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Let $X$ and $Y$ be two dependent normally distributed continuous random variables (their marginals are $\mathcal{N}(0, 1)$). I would like to find an upper bound on the probability that one is greater than the other by a given threshold, i.e. find a $\theta$ such that $P(Y > X + \delta) < \theta$, $\delta, \theta \in \mathbb{R}$

If $X$ and $Y$ were independent it would be easy as I could directly compute their joint density, but in the case were they are allowed to be dependent all I could do is find a numerical solution by discretizing the joint density (which looked far from trivial).

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If $X$ and $Y$ are jointly normal, $Y - X$ is normal with mean $0$ and variance $v = 2 - 2 \text{Cov}(X,Y)$, which can be anywhere from $0$ to $4$, and then $P(Y > X + \delta) = \Phi(-\delta/\sqrt{v})$, where $\Phi$ is the standard normal cdf.

If $X$ and $Y$ are not jointly normal, it's still true that $Y-X$ has mean $0$ and variance $v \in [0, 4]$. Now for any random variable $Z$ with mean $0$ and variance $v\le 4$, if $\delta > 0$ we have $P(Z > \delta) \le \frac{v}{v + \delta^2} \le \frac{4}{4+\delta^2}$
This comes from the fact that $\frac{ (Z + v/\delta)^2}{(\delta + v/\delta)^2} \ge \cases{1 & if $Z \ge \delta$\cr 0 & otherwise\cr}$ and $E\left[\frac{(Z+v/\delta)^2}{(\delta+v/\delta)^2}\right] = \frac{ v + v^2/\delta^2}{(\delta+v/\delta)^2} = \frac{v}{v+\delta^2}$

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    $X = -2\delta$ is certainly in X < -\delta/2 if \delta > 0. Maybe it would be easier to see if we wrote $Z = -X$. I'm saying that if $Y + Z$ is more than $\delta$, then at least one of $Y$ and $Z$ is more than $\delta/2$.2012-03-19