Concept

Cluster-robust standard error — where it appears

A variance estimate that sums residual contributions within a group before squaring them, so that observations in the same group are allowed to repeat each other. It is consistent in the number of groups rather than of rows, which is why a study with few groups needs a reference other than a normal.

Named by 3 essays across one field — each of them below, with the objects they name alongside it.

Also named here as clustered sampling — the same set of essays touches all of them, so they are one junction rather than several.

The rows are held fixed; only the clusters move. Counted coverage of four 95% intervals for a slope, at five cluster counts with the row count held at 300 throughout and a within-cluster correlation of 0.1, over 6000 draws apiece, with the sizes equal. The interval that counts rows covers 53.42% at 5 clusters — a second closed form says 2Φ(z/√D) − 1 = 54.44% for a design effect of 6.900, and reads nothing about clusters at all. The cluster-robust interval read against a normal covers 74.43% there and 94.20% at 100 clusters; read against a t on G − 1 it covers 85.08% and 94.47%. The number of independent things is the cluster count, and every quantity here is blind to how many rows were typed.

The count that is not the rows

Three hundred rows in five clusters of sixty carry 6.9000 times the variance an independent-rows calculation reports, and the interval that counts rows covers 53.42%. The same five unequal sizes laid out two ways give design effects of 9.3158 and 5.4652.

sandwich · Misspecification

One estimator, three answers, and only the reference changes. Coverage of the cluster-robust 95% interval for the slope against the number of clusters, at 30 rows in each. The estimator is identical in all three curves; what differs is the number it is compared against. At 5 clusters it covers 75.05% against a normal, 85.30% against a t on 4 degrees of freedom and 87.95% against a t on 3. At 80 clusters the three agree to within a point. The correction costs nothing: the same standard error, a different table.

The reference the sandwich is read against

The cluster-robust interval covers 75.05% at five clusters and 93.58% at eighty. The same estimate read against a t on G − 2 covers 87.95% at five, and the estimator is unchanged — three hundred rows grouped into five clusters cover 74.28% where the same three hundred grouped into seventy-five cover 94.63%.

sandwich · Misspecification

Coverage of the cluster-robust slope interval at six, ten and twenty clusters of unequal size, by reference. 5,000 studies of 300 rows at each setting, within-cluster correlation 0.2. 6 clusters, equal sizes: 89.1%, 92.6%, 96.0%; 6 clusters, 5:1, big at the edges: 80.1%, 88.1%, 95.2%; 6 clusters, 5:1, big at the centre: 93.8%, 95.4%, 96.8%; 6 clusters, 15:1, big at the edges: 76.2%, 86.8%, 94.5%; 10 clusters, equal sizes: 91.1%, 93.0%, 95.8%; 10 clusters, 5:1, big at the edges: 86.3%, 90.3%, 94.7%; 10 clusters, 5:1, big at the centre: 94.5%, 95.7%, 96.4%; 10 clusters, 15:1, big at the edges: 84.9%, 89.6%, 94.8%; 20 clusters, equal sizes: 92.9%, 93.8%, 95.1%; 20 clusters, 5:1, big at the edges: 90.9%, 92.7%, 94.8%; 20 clusters, 5:1, big at the centre: 94.5%, 95.0%, 95.6%; 20 clusters, 15:1, big at the edges: 90.6%, 92.4%, 94.7% — for the usual variance on G − 2, the bias-reduced variance on G − 2, and the bias-reduced variance on its effective count.

An effective number of clusters

Ten clusters of thirty rows, a covariate spread evenly across them, and a cluster-robust interval for the slope read against a t on G − 2 = 8 covers 91.1%. The slope needed about five degrees of freedom, not eight, because the two outermost clusters carry 49% of its variance and the middle two 0.6%. A count computed from the sizes and positions before any outcome is seen — 4.45 here, 3.32 when the big clusters sit at the edges — with the bias-reduced variance covers between 94.5% and 96.8% in all twelve layouts measured, where the usual interval on G − 2 covers between 76.2% and 94.5%.

sandwich · Misspecification

Named alongside it

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

Cluster size Clustered sampling Coverage Degrees of freedom Design effect Effective sample size Kish effective size Leverage Monte Carlo Reference distribution Sandwich estimator T interval

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