REVIEW 4 major objections 6 minor 29 references
Does Prompt Design Impact Quality of Data Imputation by LLMs?
T0 review · 4 major / 6 minor · reviewed 2026-08-07 · deepseek-v4-flash
Pith's one-line read This paper claims that restricting LLM imputation prompts to features strongly correlated with the missing column reduces token use while maintaining or improving imputation quality, with the largest gains on small class-imbalanced…
desk verdict A credible but narrow empirical result: pruning features in LLM imputation prompts helps on small datasets, but the evaluation is too indirect to fully support the general claim. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central mechanism is the group-wise CSV-style prompt combined with correlation-threshold feature selection. The prompt template begins with an instruction, feature descriptions, a header, and groups of completed examples sampled separately for each target class, so the LLM sees majority and minority distributions in a balanced way; the model is told to output only the imputed column to save tokens. Around this template sits a feature-selection step that computes pairwise association strengths (Pearson for numeric-numeric, Cramer's V for categorical-categorical, eta ratio for mixed pairs), ranks them, and chooses a correlation threshold at the elbow of the ranked plot; only features above the threshold enter the prompt. The imputation order is chosen so that the feature relevant to the most other imputation features goes first, and each completed column is folded back into the prompt for the next feature. This mechanism is what lets the method shrink the prompt while directing the LLM's attention to the information that carries the missing value.
What would settle it
For the Travel dataset, retrieve the 120 artificially removed 'FrequentFlyer' entries and compare the LLM's imputed values to these ground-truth values; if the per-value accuracy is at chance level while classification metrics remain high, the claim that imputation quality is 'maintained or improved' would be shown to rest entirely on the classification proxy rather than on factually correct imputation.
Extended reading notes
Core claim
The central claim is that discarding dataset features with weak associations with the feature being imputed yields imputation quality at least as good as using the complete dataset, and sometimes better. The paper demonstrates this with a token-aware imputation method built on a group-wise CSV-style prompt: completed records are presented in class-separated groups, and the prompt instructs the model to output only the imputed column. Features are selected by computing Pearson correlation, Cramer's V, and eta correlation ratio between the imputation feature and all other columns, ranking the absolute correlations, and picking a threshold at the elbow of the ranked plot. The imputation proceeds one feature at a time, starting with the feature relevant to the most other imputation features, and the completed column replaces the missing column in later prompts. On the Adult Income dataset, the minority-class F1 stays within 0.04 while the feature space is cut by about 76%; on the Travel dataset, omitting weakly correlated features raises minority-class F1 by about 7.46%, balanced accuracy by 4.45%, and ROC AUC from about 0.91 to 0.95.
Load-bearing premise
The entire evaluation assumes that classification performance on the imputed rows is a valid measure of imputation quality, and the paper never checks the imputed values against the true values that are known for the Travel dataset's intentionally introduced missingness.
Editorial extensions
If this is right
- LLM imputation costs drop substantially with no penalty in classification-based imputation quality when weakly correlated features are excluded, so users can fit more in-context examples within a fixed token budget.
- Small class-imbalanced datasets benefit most: Travel-style datasets with few records saw minority-class F1, balanced accuracy, and ROC AUC all improve when the prompt was pruned.
- For larger datasets such as Adult Income, aggressive pruning (around 76% of columns) preserves minority-class F1 within 0.04 and keeps balanced accuracy and ROC AUC nearly stable, suggesting the method trades tokens for little or no quality.
- Because the completed column is re-inserted into later prompts, the approach chains imputations so that the information available for successive missing features grows, which is a direct corollary of the method's design.
Reading between the lines
- The paper's evaluation never compares imputed values to ground truth, even though the Travel dataset's artificial missingness preserves the true values; a natural next step would be to measure token-level imputation accuracy directly, which could either corroborate or undercut the classification-based conclusion.
- The correlation threshold is chosen per dataset by visual elbow inspection; our inference is that a fixed or automated threshold-selection rule (e.g., a permutation-based null) would make the method more reproducible and would clarify whether the Travel gains are sensitive to the exact cutoff.
- The paper's explanation for why pruning helps small datasets—that a small sample cannot let the model filter noise by itself—suggests a testable extension: vary the number of in-context examples while holding the feature set fixed, and the benefit of pruning should shrink as sample size grows.
