# The Role of Placebo Samples in Observational Studies

arXiv: https://arxiv.org/abs/2205.10761 · HTML: https://mlsanity.com/paper/2205.10761/

## Key facts

- Cited by 1 papers in the map (data through 14 Oct 2026).
- First posted on arXiv 22 May 2022.

## Built on by (most cited first, 1 of 1)

- [Dead text or binding clause? Measuring and restoring constraint influence in black-box LLM dialogues](https://mlsanity.com/paper/2608.12599/) (2026)

## Abstract (arXiv)

In an observational study, it is common to leverage known null effect to detect bias. One such strategy is to set aside a placebo sample -- a subset of data immune from the hypothesized cause-and-effect relationship. Existence of an effect in the placebo sample raises concern of unmeasured confounding bias while absence of it corroborates the causal conclusion. This paper establishes a formal framework for using a placebo sample to detect and remove bias. We state identification assumption, and develop estimation and inference methods based on outcome regression, inverse probability weighting, and doubly-robust approaches. Simulation studies and an empirical application illustrate the finite-sample performance of the proposed methods.