' or 'n_+') would improve readability.
  2. [Algorithm 3, line 10] There appears to be a typo: the update for pi(0|x) uses '1 - 2*alpha_p' but should likely use '1 - 2*alpha_n' to match the negative-class support in Eq. (24).
  3. [Section 5.1, Algorithm 2] Algorithm 2 sets epsilon to the average K-th nearest neighbor distance over training instances, but the text states this makes the average number of neighbors 'just K'. This is only approximately true, and the approximation is not quantified; a brief clarification would help.
  4. [Figure 1] The color map has no colorbar or scale, so the reader cannot quantitatively interpret the uncertainty values shown. Adding a colorbar would improve the figure's informativeness.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the uncertainty measures are transparently restated from prior work and the central claim is an empirically tested conjecture, not a quantity forced by construction.

full rationale

The paper's derivation chain is not circular. The epistemic and aleatoric uncertainty measures in Eqs. (4)-(10) are explicitly recalled from the authors' earlier framework [19], but the paper says this is done "for the sake of completeness and self-containedness" and reproduces the full definitions, including the normalized likelihood, support degrees, and the resulting uncertainty degrees. Thus the reliance on [19] is transparent rather than a black-box self-citation. Moreover, the cited framework itself rests on external foundations (relative likelihoods of Birnbaum [2] and the likelihood-based possibility framework of Walley and Moral [23]), so it is not an unverified premise invented for this paper. The central active-learning claim, that epistemic uncertainty is a better query criterion than aleatoric uncertainty, is explicitly introduced as a conjecture ("We conjecture that, in uncertainty sampling, the usefulness of an instance is better reflected by its epistemic than by its aleatoric uncertainty") and then tested against UCI benchmark data sets in Section 5. The selection rule x* = arg max u_e(x), Eq. (11), follows from the interpretation of u_e as reducible uncertainty, but the paper does not present this as a formal derivation of empirical success; the experimental comparison supplies the evidence. No fitted parameter is renamed as a prediction: hyperparameters such as K and the regularization gamma are fixed in advance, and the method is not tuned to the active-learning outcome. The Section 5.2 observation that logistic regression's linearity bias makes epistemic uncertainty "disappear quite quickly" is an honest limitation of the approach under model misspecification, not a circular step. There is therefore no reduction of the paper's conclusions to its inputs, and no load-bearing self-citation chain.

Assumptions & free parameters 3 free parameters · 5 assumptions · 0 invented entities

The method does not introduce new physical or model entities. It inherits the uncertainty decomposition from [19] and adds algorithmic parameters (K, Q, gamma). The main burden is the assumption that normalized likelihood plausibility correctly captures epistemic uncertainty for the chosen hypothesis spaces.

free parameters (3)
  • K (neighborhood size) = sqrt(N), where N is the size of the initial training plus pool set
    Determines the Parzen window width and the scale of local uncertainty estimates (Section 5.1, Algorithm 2). Set by a rule of thumb from [10]; no sensitivity analysis is provided.
  • Q (alpha discretization count) = 50
    Number of alpha values used in the logistic regression search (Section 4.2). The approximation quality depends on this arbitrary choice.
  • gamma (L2 regularization) = 1
    Regularization coefficient in the logistic regression log-likelihood (Eq. 18). Fixed to 1 without tuning; it affects the normalized likelihood and thus u_e and u_a.
assumptions (5)
  • domain assumption Normalized likelihood pi_H(h) is a valid plausibility measure for hypotheses
    Used in Eqs. (4)-(6) from [19]; the entire uncertainty decomposition rests on this.
  • domain assumption For every x, there are hypotheses h and h' with h(x) >= 0.5 and h'(x) <= 0.5
    Footnote 1 in Section 3; required for pi(1|x) and pi(0|x) to be non-negative.
  • domain assumption Epistemic uncertainty u_e (Eq. 9) corresponds to reducible model uncertainty, while u_a (Eq. 10) is irreducible
    The interpretive link that motivates selection by u_e; if the decomposition does not align with reducibility of test error, the method loses its rationale.
  • standard math Solving the concave log-likelihood over the hyperplane constraint (Eq. 22) gives the true restricted maximum
    Valid because the L2-regularized log-likelihood is strictly concave (Eq. 18, citing [17]).
  • ad hoc to paper The global solution of the non-concave problem (19) is obtained by scanning alpha in [0.5, 1) and taking the best restricted maximum
    Section 4.2 discretizes alpha into 50 values; no proof that this finds the global optimum of (7).

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Cite this review

Pith. "Pith review of Epistemic Uncertainty Sampling." pith.science (2026). https://pith.science/paper/JS7GDTOE

@misc{pith2026190900218,
  author       = {Pith},
  title        = {Pith review of: Epistemic Uncertainty Sampling},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/JS7GDTOE}},
  note         = {Machine review of arXiv:1909.00218}
}
read the original abstract

Various strategies for active learning have been proposed in the machine learning literature. In uncertainty sampling, which is among the most popular approaches, the active learner sequentially queries the label of those instances for which its current prediction is maximally uncertain. The predictions as well as the measures used to quantify the degree of uncertainty, such as entropy, are almost exclusively of a probabilistic nature. In this paper, we advocate a distinction between two different types of uncertainty, referred to as epistemic and aleatoric, in the context of active learning. Roughly speaking, these notions capture the reducible and the irreducible part of the total uncertainty in a prediction, respectively. We conjecture that, in uncertainty sampling, the usefulness of an instance is better reflected by its epistemic than by its aleatoric uncertainty. This leads us to suggest the principle of "epistemic uncertainty sampling", which we instantiate by means of a concrete approach for measuring epistemic and aleatoric uncertainty. In experimental studies, epistemic uncertainty sampling does indeed show promising performance.

Figures

Figures reproduced from arXiv: 1909.00218 by the authors.

Figure 1
Figure 1. From left to right: Epistemic, aleatoric, and total uncertainty (epistemic + [PITH_FULL_IMAGE:figures/full_fig_p007_1.png] view at source ↗
Figure 2
Figure 2. Average accuracies (y-axis) for the Parzen window classifier as a function of the [PITH_FULL_IMAGE:figures/full_fig_p012_2.png] view at source ↗
Figure 3
Figure 3. Average accuracies (y-axis) for the decision tree classifier as a function of the [PITH_FULL_IMAGE:figures/full_fig_p013_3.png] view at source ↗
Figures from the paper (1 more)
Figure 4
Figure 4. Figure 4: Average accuracies (y-axis) for logistic regression as a function of the number [PITH_FULL_IMAGE:figures/full_fig_p014_4.png]

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Reference graph

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