- Since only categorical imputation features were tested, the method's applicability to numeric missing values is open; our inference is that the same correlation-based pruning should carry over, but numeric imputation errors are more likely to be visible to a numeric metric, so the classification proxy may overstate quality.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This paper proposes a token-aware LLM-based imputation method for class-imbalanced tabular data. The method uses group-wise CSV-style prompts (building on Kim et al.'s EPIC) and prunes features that are weakly correlated with the feature being imputed, based on an elbow of sorted absolute correlations (Pearson/Cramér's V/η). The authors evaluate on Adult Income and Travel datasets, training XGBoost and Random Forest on complete rows and testing on imputed rows. They report precision/recall/F1, balanced accuracy, and ROC AUC at correlation thresholds 0, 0.15/0.2. The central claim is that feature pruning maintains or improves imputation quality while reducing prompt tokens, with a stronger benefit on the smaller Travel dataset.
Significance. If the claims held, the work would be a practical contribution: cheaper LLM imputation for imbalanced datasets by omitting irrelevant features. The paper is transparent about its setup and includes a grouped-vs-ungrouped prompt comparison (Table 2) that supports the benefit of structured grouping. However, the evaluation is indirect (classifier scores rather than fidelity of imputed values), single-run, and the Adult results show a small decline rather than maintenance; the Limitations section acknowledges the categorical-only scope. The evidence is therefore suggestive rather than conclusive.
major comments (4)
- [Data Imputation Technique / Experiments and Insights (Tables 3-4)] The central claim that pruning weakly associated features achieves 'an imputation quality at least as good as using the complete dataset information' is not directly tested. The Travel dataset includes artificially introduced missingness with known ground-truth values, yet the paper never compares the imputed FrequentFlyer values against those true values. Because pruning changes the feature space seen by the downstream classifiers, classification metrics can improve or decline even if the imputed values are equally correct. The improvements in Tables 3-4 therefore do not by themselves establish improved imputation quality. Please report direct accuracy metrics for the Travel dataset (e.g., imputation accuracy, per-class confusion between true and imputed values), and at least discuss the relationship between the proxy metrics and imputation fidelity.
- [Tables 5-6] The description of the Adult Income results as 'maintained' is contradicted by the reported numbers. At threshold 0.2, XGBoost minority-class F1 drops from 0.77 (threshold 0) to 0.74, and balanced accuracy from 0.814 to 0.792; Random Forest shows the same pattern (F1 0.78 to 0.74, balanced accuracy 0.813 to 0.790). A drop of 0.03-0.04 F1 and about 2 percentage points in balanced accuracy is a decline, not maintenance. Please either revise the claim to 'minor decline' or provide statistical evidence (e.g., confidence intervals) that these differences are within noise.
- [Experiments and Insights (Tables 3-6)] All reported metrics are point estimates from a single LLM run per condition, with no multiple seeds, random restarts, or confidence intervals. Since the prompt examples are obtained by random sampling and GPT-4.1 is stochastic, observed differences such as the Travel F1 increase from 0.67 to 0.72 (Table 3) could reflect run-to-run variance rather than a true effect. Please repat the imputation several times per condition and report means with standard deviations or confidence intervals, or justify why a single run is sufficient.
- [Determining Correlation Threshold] The correlation threshold is chosen from an elbow plot of the same dataset on which the method is then evaluated, and the comparison is made between thresholds on that same test set (e.g., 0, 0.15, 0.2). This data-dependent selection makes it difficult to separate the benefit of pruning from selection bias. To support a general claim, either fix the threshold a priori (e.g., a default of 0.2) or select it on a held-out validation set and apply it to the test data. At minimum, state explicitly that the elbow heuristic may overfit the current datasets.
minor comments (6)
- [Throughout] There are several typos and wording issues: 'irrelevent' in the Introduction, 'atleast' in the Data Imputation Technique section, and 'the the F1' in the Experiments section.
- [Table 1] The Adult Income row lists 15 features while the text states the dataset contains 14 features; please clarify whether the target column is included in the feature count.
