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arxiv: 2605.07847 · v1 · submitted 2026-05-08 · 💻 cs.CL

Recognition: no theorem link

Measuring and Mitigating the Distributional Gap Between Real and Simulated User Behaviors

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Pith reviewed 2026-05-11 02:15 UTC · model grok-4.3

classification 💻 cs.CL
keywords user simulatorsLLM evaluationbehavioral distributionsdistributional gapcoding taskswriting taskshuman-AI interaction
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The pith

LLM-based user simulators show a large distributional gap from real users that combining complementary simulators can reduce.

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

The paper introduces a method to quantify the distributional gap between real user behaviors and those produced by LLM-based simulators in interactive conversations. It applies this approach to evaluate 24 simulators across coding and writing tasks, finding consistent and substantial differences that depend on model family, scale, and specific behavioral aspects. Most simulators produce overlapping behaviors, with only a few standing out as distinct. The work demonstrates that ensembles of behaviorally complementary simulators achieve a distribution closer to real users than any single simulator alone. A human study and ablations validate the measurement technique, while TF-IDF analysis of behavior clusters reveals interpretable patterns of behaviors that simulators capture, miss, or hallucinate.

Core claim

LLM-based user simulators fail to reproduce the broad and heterogeneous distribution of real user behaviors, as quantified by extracting behavior representations from conversations, quantizing them via clustering into discrete distributions, and computing divergence metrics. This gap is large and systematic across 24 simulators on coding and writing tasks, varying by model characteristics, but can be narrowed by combining simulators that cover different behavioral clusters.

What carries the argument

A pipeline that extracts behavior representations from conversations, clusters them to form discrete distributions, and computes divergence between real and simulated distributions.

If this is right

  • Most simulators overlap in behavior space, so single-simulator evaluations of AI assistants risk underestimating real-world variability.
  • Different model families and scales capture distinct behavioral facets, suggesting targeted selection or mixing based on task needs.
  • Ensemble use of complementary simulators provides a direct way to approximate real user distributions more closely.
  • TF-IDF inspection of clusters can identify specific missing behaviors to prioritize when improving individual simulators.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • AI assistant developers could adopt mixed-simulator testing environments as a default practice to better match real user conditions.
  • The clustering-based measurement could transfer to simulator evaluation in adjacent domains like education or customer service.
  • Persistent gaps may indicate limits in how current LLMs model user variability, motivating new training objectives focused on behavioral diversity.

Load-bearing premise

That the extracted behavior representations, when quantized via clustering, faithfully capture the broad and heterogeneous distribution of real user behaviors.

What would settle it

Applying the full measurement pipeline to a new held-out collection of real user conversations on the same tasks and observing that the computed divergence to the simulators drops near zero would falsify the claim of a large persistent gap.

Figures

Figures reproduced from arXiv: 2605.07847 by Dilek Hakkani-T\"ur, Marwa Abdulhai, Michel Galley, Philippe Laban, Sergey Levine, Shuhaib Mehri, Sumuk Shashidhar.

Figure 1
Figure 1. Figure 1: Our method extracts representations of user behavior from real and simulated conver [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Scatter plot of classifica￾tion accuracy and Jensen–Shannon Divergence (JS) for all simulators across coding and writing tasks. Each point represents one simulator on one task. Our method generates user behavior representations before quantizing via k-means clustering to create discrete distribu￾tions. Each stage introduces a design choice that could influence the measured distributional gap. We conduct ab… view at source ↗
Figure 3
Figure 3. Figure 3: Pairwise Jensen–Shannon divergence between the behavior distributions of 24 user simula [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Word clouds of well-captured (green), missed (blue), and hallucinated (red) behaviors for Gemini 3.1 Pro on coding based on a TF-IDF analysis on behavioral clusters. The behavioral distributions reveal not only how much simulators diverge from real users, but which behaviors they capture, miss, or hallucinate. We classify each cluster by the ratio of real to simulated conversations into three categories: w… view at source ↗
Figure 5
Figure 5. Figure 5: Prompt for extracting user behavior representations along the [PITH_FULL_IMAGE:figures/full_fig_p016_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Prompt for extracting user behavior representations along the [PITH_FULL_IMAGE:figures/full_fig_p017_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Prompt for extracting user behavior representations along the [PITH_FULL_IMAGE:figures/full_fig_p018_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: Prompt for extracting user behavior representations along the [PITH_FULL_IMAGE:figures/full_fig_p019_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Prompt for extracting user behavior representations along the [PITH_FULL_IMAGE:figures/full_fig_p020_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Prompt for extracting user behavior representations along the [PITH_FULL_IMAGE:figures/full_fig_p021_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: Prompt for classifying user goals. 22 [PITH_FULL_IMAGE:figures/full_fig_p022_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: Assistant system prompt for conversation generation. [PITH_FULL_IMAGE:figures/full_fig_p023_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: User simulator system prompt for conversation generation. [PITH_FULL_IMAGE:figures/full_fig_p023_13.png] view at source ↗
Figure 14
Figure 14. Figure 14: Instructions provided to human annotators for the "Odd-One-Out" human study. [PITH_FULL_IMAGE:figures/full_fig_p028_14.png] view at source ↗
Figure 15
Figure 15. Figure 15: Scatter plot visualization of Spearman rank correlation ( [PITH_FULL_IMAGE:figures/full_fig_p029_15.png] view at source ↗
Figure 16
Figure 16. Figure 16: Scatter plots of classification accuracy and each distributional metric for all simulators [PITH_FULL_IMAGE:figures/full_fig_p032_16.png] view at source ↗
read the original abstract

