One Pipeline, Many Transformers: Pattern-Specific Imputation Specialists for Tabular Missing Data
Missing data in tabular datasets forces practitioners into a hard choice: deploy a general-purpose imputer that may perform poorly for the problem at hand, or wait for someone to design a specialized algorithm. This problem is worsened by the fact that real-world missingness rarely satisfies the textbook missing completely at random (MCAR) assumption, as entries are often missing not at random (MNAR), where the probability of being observed depends on the missing data itself. We collapse this trade-off into a single pre-training pipeline that builds transformer-based imputation specialists through three components: an entry-wise featurization that recasts imputation as supervised prediction over row--column context, a synthetic data generator with pluggable missingness modules, and prior-data fitting on millions of synthetic tables. Swapping only the missingness module, with no changes to architecture, loss, or training, yields a pattern-specific specialist that outperforms methods purpose-built for that pattern. We validate this on MissBench, a new benchmark of 42 OpenML datasets and 11 missingness patterns (including 9 MNAR variants) spanning medicine, finance, and engineering. Remarkably, training exclusively on MCAR yields a default model---TabImpute---robust across all tested patterns. In addition, the pattern-specific specialists produced by our pipeline outperform the 14 established baselines on their target patterns. We open-source the pipeline, models, and benchmark.