In a clinical trial, the treatment effect $\Delta$ is assessed via a likelihood ratio test. If the log-likelihood is $\ell(\Delta)$, and we compare a constrained model $\Delta = 0$ to an unconstrained one, what property of the statistic $\lambda = -2[\ell(0) - \ell(\hat{\Delta})]$ allows us to determine significance without knowing the exact distribution of $\hat{\Delta}$?