Improved Online Conformal Prediction via Strongly Adaptive Online Learning

Aadyot BhatnagarHuan WangCaiming XiongYu Bai

article2023ICML110 citations

Proposes a strongly adaptive online conformal prediction framework that maintains multiple localized experts to guarantee valid coverage and near-optimal regret across all time intervals simultaneously, improving uncertainty quantification under dynamic distribution shifts.

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Modern machine learning models are widely deployed in high-stakes environments where understanding prediction uncertainty is essential for managing risk and ensuring safety. A standard way to express this uncertainty is through prediction sets—such as confidence intervals for numerical forecasts or candidate label groups for classification—that aim to cover the true outcome with a user-defined target rate (such as 90%). However, standard conformal prediction methods rely on the assumption that data points are independent and identically distributed. In real-world dynamic environments, such as live time series feeds or changing visual conditions, data distributions shift unpredictably over time, causing traditional uncertainty estimates to produce invalid coverage or overly broad, uninformative intervals.

The article develops and evaluates new online conformal prediction methods designed to provide robust, localized uncertainty quantification under arbitrary distribution shifts. Specifically, it introduces Strongly Adaptive Online Conformal Prediction (SAOCP) and Scale-Free Online Gradient Descent (SF-OGD) to guarantee valid coverage and minimal prediction regret simultaneously across all continuous time windows of any duration.

To accomplish this, the authors adapt advanced online learning concepts into a sequential calibration framework. SAOCP operates as a meta-algorithm managing multiple base learners, where each expert learner handles a localized active time interval rather than the entire data history. The individual experts are instantiated using SF-OGD, an algorithm that dynamically adjusts its learning rate according to past gradient sizes. The authors evaluate this framework mathematically, establishing near-optimal regret bounds, and test it empirically across more than 5,100 real-world time series datasets (including industrial, demographic, financial, and banking data) as well as perturbed image datasets under sudden and gradual corruptions.

The findings show that SAOCP consistently matches the target coverage rate while maintaining superior local stability compared to existing methods. In time series forecasting, SAOCP achieved the lowest or second-lowest local coverage error and interval width across diverse underlying models, preventing large miscoverage spikes during sudden shifts. In image classification benchmarks, both SAOCP and SF-OGD adapted significantly faster to abrupt changes in corruption levels than baseline approaches, with SAOCP delivering equivalent or better coverage using noticeably smaller prediction set sizes.

These results demonstrate that online calibration can maintain reliable uncertainty bounds in non-stationary real-world environments without sacrificing prediction efficiency. By maintaining localized experts, systems can prevent critical short-term safety failures caused by delayed adaptation to shifts. Organizations can also avoid the trade-off of using excessively wide, uninformative prediction sets to compensate for unknown distribution drift.

Organizations deploying machine learning in streaming or changing environments should implement localized adaptive calibration, particularly direct radius tracking methods like SAOCP, to enhance system reliability. For future development, technical teams should evaluate these methods in live pilot workflows, explore extended coverage bounds under varied real-world assumptions, and determine optimal interval weighting when integrating with existing ensemble architectures.

The analysis assumes bounded prediction radii and mild distributional regularity conditions for theoretical coverage guarantees. While empirical performance remained robust across thousands of benchmarks, teams should exercise standard caution and validate operational bounds when applying these algorithms to heavily tailed or unconstrained domain outputs.

arXiv: 2302.07869
  • Paper: Conformal Inference for Online Prediction with Arbitrary Distribution Shifts, Isaac Gibbs et al. (2024). It develops dynamically-tuned adaptive conformal inference to guarantee local coverage under arbitrary distribution shifts without pre-tuned parameters, directly continuing the source's line of inquiry.
  • Paper: Replicable Conformal Prediction, Marios Papamichalis et al. (2026). It extends conformal prediction theory to ensure replicability and stability across calibrations, advancing post-hoc calibration guarantees in dynamic deployments.
Cover for Improved Online Conformal Prediction via Strongly Adaptive Online Learning

Abstract

We study the problem of uncertainty quantification via prediction sets, in an online setting where the data distribution may vary arbitrarily over time. Recent work develops online conformal prediction techniques that leverage regret minimization algorithms from the online learning literature to learn prediction sets with approximately valid coverage and small regret. However, standard regret minimization could be insufficient for handling changing environments, where performance guarantees may be desired not only over the full time horizon but also in all (sub-)intervals of time. We develop new online conformal prediction methods that minimize the strongly adaptive regret, which measures the worst-case regret over all intervals of a fixed length. We prove that our methods achieve near-optimal strongly adaptive regret for all interval lengths simultaneously, and approximately valid coverage. Experiments show that our methods consistently obtain better coverage and smaller prediction sets than existing methods on real-world tasks, such as time series forecasting and image classification under distribution shift.