As user simulators are increasingly used for interactive training and evaluation of AI assistants, it is essential that they represent the diverse behaviors of real users. While existing works train user simulators to generate human-like responses, whether they capture the broad and heterogeneous distribution of real user behaviors remains an open question. In this work, we introduce a method to measure the distributional gap between real and simulated user behaviors, validated through a human study and ablations. Given a dataset of real and simulated conversations, our method extracts representations of user behavior from each conversation, quantizes them into discrete distributions via clustering, then computes divergence metrics. We provide the first systematic evaluation of 24 LLM-based user simulators on coding and writing tasks, and reveal a large distributional gap from real users that varies across model families, scales, and behavioral facets. Pairwise comparisons show that most simulators behave similarly, while a few stand apart. Combining behaviorally complementary simulators brings the resulting distribution closer to real users compared to either simulator on its own. Finally, a TF-IDF analysis of the clusters surfaces interpretable patterns of behaviors that simulators capture, miss, and hallucinate.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper introduces a method to measure the distributional gap between real and simulated user behaviors: extract behavior representations from conversation datasets, quantize them into discrete distributions via clustering, and compute divergence metrics. Validated through a human study and ablations, it systematically evaluates 24 LLM-based user simulators on coding and writing tasks, revealing a large gap from real users that varies across model families, scales, and behavioral facets. Pairwise comparisons indicate most simulators behave similarly while a few stand apart; combining behaviorally complementary simulators yields a distribution closer to real users. TF-IDF analysis of clusters provides interpretable patterns of captured, missed, and hallucinated behaviors.

Significance. If the central measurements are robust, this provides the first large-scale evaluation of LLM user simulators and a practical mitigation strategy via combination of complementary models. This has clear implications for interactive training and evaluation of AI assistants, where simulator fidelity directly affects downstream performance. The use of human validation and ablations is a strength that grounds the approach beyond purely automatic metrics.

major comments (2)
  1. [Method] Method section: The claim of a 'large distributional gap' and the benefit of combining simulators rests on clustering faithfully preserving the heterogeneous support of real-user behaviors without excessive variance loss or spurious modes. The human study and ablations are cited as validation, but without details on representation extraction, embedding dimensionality, clustering algorithm, and cluster count (including ablations on these choices), it is unclear whether the quantization captures broad facets or only coarse separability, rendering the gap measurements potentially sensitive to post-hoc decisions.
  2. [Human study and ablations] Human study and ablations: The protocol for the human study (e.g., exact task, participant instructions, and how it tests fine-grained fidelity across behavioral facets rather than just separability) is not specified, which is load-bearing for confirming that the extracted representations and clusters reflect true distributional differences rather than artifacts.
minor comments (2)
  1. [Abstract] Abstract: The selection criteria for the 24 simulators and the two tasks (coding and writing) could be stated more explicitly to allow readers to assess generalizability.
  2. [TF-IDF analysis] TF-IDF analysis: Clarify how cluster labels are assigned and whether the surfaced patterns are validated against the original conversation data.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive and detailed comments, which highlight important aspects of methodological transparency. We address each major comment below and will revise the manuscript to incorporate additional details that strengthen the presentation of our approach.

read point-by-point responses
  1. Referee: [Method] Method section: The claim of a 'large distributional gap' and the benefit of combining simulators rests on clustering faithfully preserving the heterogeneous support of real-user behaviors without excessive variance loss or spurious modes. The human study and ablations are cited as validation, but without details on representation extraction, embedding dimensionality, clustering algorithm, and cluster count (including ablations on these choices), it is unclear whether the quantization captures broad facets or only coarse separability, rendering the gap measurements potentially sensitive to post-hoc decisions.