- [Abstract / Data Imputation Technique] The paper claims a token-aware method and significant input prompt reduction, but reports only feature-space reduction (Tables 7-8) and never measures actual token counts. Reporting token usage per condition would substantiate the central token-efficiency claim.
- [Discussion] The statement that the LLM 'encounters both the majority and minority class samples in a proportionate manner' is not fully supported, since the group-wise sampling randomly draws a fixed number of examples per class; clarify how proportionality is guaranteed and how the group sizes are chosen.
- [Appendix Table 9] The example prompt contains inconsistent spellings ('Doesnt' vs 'Doesn’t', 'No record' vs 'No Record') and a duplicated 'Set Header' instruction; please standardize the template and ensure the header matches the feature list.
- [References] The reference to Kim, Kim, and Choo (2024b) is duplicated, and the second entry cites 'Advances in Neural Information Processing Systems, 33: 1877–1901' which appears to be the volume for NeurIPS 2020; please verify the correct venue, volume, and page numbers.
Circularity Check
No circularity: the feature-pruning claim is an empirical comparison, not a fitted-prediction reduction.
full rationale
Most of the paper is an empirical comparison. The claimed result that pruning weakly correlated features maintains or improves imputation quality follows from measured XGBoost and Random Forest scores (Tables 3–6), not from an equation that defines the outcome in terms of the pruning rule. The group-wise CSV prompting method is borrowed from an external prior work (Kim et al.), not from the authors' own earlier papers, so no self-citation chain is load-bearing. The correlation threshold is selected from an elbow plot on the same datasets used for evaluation; this is data-dependent threshold selection, which can bias the comparison, but it is not a circular derivation because the reported improvements are not forced by the selection rule—the LLM outputs could have been worse. The main validity concern is that 'imputation quality' is operationalized as downstream classifier performance rather than direct accuracy against known missing values (which exist for the Travel dataset), but this is a measurement-validity/proxy issue, not a case of the conclusion being equivalent to the input by definition. No circular step satisfying the quote-and-reduction standard was found.
Assumptions & free parameters
free parameters (2)
- Correlation threshold for feature selection =
0.2 (with 0.1 and 0.15 tested for comparison)
- Number of completed examples per group in the prompt
assumptions (5)
- domain assumption Classification metrics on imputed rows measure imputation quality.
- ad hoc to paper The elbow point in the sorted absolute correlation plot identifies a meaningful feature-selection threshold.
- domain assumption Classifiers trained on complete rows transfer to imputed rows.
- domain assumption GPT-4.1 imputation results represent LLM behavior in general.
- standard math Pearson correlation, Cramer's V, and eta ratio are appropriate measures of association for the feature types used.
Cite this review
Pith. "Pith review of Does Prompt Design Impact Quality of Data Imputation by LLMs?." pith.science (2026). https://pith.science/paper/MIGFJLZP
@misc{pith2026250604172,
author = {Pith},
title = {Pith review of: Does Prompt Design Impact Quality of Data Imputation by LLMs?},
year = {2026},
howpublished = {\url{https://pith.science/paper/MIGFJLZP}},
note = {Machine review of arXiv:2506.04172}
}
read the original abstract
Generating realistic synthetic tabular data presents a critical challenge in machine learning. It adds another layer of complexity when this data contain class imbalance problems. This paper presents a novel token-aware data imputation method that leverages the in-context learning capabilities of large language models. This is achieved through the combination of a structured group-wise CSV-style prompting technique and the elimination of irrelevant contextual information in the input prompt. We test this approach with two class-imbalanced binary classification datasets and evaluate the effectiveness of imputation using classification-based evaluation metrics. The experimental results demonstrate that our approach significantly reduces the input prompt size while maintaining or improving imputation quality compared to our baseline prompt, especially for datasets that are of relatively smaller in size. The contributions of this presented work is two-fold -- 1) it sheds light on the importance of prompt design when leveraging LLMs for synthetic data generation and 2) it addresses a critical gap in LLM-based data imputation for class-imbalanced datasets with missing data by providing a practical solution within computational constraints. We hope that our work will foster further research and discussions about leveraging the incredible potential of LLMs and prompt engineering techniques for synthetic data generation.
Figures
Reference graph
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Reviewed August 7, 2026 · model on record in the stance chip above.
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