Table of Contents

  • 1. Introduction
  • 1.1. Related work
  • 2. Preliminaries
  • 3. Strongly Adaptive Online Conformal Prediction
  • 4. Theory
  • 4.1. Strongly Adaptive Regret
  • 4.2. Coverage
  • 5. Experiments
  • 5.1. Time Series Forecasting
  • 5.2. Image Classification Under Distribution Shift
  • 6. Conclusion
  • References
  • A. Basic Properties of Online Conformal Prediction Algorithms
  • A.1. Properties of SF-OGD
  • A.2. Example of 'Trivial' Algorithm with Coverage Guarantee
  • B. Proofs for Section 4
  • B.1. Proof of Proposition 4.1
  • B.2. Dynamic Regret for SAOCP
  • B.3. Proof of Theorem 4.2
  • B.4. Coverage of SAOCP
  • B.4.1. DISCUSSIONS & SF-OGD AS A SPECIAL CASE
  • C. Distribution-Aware Coverage Guarantees for SAOCP
  • D. Additional Time Series Experiments
  • E. Additional Experimental Details
  • F. Time Series Experiments with Ensemble Models
  • G. Image Classification on ImageNet/ImageNet-C

Knowls

  1. Knowl 1 — Strongly Adaptive Online Conformal Prediction Algorithm

    algorithm

    Strongly Adaptive Online Conformal Prediction (SAOCP) is a meta-algorithm that maintains multiple online learning experts, each active over a geometrically scheduled finite time interval, to adaptively adjust the prediction set radius s^t\hat{s}_t in online conformal prediction tasks.

    At each step t∈[T]t \in [T], a new expert At\mathcal{A}_t running Scale-Free Online Gradient Descent (SF-OGD) is initialized with lifetime L(t)=g⋅max⁡n∈Z{2n:t≡0(mod2n)}L(t) = g \cdot \max_{n \in \mathbb{Z}} \{2^n : t \equiv 0 \pmod{2^n}\}, where g∈Z≥1g \in \mathbb{Z}_{\ge 1} is a lifetime multiplier. The active set of experts at time tt is Active(t)={i∈[t]:t−L(i)<i≤t}\text{Active}(t) = \{i \in [t] : t - L(i) < i \le t\}. The algorithm assigns prior probabilities πi∝i−2(1+⌊log⁡2i⌋)−1I[i∈Active(t)]\pi_i \propto i^{-2} (1 + \lfloor \log_2 i \rfloor)^{-1} \mathbb{I}[i \in \text{Active}(t)] and computes betting weights wt,iw_{t,i} via a coin betting scheme to construct normalized mixture weights pt,ip_{t,i}. The overall radius prediction s^t\hat{s}_t is the weighted combination of the active experts' predictions. With g=Θ(1)g = \Theta(1), at most g⌊log⁡2t⌋g \lfloor \log_2 t \rfloor experts are active simultaneously, yielding a total time complexity of O(Tlog⁡T)\mathcal{O}(T \log T) over horizon TT.

