Concept

Doubly robust — where it appears

The property of an estimator that stays consistent when either of two models it uses is right, even if the other is wrong. The two are a model for the assignment and a model for the outcome, it gives no protection when both are wrong, and weights fitted to balance the covariates have it built in.

Named by 4 essays across one field — each of them below, with the objects they name alongside it.

Either model is enough; neither is not. The bias of three estimators of an average effect of 1.0000, over 600 samples of 600 units with the assignment rule at strength 1, in each of the four cells made by getting each nuisance model right or wrong. The wrong model in both cases is one that omits the second covariate, which the outcome and the assignment both depend on. The outcome model alone is off by 0.8064 whenever it is the wrong one; weighting alone is off by 0.8190 whenever the propensity model is. The augmented estimator built from both is off by -0.0085, -0.0083 and -0.0016 in the three cells where at least one of them is right, and by 0.8118 in the fourth — which is between its two components rather than better than either.

Either model, but not neither

The augmented estimator's bias is −0.0085, −0.0083 and −0.0016 wherever one nuisance model is right, against components off by 0.8064 and 0.8190. One step past the overlap sweep it is the least biased estimator on the table at 0.0857 and the worst on it at 1.9265.

weights · Weighting

Weights that balance a sample by construction. What three sets of weights leave of the standardised difference between the arms on each covariate, as a root mean square over 1200 samples of 600 units. The true propensity leaves 0.1317 and 0.1186 — a sampling error, since it is right about the population and knows nothing of the draw. A likelihood fit leaves 0.0770 and 0.0657, having absorbed part of the draw's imbalance as a side effect of fitting the treatment. Weights fitted so that each arm's weighted means are the sample's leave 1.4e-14 and 1.2e-14, which is the arithmetic's floor rather than a small number: the largest gap between a weighted arm mean and the sample mean in any draw is 9.8e-14.

A weight fitted to balance

Weights fitted so that each arm's weighted covariate means equal the sample's leave a difference of 1.4×10⁻¹⁴ between the arms and give the estimate a third of the variance of weights fitted by likelihood — 0.011883, within a relative 5.8% of the bound no estimator can beat. In the world where the assignment carries a square nobody named, the same exact balance leaves the square further apart than no weighting at all, and where the outcome carries it too the estimate is wrong by 0.6973 with an interval that covers 1.5%.

weights · Weighting

One weighting told the means and one told the second moments, in five worlds. The bias of the fit to balance over 600 samples of 600 units in each world, fitted to the covariates' means and fitted to their means, squares and product. Told the means it is off by -0.0004, -0.0020, 0.0103, 0.6973, 0.2698 in the worlds with no square, a square in the assignment, a square in the outcome, a square in both and a cube in both; told the second moments, by -0.0009, -0.0000, 0.0009, -0.0045, 0.3099. Its interval covers 94.0%, 94.7%, 95.0%, 1.5%, 51.0% and 93.7%, 89.8%, 94.0%, 91.0%, 48.3%.

The moments a balance is told

Weights fitted to balance the covariates' means were wrong by 0.6973 in the world where both the assignment and the outcome carry a square. Told the squares and the product as well, the same construction is off by −0.0045 there and its interval covers 91.0%. The failure moves up a moment rather than away: with a cube in both, the second-moment balance is off by 0.3099 and leaves the cube twice as far apart as no weighting. And where overlap is thin, 37.0% of samples have no such weights at all.

weights · Weighting

The bias a tolerance on the second moments leaves where a square drives both the assignment and the outcome. 600 samples a point. Bias at δ = 0, 0.02, 0.05, 0.1, 0.2, 0.5: −0.0074, 0.0500, 0.1353, 0.2782, 0.5473, 0.6983; coverage 89.7%, 89.5%, 72.4%, 26.5%, 1.0%, 1.0%. The tolerance spent in full at the outcome's slope on the square would give 2·δ·√2: 0.0566, 0.1414, 0.2828, 0.5657 at the four smaller values; balancing the means alone gives 0.6973.

A balance allowed a tolerance

Weights that balance the means exactly and the second moments only to within a tenth of a standard deviation were supposed to keep the second-moment repair and recover the samples exact balance loses. Where a square drives both the assignment and the outcome they keep neither half of the repair: the bias is 0.2782 against −0.0074 with exact balance, and the interval covers 26.5%, because the weights spend the whole tolerance — every second-moment gap sits at the edge of its band — and the outcome converts a gap of a tenth into bias at its own slope, 2·δ·√2 = 0.283. At thin overlap the same tolerance does recover samples: 89.0% have weights at all, against 62.0% with exact balance.

weights · Weighting

Named alongside it

The objects these essays reach for when they reach for this one.

Inverse-probability weighting Model misspecification Propensity score Covariate balance Overlap Standardised difference Closed form Effective sample size Efficiency bound Estimand Positivity Augmented estimator

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