Knockoffs for exchangeable categorical covariates
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2024
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| author | Dreassi, Emanuela Pratelli, Luca Rigo, Pietro |
| author_facet | Dreassi, Emanuela Pratelli, Luca Rigo, Pietro |
| contents | Let $X=(X_1,\ldots,X_p)$ be a $p$-variate random vector and $F$ a fixed finite set. In a number of applications, mainly in genetics, it turns out that $X_i\in F$ for each $i=1,\ldots,p$. Despite the latter fact, to obtain a knockoff $\widetilde{X}$ (in the sense of \cite{CFJL18}), $X$ is usually modeled as an absolutely continuous random vector. While comprehensible from the point of view of applications, this approximate procedure does not make sense theoretically, since $X$ is supported by the finite set $F^p$. In this paper, explicit formulae for the joint distribution of $(X,\widetilde{X})$ are provided when $P(X\in F^p)=1$ and $X$ is exchangeable or partially exchangeable. In fact, when $X_i\in F$ for all $i$, there seem to be various reasons for assuming $X$ exchangeable or partially exchangeable. The robustness of $\widetilde{X}$, with respect to the de Finetti's measure $π$ of $X$, is investigated as well. Let $\mathcal{L}_π(\widetilde{X}\mid X=x)$ denote the conditional distribution of $\widetilde{X}$, given $X=x$, when the de Finetti's measure is $π$. It is shown that $$\norm{\mathcal{L}_{π_1}(\widetilde{X}\mid X=x)-\mathcal{L}_{π_2}(\widetilde{X}\mid X=x)}\le c(x)\,\norm{π_1-π_2}$$ where $\norm{\cdot}$ is total variation distance and $c(x)$ a suitable constant. Finally, a numerical experiment is performed. Overall, the knockoffs of this paper outperform the alternatives (i.e., the knockoffs obtained by giving $X$ an absolutely continuous distribution) as regards the false discovery rate but are slightly weaker in terms of power. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2410_09835 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | Knockoffs for exchangeable categorical covariates Dreassi, Emanuela Pratelli, Luca Rigo, Pietro Statistics Theory Methodology 62E10, 62H05, 60E05, 62J02 Let $X=(X_1,\ldots,X_p)$ be a $p$-variate random vector and $F$ a fixed finite set. In a number of applications, mainly in genetics, it turns out that $X_i\in F$ for each $i=1,\ldots,p$. Despite the latter fact, to obtain a knockoff $\widetilde{X}$ (in the sense of \cite{CFJL18}), $X$ is usually modeled as an absolutely continuous random vector. While comprehensible from the point of view of applications, this approximate procedure does not make sense theoretically, since $X$ is supported by the finite set $F^p$. In this paper, explicit formulae for the joint distribution of $(X,\widetilde{X})$ are provided when $P(X\in F^p)=1$ and $X$ is exchangeable or partially exchangeable. In fact, when $X_i\in F$ for all $i$, there seem to be various reasons for assuming $X$ exchangeable or partially exchangeable. The robustness of $\widetilde{X}$, with respect to the de Finetti's measure $π$ of $X$, is investigated as well. Let $\mathcal{L}_π(\widetilde{X}\mid X=x)$ denote the conditional distribution of $\widetilde{X}$, given $X=x$, when the de Finetti's measure is $π$. It is shown that $$\norm{\mathcal{L}_{π_1}(\widetilde{X}\mid X=x)-\mathcal{L}_{π_2}(\widetilde{X}\mid X=x)}\le c(x)\,\norm{π_1-π_2}$$ where $\norm{\cdot}$ is total variation distance and $c(x)$ a suitable constant. Finally, a numerical experiment is performed. Overall, the knockoffs of this paper outperform the alternatives (i.e., the knockoffs obtained by giving $X$ an absolutely continuous distribution) as regards the false discovery rate but are slightly weaker in terms of power. |
| title | Knockoffs for exchangeable categorical covariates |
| topic | Statistics Theory Methodology 62E10, 62H05, 60E05, 62J02 |
| url | https://arxiv.org/abs/2410.09835 |