    Input: Target coverage level 1−α∈(0,1)1 - \alpha \in (0, 1), maximum radius bound D>0D > 0, lifetime multiplier g∈Z≥1g \in \mathbb{Z}_{\ge 1}
    for t=1,…,Tt = 1, \dots, T do
        Initialize new expert At=SF-OGD(α,η=D/3,s^1=s^t−1)\mathcal{A}_t = \text{SF-OGD}(\alpha, \eta = D/\sqrt{3}, \hat{s}_1 = \hat{s}_{t-1}) and set weight wt,t=0w_{t,t} = 0
        Compute active set Active(t)={i∈[t]:t−L(i)<i≤t}\text{Active}(t) = \{i \in [t] : t - L(i) < i \le t\} where L(i)=g⋅max⁡n∈Z{2n:i≡0(mod2n)}L(i) = g \cdot \max_{n \in \mathbb{Z}} \{2^n : i \equiv 0 \pmod{2^n}\}
        Compute prior πi∝i−2(1+⌊log⁡2i⌋)−1I[i∈Active(t)]\pi_i \propto i^{-2} (1 + \lfloor \log_2 i \rfloor)^{-1} \mathbb{I}[i \in \text{Active}(t)]
        Compute unnormalized probabilities p^i=πi[wt,i]+\hat{p}_i = \pi_i [w_{t,i}]_+ for all i∈[t]i \in [t]
        if ∥p^∥1>0\|\hat{p}\|_1 > 0 then
            pt=p^/∥p^∥1p_t = \hat{p} / \|\hat{p}\|_1
        else
            pt=πp_t = \pi
        if t=1t = 1 then
            s^t=0\hat{s}_t = 0
        else
            s^t=∑i∈Active(t)pt,is^i,t\hat{s}_t = \sum_{i \in \text{Active}(t)} p_{t,i} \hat{s}_{i,t}
        Observe input Xt∈XX_t \in \mathcal{X} and return prediction set C^t(Xt,s^t)\hat{C}_t(X_t, \hat{s}_t)
        Observe true label Yt∈YY_t \in \mathcal{Y}, compute true radius St=inf⁡{s∈R:Yt∈C^t(Xt,s)}S_t = \inf\{s \in \mathbb{R} : Y_t \in \hat{C}_t(X_t, s)\}, and loss ℓ(t)(⋅)=ℓ1−α(St,⋅)\ell^{(t)}(\cdot) = \ell_{1-\alpha}(S_t, \cdot)
        for i∈Active(t)i \in \text{Active}(t) do
            Update expert Ai\mathcal{A}_i with (Xt,Yt)(X_t, Y_t) to obtain radius s^i,t+1\hat{s}_{i, t+1}
            if wi,t>0w_{i,t} > 0 then
                gi,t=1D(ℓ(t)(s^t)−ℓ(t)(s^i,t))g_{i,t} = \frac{1}{D} (\ell^{(t)}(\hat{s}_t) - \ell^{(t)}(\hat{s}_{i,t}))
            else
                gi,t=1D[ℓ(t)(s^t)−ℓ(t)(s^i,t)]+g_{i,t} = \frac{1}{D} [\ell^{(t)}(\hat{s}_t) - \ell^{(t)}(\hat{s}_{i,t})]_+
            Update expert weight wi,t+1=1t−i+1(∑j=itgi,j)(1+∑j=itwi,jgi,j)w_{i, t+1} = \frac{1}{t - i + 1} (\sum_{j=i}^t g_{i,j}) (1 + \sum_{j=i}^t w_{i,j} g_{i,j})
  2. Knowl 2 — Scale-Free Online Gradient Descent for Online Conformal Prediction

    algorithm

    Scale-Free Online Gradient Descent (SF-OGD) adaptively tunes the conformal prediction radius parameter s^t\hat{s}_t without requiring a fixed time horizon or manual learning rate schedule, scaling the effective step size by the cumulative norm of past subgradients.

    At each time step tt, the algorithm outputs a prediction set C^t(Xt,s^t)\hat{C}_t(X_t, \hat{s}_t) parameterized by radius s^t\hat{s}_t. Upon observing the true label YtY_t and determining whether miscoverage occurred (errt=I[Yt∉C^t(Xt,s^t)]\text{err}_t = \mathbb{I}[Y_t \notin \hat{C}_t(X_t, \hat{s}_t)]), the subgradient of the pinball loss ∇ℓ(t)(s^t)=α−errt\nabla \ell^{(t)}(\hat{s}_t) = \alpha - \text{err}_t is evaluated. SF-OGD updates the radius according to:

    s^t+1=s^t−η∇ℓ(t)(s^t)∑i=1t∥∇ℓ(i)(s^i)∥22=s^t+ηerrt−α∑i=1t(erri−α)2\hat{s}_{t+1} = \hat{s}_t - \eta \frac{\nabla \ell^{(t)}(\hat{s}_t)}{\sqrt{\sum_{i=1}^t \|\nabla \ell^{(i)}(\hat{s}_i)\|_2^2}} = \hat{s}_t + \eta \frac{\text{err}_t - \alpha}{\sqrt{\sum_{i=1}^t (\text{err}_i - \alpha)^2}}

