# Large sample theory

Delta methods, influence functions, and so on. Convolution theorems, local asymptotic minimax theorems.

A convenient feature of M-estimation, and especially maximum likelihood esteimation is simple behaviour of estimators in the asymptotic large-sample-size limit, which can give you, e.g. variance estimates, or motivate information criteria, or robust statistics, optimisation etc.

In the most celebrated and convenient cases case asymptotic bounds are about normally-distributed errors, and these are typically derived through Local Asymptotic Normality theorems. A simple and general introduction is given in Andersen et al. (1997) page 594., which applies to both i.i.d. data and dependent_data in the form of point processes. For all that it is applied, it is still stringent.

## Fisher Information

Used in ML theory and kinda-sorta in robust estimation. A matrix that tells you how much a new datum affects your parameter estimates. (It is related, I am told, to garden-variety Shannon information, and when that non-obvious fact is more clear to me I shall expand how precisely this is so.) 🏗

## Convolution Theorem

The unhelpfully-named convolution theorem of Hájek (1970).

Suppose $$\hat{\theta}$$ is an efficient estimator of $$\theta$$ and $$\tilde{\theta}$$ is another, not fully efficient, estimator. The convolution theorem says that, if you rule out stupid exceptions, asymptotically $$\tilde{\theta} = \hat{\theta} + \varepsilon$$ where $$\varepsilon$$ is pure noise, independent of $$\hat{\theta}.$$

The reason that’s almost obvious is that if it weren’t true, there would be some information about $$\theta$$ in $$\tilde{\theta}-\hat{\theta}$$, and you could use this information to get a better estimator than $$\hat{\theta}$$, which (by assumption) can’t happen. The stupid exceptions are things like the Hodges superefficient estimator that do better at a few values of $$\hat{\theta}$$ but much worse at neighbouring values.

## References

Andersen, Per Kragh, Ornulf Borgan, Richard D. Gill, and Niels Keiding. 1997. Statistical models based on counting processes. Corr. 2. print. Springer series in statistics. New York, NY: Springer.
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