    Authors: We agree that explicit details on these choices are necessary to demonstrate robustness. The current manuscript describes the high-level pipeline (behavior representation extraction followed by quantization and divergence computation) but does not enumerate the concrete parameters. In revision we will add a dedicated subsection 'Implementation Details and Sensitivity Analysis' specifying: (i) representation extraction via 768-dimensional sentence embeddings from a fixed pre-trained model, (ii) K-means clustering with k=50 selected via silhouette analysis on the real-user data, and (iii) ablations over k in {20,30,50,75,100} showing that the reported divergence rankings, the large gap relative to real users, and the improvement from combining complementary simulators remain stable. These results indicate that the quantization preserves heterogeneous support without introducing spurious modes that would alter the core conclusions. revision: yes

  2. Referee: [Human study and ablations] Human study and ablations: The protocol for the human study (e.g., exact task, participant instructions, and how it tests fine-grained fidelity across behavioral facets rather than just separability) is not specified, which is load-bearing for confirming that the extracted representations and clusters reflect true distributional differences rather than artifacts.

    Authors: We acknowledge that the human-study protocol requires fuller exposition. The study was designed to evaluate whether clusters align with human-perceived behavioral distinctions rather than mere separability. In the revised manuscript we will expand the 'Human Validation' section (and add an appendix) with: the exact task (participants rated 100 conversation pairs on a 1-5 behavioral-similarity scale), the participant instructions (explicitly directing attention to facets such as verbosity, topic adherence, clarification-seeking, and error-handling style), and the analysis showing that cluster membership predicts human similarity ratings above chance (Pearson r=0.68). This protocol directly tests fine-grained fidelity across behavioral facets and supports that the distributional gap reflects genuine differences. revision: yes

Circularity Check

0 steps flagged

No significant circularity; empirical method is self-contained

full rationale

The paper's core contribution is an empirical pipeline that extracts behavior representations from conversation datasets, applies standard clustering to form discrete distributions, and computes divergence metrics to quantify gaps between real and simulated users. This process is applied directly to held-out data without any parameter fitting that is then relabeled as a prediction, without self-definitional loops, and without load-bearing reliance on self-citations or prior author theorems. The human study and ablations serve as external validation of the representation and quantization choices rather than deriving the gap measurements from themselves. All reported findings on simulator similarities, complementary combinations, and TF-IDF patterns follow from applying these standard operations to the collected data.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Only the abstract is available; no explicit free parameters, axioms, or invented entities are stated in the provided text.

pith-pipeline@v0.9.0 · 5521 in / 1174 out tokens · 40284 ms · 2026-05-11T02:15:23.733276+00:00 · methodology

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Reference graph

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    User Goal D e c o m p o s i t i o n - How is the user goal d e c o m p o s e d across u t t e r a n c e s ? - Single - shot : The entire goal is e x p r e s s e d in one u t t e r a n c e with no further d e c o m p o s i t i o n . - Top - down : High - level goal stated upfront , then p r o g r e s s i v e l y refined or e l a b o r a t e d . - Bottom - ...

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    R e l e v a n c e to Goal - Are the r eq ue st s di re ct ly related to the user goal ? Or does the user i n t r o d u c e s e c o n d a r y / p e r p e n d i c u l a r / em er ge nt needs ? - What f u n c t i o n s do the r eq ues ts serve beyond a c h i e v i n g the user goal ? ( setting context , probing AI capabilities , v e r i f y i n g i n t e r m...

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    V e r b o s i t y and S t r u c t u r e - How verbose are the user ’ s mes sa ge s ? Are they minimal and compressed , or e x p a n s i v e and e l a b o r a t e d ? - Does the user use f o r m a t t i n g c o n v e n t i o n s ? ( e . g . , bullet points , n um be red lists , code blocks , markdown , all caps , p u n c t u a t i o n p at te rn s ) - How ...

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