    Input: Target miscoverage rate α∈(0,1)\alpha \in (0, 1), learning rate parameter η>0\eta > 0, initialization s^1∈R\hat{s}_1 \in \mathbb{R}
    for t=1,2,…t = 1, 2, \dots do
        Observe input Xt∈XX_t \in \mathcal{X}
        Return prediction set C^t(Xt,s^t)\hat{C}_t(X_t, \hat{s}_t)
        Observe true label Yt∈YY_t \in \mathcal{Y} and compute St=inf⁡{s∈R:Yt∈C^t(Xt,s)}S_t = \inf\{s \in \mathbb{R} : Y_t \in \hat{C}_t(X_t, s)\}
        Evaluate subgradient ∇ℓ(t)(s^t)=α−I[Yt∉C^t(Xt,s^t)]\nabla \ell^{(t)}(\hat{s}_t) = \alpha - \mathbb{I}[Y_t \notin \hat{C}_t(X_t, \hat{s}_t)]
        Update predicted radius s^t+1=s^t−η∇ℓ(t)(s^t)∑i=1t∥∇ℓ(i)(s^i)∥22\hat{s}_{t+1} = \hat{s}_t - \eta \frac{\nabla \ell^{(t)}(\hat{s}_t)}{\sqrt{\sum_{i=1}^t \|\nabla \ell^{(i)}(\hat{s}_i)\|_2^2}}
  3. Knowl 3 — Strongly Adaptive Regret Bound for SAOCP

    theoretical result

    Let the true radius St=inf⁡{s∈R:Yt∈C^t(Xt,s)}S_t = \inf\{s \in \mathbb{R} : Y_t \in \hat{C}_t(X_t, s)\} be bounded such that St∈[0,D]S_t \in [0, D] almost surely for all t∈[T]t \in [T]. For any interval length k∈[T]k \in [T], the strongly adaptive regret of an algorithm over horizon TT is defined as:

    SAReg(T,k):=max⁡[τ,τ+k−1]⊆[T](∑t=ττ+k−1ℓ1−α(St,s^t)−inf⁡s⋆∈R∑t=ττ+k−1ℓ1−α(St,s⋆))\text{SAReg}(T, k) := \max_{[\tau, \tau+k-1] \subseteq [T]} \left( \sum_{t=\tau}^{\tau+k-1} \ell_{1-\alpha}(S_t, \hat{s}_t) - \inf_{s^\star \in \mathbb{R}} \sum_{t=\tau}^{\tau+k-1} \ell_{1-\alpha}(S_t, s^\star) \right)

    where ℓ1−α(St,s)=max⁡{(1−α)(St−s),α(s−St)}\ell_{1-\alpha}(S_t, s) = \max\{(1 - \alpha)(S_t - s), \alpha(s - S_t)\} is the pinball loss.

    The Strongly Adaptive Online Conformal Prediction algorithm (SAOCP), instantiated with Scale-Free Online Gradient Descent (SF-OGD) base experts with base learning rate η=D/3\eta = D/\sqrt{3}, achieves the strongly adaptive regret bound:

    SAReg(T,k)≤15Dk(log⁡T+1)=O~(Dk)\text{SAReg}(T, k) \le 15D \sqrt{k (\log T + 1)} = \tilde{\mathcal{O}}(D\sqrt{k})

    simultaneously for all interval lengths k∈[T]k \in [T]. This rate is near-optimal in kk up to logarithmic factors in TT, matching the Ω(Dk)\Omega(D\sqrt{k}) lower bound for online convex optimization on intervals of length kk.

  4. Knowl 4 — Distribution-Free Coverage Error Bound for SF-OGD

    theoretical result

    Suppose the true radii satisfy St=inf⁡{s∈R:Yt∈C^t(Xt,s)}∈[0,D]S_t = \inf\{s \in \mathbb{R} : Y_t \in \hat{C}_t(X_t, s)\} \in [0, D] almost surely for all t∈[T]t \in [T], and prediction sets C^t(x,s)\hat{C}_t(x, s) are nested in ss. Scale-Free Online Gradient Descent (SF-OGD) initialized at s^1∈[0,D]\hat{s}_1 \in [0, D] with learning rate η=Θ(D)\eta = \Theta(D) guarantees that for any time horizon T≥1T \ge 1, without requiring any distributional or exchangeability assumptions on (Xt,Yt)(X_t, Y_t):

    CovErr(T):=∣1T∑t=1Terrt−α∣≤O(α−2T−1/4log⁡T)\text{CovErr}(T) := \left| \frac{1}{T} \sum_{t=1}^T \text{err}_t - \alpha \right| \le \mathcal{O}\left(\alpha^{-2} T^{-1/4} \log T\right)

    where errt=I[Yt∉C^t(Xt,s^t)]\text{err}_t = \mathbb{I}[Y_t \notin \hat{C}_t(X_t, \hat{s}_t)] indicates miscoverage at time step tt, and α∈(0,1)\alpha \in (0, 1) is the target miscoverage level.

