# Local asymptotic minimax, and nearly-true models

I’ve written a bunch of times about nearly-true models. The idea is that you have some regression model for $$Y|X$$ you’re trying to fit with data from a two-phase sample with known sampling probabilities $$\pi_i$$ for individual $$i$$. You know $$Y$$ and some auxililary variables $$A$$ for everyone, but you know $$X$$ only for the subsample. If you had complete data, you’d fit a particular parametric model for $$Y|X$$, with parameters $$\theta$$ you’re interested in and nuisance parameters $$\eta$$, call it $$P_{\theta,\eta}$$.

You can assume

• the sampling model: just that the sampling probabilities are known
• the sampling+outcome model: that, in addition, $$Y|X$$ truly follows $$P_{\theta,\eta}$$

Under only the sampling model, the best estimator of $$\theta$$ is the optimal AIPW estimator $$\hat\theta_w$$ : it weights the observations where we know $$X$$ by probabilities based on $$\pi_i$$ but adjusted using $$A$$. Under the sampling+outcome models you can do better, and we’ll write $$\hat\theta_e$$ for the semiparametric-efficient estimator.

I’m interested in how the estimators compare when the outcome model is nearly true. That is, the data actually come from a model $$Q$$ which is close to $$P_{\theta,\eta}$$ for some $$(\theta,\eta)$$, close enough that you wouldn’t be able to tell the difference given the amount of data you have. As $$n$$ increases, you can tell the difference better, so $$Q$$ needs to move closer: we have a sequence $$Q_n$$ contiguous to the ‘nearly true’ $$P_n=P_{\theta_0,\eta_0}$$.

I’m defining the ‘true’ value of $$\theta$$ as the value you would estimate with complete data, where the two estimators agree. What I’ve shown in the past is that (for large enough $$n$$) you can always find $$Q_n$$ where the outcome model can’t be reliably rejected but where $$\hat\theta_w$$ has higher mean squared error than $$\hat\theta_e$$

The standard theoretical result in this direction is the local asymptotic minimax theorem. Here’s the version from van der Vaart & Wellner’s book

3.11.5 Theorem (Minimax theorem). Let the sequence of experiments $$(X_n,{\cal A}_n, P_{n,h} :h\in H)$$ be asymptotically normal and the sequence of parameters $$\kappa_n(h)$$ be regular. Suppose a tight, Borel measurable Gaussian element $$G$$, as in the statement of the convolution theorem, exists. Then for every asymptotically $$B'$$-measurable estimator sequence $$T_n$$ and $$\tau(B')$$-subconvex function $$\ell$$, $\sup_{I\subset H} \liminf_{n\to\infty}\sup_{h\in I}E_{h\star}[r_n(T_n-\kappa_n(h))]\geq E[\ell(G)]$ Here the first supremum is taken over all finite subsets $$I$$ of $$H$$.

That might need a little translation. $$H$$ is the model space, which is a (possibly infinite-dimensional) vector space. $$P_{n,h}$$ is a way to define a distribution near some distribution $$P$$; think of it as being different by $$h/\sqrt{n}$$. In our setting $$r_n=\sqrt{n}$$; it says how fast everything needs to scale to be just interesting different. The parameters $$\kappa_n(h)$$ are the parameters you’re interested in: in our case, $$\kappa_n(h)$$ is the ‘true’ value of $$\theta$$ for the distribution $$P_{n,h}$$. We can tiptoe past the measurability assumption, because we’re not Jon Wellner, and $$\tau(B')$$-subconvex function in our case is just the Euclidean squared error – the point of having more generality in the theorem is to show that the result isn’t something weird about squared-error loss. $$E_h$$ is the expectation under $$P_{n,h}$$ and $$E$$ is the expectation under the limiting $$P$$. Finally, $$G$$ is the limiting Normal distribution of the efficient estimator in whatever model we’re working in.

If we were working in a parametric model you could take $$I$$ to be a ball of some radius $$\delta$$, and the result would then say that for any $$\delta_n\to\infty$$ there’s some sequence of $$P_{n,h}$$ at distances $$\delta_n/\sqrt{n}$$ from $$P$$ where the mean squared error of $$T_n$$ is asymptotically no better than it is for the efficient estimator. That is, $$T_n$$ can’t be better than the efficient estimator uniformly even over very small neighbourhoods of a point. For technical reasons you can’t use balls of small radius as small neighbourhoods in the infinite-dimensional case, but the idea is the same.

We’d normally use the local asymptotic minimax theorem with the sampling+outcome model being $$H$$, in which case it would show us that no estimator $$T_n$$ could beat $$\hat\theta_e$$. Instead, we’re going to use it with the sampling model being $$H$$ (or some well-behaved large submodel that I won’t try to specify here). The efficient estimator is $$\hat\theta_w$$, and $$\hat\theta_e$$ is our alternative estimator $$T_n$$, and we’re working at a point $$P$$ where the sampling+outcome model is true.

The theorem now talks about nearby distributions $$P_{n,h}$$ where the sampling model is true but the outcome model isn’t. There are sequences of $$P_{n,h}$$ converging to $$P$$ where $$\hat\theta_e$$ (ie, $$T_n$$) doesn’t beat the weighted estimator $$\hat\theta_w$$. The efficiency advantage of $$\hat\theta_e$$ doesn’t generalise even a very little way away from where the outcome model is true.

That’s less persuasive (I think) than my construction. First, it doesn’t show $$\hat\theta_w$$ is better, just that it’s no worse. Second, the distance between the true and nearly-true model is $$\delta_n/\sqrt{n}$$ for $$\delta_n$$ potentially diverging to infinity. In my construction, we reach equal mean squared error at an explicit, finite $$\delta$$, and it keeps getting worse for larger $$\delta$$.

The reason I can do better is because of regularity. The full power of the local asymptotic minimax theorem is needed for estimators with unsmooth behaviour as a function of $$h$$: these can be silly counterexamples like Hodges’s superefficient estimator or useful ideas like the lasso, or something in between, like a regression estimator that adjusts only for statistically significant confounders.

A compromise estimator based on testing goodness of fit of the outcome model could mitigate the breakdown of $$\hat\theta_e$$. It still couldn’t do uniformly better than $$\hat\theta_w$$ when the model was only nearly true – the local asymptotic minimax theorem guarantees that. It’s concievable that it could do well enough to be preferable.