  5. Knowl 5 — Distribution-Aware Interval Coverage and Quantile Estimation Bounds for SAOCP

    theoretical result

    Let Ft=σ({(Xi,s^i,Si)}i=1t−1,Xt)\mathcal{F}_t = \sigma(\{(X_i, \hat{s}_i, S_i)\}_{i=1}^{t-1}, X_t) denote the history up to the arrival of XtX_t. Suppose the true radii St∈[0,D]S_t \in [0, D] satisfy:

    1. Density upper bound: For each t∈[T]t \in [T], the conditional density ftf_t of St∣FtS_t \mid \mathcal{F}_t satisfies ft(s)≤L/Df_t(s) \le L/D for all s∈[0,D]s \in [0, D] for some constant L>0L > 0.
    2. Density lower bound: For each t∈[T]t \in [T], with probability 1, there exist constants b>0,q≥2,Δt>0b > 0, q \ge 2, \Delta_t > 0 such that ft(s)≥2bD∣2(s−st⋆)D∣q−2f_t(s) \ge \frac{2b}{D} \left|\frac{2(s - s_t^\star)}{D}\right|^{q-2} for all s∈[st⋆−Δt,st⋆+Δt]s \in [s_t^\star - \Delta_t, s_t^\star + \Delta_t], where st⋆=Q1−α(St∣Ft)s_t^\star = Q_{1-\alpha}(S_t \mid \mathcal{F}_t) is the (1−α)(1-\alpha)-conditional quantile.

    For any sub-interval I=[τ,τ+k−1]⊆[T]I = [\tau, \tau+k-1] \subseteq [T], let ΔI=min⁡t∈IΔt\Delta_I = \min_{t \in I} \Delta_t and define the interval quantile variation:

    VarI:=∑t=ττ+k−1E[(st⋆D−1kD∑i=ττ+k−1E[si⋆∣Fτ])2]\text{Var}_I := \sum_{t=\tau}^{\tau+k-1} \mathbb{E}\left[ \left( \frac{s_t^\star}{D} - \frac{1}{k D} \sum_{i=\tau}^{\tau+k-1} \mathbb{E}[s_i^\star \mid \mathcal{F}_\tau] \right)^2 \right]

    Then SAOCP achieves an average quantile estimation error of:

    1∣I∣∑t∈IE[∣s^t−st⋆D∣]≤O(1b1/qDΔI(log⁡T∣I∣)1/(2q))+O(L1/qb1/qDΔI(VarI∣I∣)1/q)\frac{1}{|I|} \sum_{t \in I} \mathbb{E}\left[ \left| \frac{\hat{s}_t - s_t^\star}{D} \right| \right] \le \mathcal{O}\left( \frac{1}{b^{1/q}} \frac{D}{\Delta_I} \left( \frac{\log T}{|I|} \right)^{1/(2q)} \right) + \mathcal{O}\left( \frac{L^{1/q}}{b^{1/q}} \frac{D}{\Delta_I} \left( \frac{\text{Var}_I}{|I|} \right)^{1/q} \right)

    and an average interval miscoverage error of:

    1∣I∣∑t∈I∣P(Yt∈C^t(Xt))−(1−α)∣≤O(Lb1/qDΔI(log⁡T∣I∣)1/(2q))+O(L1+1/qb1/qDΔI(VarI∣I∣)1/q)\frac{1}{|I|} \sum_{t \in I} \left| \mathbb{P}(Y_t \in \hat{C}_t(X_t)) - (1 - \alpha) \right| \le \mathcal{O}\left( \frac{L}{b^{1/q}} \frac{D}{\Delta_I} \left( \frac{\log T}{|I|} \right)^{1/(2q)} \right) + \mathcal{O}\left( \frac{L^{1 + 1/q}}{b^{1/q}} \frac{D}{\Delta_I} \left( \frac{\text{Var}_I}{|I|} \right)^{1/q} \right)

  6. Knowl 6 — Dynamic Regret Bound for SAOCP

    theoretical result

    For true radii St∈[0,D]S_t \in [0, D] almost surely, SAOCP achieves a worst-case dynamic regret bound on any sub-interval [τ,τ+k−1]⊆[T][\tau, \tau+k-1] \subseteq [T] of length k∈[T]k \in [T] with respect to the pinball loss ℓ(t)(⋅)=ℓ1−α(St,⋅)\ell^{(t)}(\cdot) = \ell_{1-\alpha}(S_t, \cdot):

    ∑t=ττ+k−1ℓ(t)(s^t)−min⁡sτ:τ+k−1⋆∑t=ττ+k−1ℓ(t)(st⋆)=∑t=ττ+k−1[ℓ(t)(s^t)−ℓ(t)(St)]≤O~(D[V[τ,τ+k−1]1/3k2/3+k])\sum_{t=\tau}^{\tau+k-1} \ell^{(t)}(\hat{s}_t) - \min_{s^\star_{\tau:\tau+k-1}} \sum_{t=\tau}^{\tau+k-1} \ell^{(t)}(s^\star_t) = \sum_{t=\tau}^{\tau+k-1} \left[ \ell^{(t)}(\hat{s}_t) - \ell^{(t)}(S_t) \right] \le \tilde{\mathcal{O}}\left( D \left[ V_{[\tau, \tau+k-1]}^{1/3} k^{2/3} + \sqrt{k} \right] \right)

    where V[τ,τ+k−1]:=∑t=τ+1τ+k−1∣St−St−1∣V_{[\tau, \tau+k-1]} := \sum_{t=\tau+1}^{\tau+k-1} |S_t - S_{t-1}| is the path length of the true radii sequence over the sub-interval. On an average per-step basis over [τ,τ+k−1][\tau, \tau+k-1], this yields:

    1k∑t=ττ+k−1[ℓ(t)(s^t)−ℓ(t)(St)]≤O~(D[(V[τ,τ+k−1]k)1/3+1k])\frac{1}{k} \sum_{t=\tau}^{\tau+k-1} \left[ \ell^{(t)}(\hat{s}_t) - \ell^{(t)}(S_t) \right] \le \tilde{\mathcal{O}}\left( D \left[ \left(\frac{V_{[\tau, \tau+k-1]}}{k}\right)^{1/3} + \frac{1}{\sqrt{k}} \right] \right)

    simultaneously for all interval lengths kk and starting steps τ\tau.

  7. Knowl 7 — Distribution-Free Coverage Error Bound for Randomized SAOCP

    theoretical result

    Consider a randomized version of SAOCP where at each time step tt, the predicted radius is set by sampling an expert index i∼pt∈Δ([t])i \sim p_t \in \Delta([t]) and outputting s^t=s^t,i\hat{s}_t = \hat{s}_{t,i}. The expected miscoverage error at step tt is err~t:=∑i=1tpt,iI[s^t,i<St]\widetilde{\text{err}}_t := \sum_{i=1}^t p_{t,i} \mathbb{I}[\hat{s}_{t,i} < S_t].

    For any T≥1T \ge 1, the empirical miscoverage error satisfies:

    ∣1T∑t=1Terr~t−α∣≤O(inf⁡β∈(1/2,1)(T1/2−β+Tβ−1Sβ(T)))\left| \frac{1}{T} \sum_{t=1}^T \widetilde{\text{err}}_t - \alpha \right| \le \mathcal{O}\left( \inf_{\beta \in (1/2, 1)} \left( T^{1/2-\beta} + T^{\beta-1} S_\beta(T) \right) \right)

    where Sβ(T):=1+∑j=2⌈T1−β⌉max⁡t∈Gj∑i=1t∣pt,i−ptj−1,iGi:tj−1iGi:ti∣S_\beta(T) := 1 + \sum_{j=2}^{\lceil T^{1-\beta} \rceil} \max_{t \in G_j} \sum_{i=1}^t \left| p_{t,i} - p_{t_{j-1},i} \frac{G_{i:t_{j-1}}^i}{G_{i:t}^i} \right|, {Gj}j=1⌈T1−β⌉\{G_j\}_{j=1}^{\lceil T^{1-\beta} \rceil} is an even partition of [T][T] into contiguous groups of size at most ⌈Tβ⌉\lceil T^\beta \rceil, and Gi:ti:=∑τ=it(errτ,i−α)2G_{i:t}^i := \sqrt{\sum_{\tau=i}^t (\text{err}_{\tau,i} - \alpha)^2} denotes the cumulative subgradient norm experienced by expert Ai\mathcal{A}_i up to step tt.

  8. Knowl 8 — Online Conformal Prediction Parameterization and Quantile Loss Formulation

    definition

    In online conformal prediction, examples (Xt,Yt)∈X×Y(X_t, Y_t) \in \mathcal{X} \times \mathcal{Y} arrive sequentially for t=1,…,Tt = 1, \dots, T. Before observing YtY_t, the learner predicts a set C^t(Xt)⊂Y\hat{C}_t(X_t) \subset \mathcal{Y} from a parameterized family of nested sets Ct={C^t(x,s)}x∈X,s∈R\mathcal{C}_t = \{\hat{C}_t(x, s)\}_{x \in \mathcal{X}, s \in \mathbb{R}}, where s≤s′s \le s' implies C^t(x,s)⊆C^t(x,s′)\hat{C}_t(x, s) \subseteq \hat{C}_t(x, s').

    The true radius StS_t is the minimal parameter value that covers the observed label:

    St:=inf⁡{s∈R:Yt∈C^t(Xt,s)}S_t := \inf\{s \in \mathbb{R} : Y_t \in \hat{C}_t(X_t, s)\}

    Given target coverage level 1−α1 - \alpha, radius prediction is framed as minimizing the (1−α)(1-\alpha)-quantile loss (pinball loss):

    ℓ(t)(s^)=ℓ1−α(St,s^):=max⁡{(1−α)(St−s^),α(s^−St)}\ell^{(t)}(\hat{s}) = \ell_{1-\alpha}(S_t, \hat{s}) := \max\{(1 - \alpha)(S_t - \hat{s}), \alpha(\hat{s} - S_t)\}

    with subgradient:

    ∇ℓ(t)(s^t)=α−I[s^t<St]=α−I[Yt∉C^t(Xt,s^t)]=α−errt\nabla \ell^{(t)}(\hat{s}_t) = \alpha - \mathbb{I}[\hat{s}_t < S_t] = \alpha - \mathbb{I}[Y_t \notin \hat{C}_t(X_t, \hat{s}_t)] = \alpha - \text{err}_t

    where errt∈{0,1}\text{err}_t \in \{0, 1\} indicates whether a coverage error occurred at time step tt.

  9. Knowl 9 — Empirical Comparison on Time Series Forecasting Benchmarks

    data/table

    The empirical performance of online conformal prediction algorithms was evaluated on multi-horizon time series forecasting across M4 Competition datasets (Hourly with 414 time series, Daily with 4227 time series) at target coverage 1−α=0.901-\alpha = 0.90. The metrics include overall empirical coverage, median prediction interval width, worst-case local coverage error LCEk=max⁡[τ,τ+k−1]⊆[1,T]∣α−1k∑t=ττ+k−1errt∣\text{LCE}_k = \max_{[\tau, \tau+k-1] \subseteq [1, T]} |\alpha - \frac{1}{k}\sum_{t=\tau}^{\tau+k-1} \text{err}_t|, and strongly adaptive regret SARegk\text{SAReg}_k, all evaluated with window length k=20k = 20.

    M4 Hourly (LGBM Base Predictor, MAE = 0.06) M4 Daily (LGBM Base Predictor, MAE = 0.13)
    Method Coverage Width LCE20\text{LCE}_{20} SAReg20\text{SAReg}_{20} Coverage Width LCE20\text{LCE}_{20} SAReg20\text{SAReg}_{20}
    SCP .844 .127 .252 .017 .769 .184 .466 .031
    NExCP .875 .134 .197 .013 .818 .183 .420 .015
    FACI .866 .113 .180 .009 .846 .169 .308 .008
    SF-OGD .889 .138 .154 .011 .873 .173 .246 .011
    FACI-S .883 .128 .163 .010 .875 .169 .240 .010
    SAOCP .882 .121 .143 .009 .869 .162 .213 .007

    SAOCP consistently achieves empirical global coverage within the acceptable range (0.85,0.95)(0.85, 0.95) while obtaining the lowest worst-case local coverage error LCE20\text{LCE}_{20} and strongly adaptive regret SAReg20\text{SAReg}_{20} across base predictors (LGBM, ARIMA, Prophet). Methods predicting s^t+1\hat{s}_{t+1} directly in radius space (SAOCP, SF-OGD, FACI-S) maintain valid coverage when base predictors have higher MAE, whereas methods predicting score quantiles (SCP, NExCP, FACI) experience significant coverage drops on M4 Daily.

  10. Knowl 10 — Local Coverage and Prediction Set Adaptability Under Image Distribution Shifts

    empirical result

    When evaluated on image classification under structured distribution shifts using a pre-trained ResNet-50 classifier on TinyImageNet-C and ImageNet-C across 15 corruption types with severity levels 0 through 5, SAOCP and SF-OGD maintain local coverage closest to the 90% nominal target (1−α=0.91-\alpha = 0.9) across both sudden shifts (alternating between severity 0 and 5 every 500 steps) and gradual shifts (increasing severity from 0 to 5).

    Prediction sets are constructed via regularized Adaptive Prediction Sets: C^t(Xt)={y:St(Xt,y)≤s^t}\hat{C}_t(X_t) = \{y : S_t(X_t, y) \le \hat{s}_t\}, where St(x,y)=λ[ky−kreg]++Utf^y(x)+∑i=1ky−1f^π(i)(x)S_t(x, y) = \lambda \sqrt{[k_y - k_{\text{reg}}]_+} + U_t \hat{f}_y(x) + \sum_{i=1}^{k_y - 1} \hat{f}_{\pi(i)}(x) with Ut∼Unif[0,1]U_t \sim \text{Unif}[0, 1]. Under sudden shifts on TinyImageNet-C, SAOCP achieves a worst-case local coverage error LCE100=0.06\text{LCE}_{100} = 0.06, compared to 0.07 for SF-OGD, 0.11 for FACI-S, 0.16 for FACI, 0.13 for NExCP, and 0.58 for SCP. SAOCP adapts set sizes rapidly at transition points without lagging behind changes in oracle set size.

Coverage note — Omitted material includes intermediate technical lemmas from the appendix (such as bounded iterate lemmas, power-mean inequalities, and detailed coin betting derivation steps) that exist solely to prove the primary theorems, as well as extended table repetitions across additional datasets (M4 Weekly, NN5 Daily, EnbPI ensembles) whose quantitative findings mirror those captured in the representative time series and image classification knowls.

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Citation

MLA
Bhatnagar, A., et al. “Improved Online Conformal Prediction via Strongly Adaptive Online Learning”. International Conference on Machine Learning, vol. 202, 2023, pp. 2337–63, https://proceedings.mlr.press/v202/bhatnagar23a.html.
APA
Bhatnagar, A., Wang, H., Xiong, C., & Bai, Y. (2023). Improved Online Conformal Prediction via Strongly Adaptive Online Learning. International Conference on Machine Learning, 202, 2337–2363. https://proceedings.mlr.press/v202/bhatnagar23a.html
Chicago
Bhatnagar, A., H. Wang, C. Xiong, and Y. Bai. 2023. “Improved Online Conformal Prediction via Strongly Adaptive Online Learning”. International Conference on Machine Learning 202: 2337–63. https://proceedings.mlr.press/v202/bhatnagar23a.html.
Harvard
Bhatnagar, A. et al. (2023) “Improved Online Conformal Prediction via Strongly Adaptive Online Learning”, International Conference on Machine Learning. PMLR, pp. 2337–2363. Available at: https://proceedings.mlr.press/v202/bhatnagar23a.html.
Vancouver
1. Bhatnagar A, Wang H, Xiong C, Bai Y (2023) Improved Online Conformal Prediction via Strongly Adaptive Online Learning. In: International Conference on Machine Learning. PMLR, pp 2337–2363

BibTeX

@InProceedings{pmlr-v202-bhatnagar23a,
  title = 	 {Improved Online Conformal Prediction via Strongly Adaptive Online Learning},
  author =       {Bhatnagar, Aadyot and Wang, Huan and Xiong, Caiming and Bai, Yu},
  booktitle = 	 {Proceedings of the 40th International Conference on Machine Learning},
  pages = 	 {2337--2363},
  year = 	 {2023},
  editor = 	 {Krause, Andreas and Brunskill, Emma and Cho, Kyunghyun and Engelhardt, Barbara and Sabato, Sivan and Scarlett, Jonathan},
  volume = 	 {202},
  series = 	 {Proceedings of Machine Learning Research},
  month = 	 {23--29 Jul},
  publisher =    {PMLR},
  pdf = 	 {https://proceedings.mlr.press/v202/bhatnagar23a/bhatnagar23a.pdf},
  url = 	 {https://proceedings.mlr.press/v202/bhatnagar23a.html},
  abstract = 	 {We study the problem of uncertainty quantification via prediction sets, in an online setting where the data distribution may vary arbitrarily over time. Recent work develops online conformal prediction techniques that leverage regret minimization algorithms from the online learning literature to learn prediction sets with approximately valid coverage and small regret. However, standard regret minimization is insufficient for handling changing environments, where performance guarantees may be desired not only over the full time horizon but also in all (sub-)intervals of time. We develop new online conformal prediction methods that minimize the strongly adaptive regret, which measures the worst-case regret over all intervals of a fixed length. We prove that our methods achieve near-optimal strongly adaptive regret for all interval lengths simultaneously, and approximately valid coverage. Experiments show that our methods consistently obtain better coverage and smaller prediction sets than existing methods on real-world tasks such as time series forecasting and image classification under distribution shift.}
}
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