Normal Distribution
nothing is called 95% until it has been counted
Concept

Analysis of covariance — where it appears

A regression that compares groups on an outcome while adjusting for a measured covariate such as a baseline value. It estimates how strongly the covariate predicts from the data themselves, which is why it removes more of the noise than a change score does.

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

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

The sample a trial needs for the same power, as a share of the unadjusted analysis's, by how strongly a baseline covariate predicts the outcome. Adjusting for the covariate as measured needs one minus the squared correlation of the unadjusted sample; cut at the median, one minus 2/π of the squared correlation. At a correlation of 0.7 those are 0.510 and 0.688; cut in three, 0.611; the change from baseline, 0.600. The change score needs fewer patients than the median-cut adjustment once the correlation passes 0.624.

A baseline cut in two

Adjusting a trial's result for a baseline measurement that predicts the outcome shrinks the sample it needs to 1 − ρ² of the unadjusted one. Adjusting for whether that measurement was above its median shrinks it only to 1 − (2/π)ρ² — the cut keeps 63.7% of what the covariate could remove, exactly the fraction a cut outcome keeps. In patients the loss grows with the covariate's strength: a cut costs 12% more patients at a correlation of 0.5, 35% at 0.7 and 2.55 times as many at 0.9, where a simple change from baseline would have done better than the cut adjustment.

planned · Power What four adjusted analyses report about their own precision and what they have, 50 patients an arm, baseline correlated 0.7. Reported against actual variance of the treatment estimate, as shares of the unadjusted analysis's, over 20,000 trials of 50 patients an arm. Adjusted for the baseline as measured: 0.529 reported, 0.518 actual. Median cut fixed in advance: 0.716 and 0.700. The decile cut with the best fit: 0.671 and 0.736. The decile cut with the smallest p-value: 0.762 and 1.615.

The cut that fitted best

A trial that adjusts for its baseline cut at whichever of the sample's nine deciles fits the outcomes best — without ever looking at the treatment difference — reports a variance 0.671 of the unadjusted analysis's where its estimates actually have 0.736, which is worse than the 0.700 a median fixed in advance delivers. The sample says the chosen cut kept 72.4% of what the baseline could remove; the population says it kept less than a median. A true null is rejected 5.87% of the time at fifty patients an arm and 9.31% at ten, and a cut chosen for its p-value rejects 14.22%.

planned · Power The variance of the baseline-adjusted estimate for four arrangements of visits, against the correlation between visits. At a correlation of 0.5 between visits, the adjusted analysis has 0.750 of the unadjusted variance with one visit before treatment and one after, 0.667 with two before, 0.500 with two after and 0.417 with two of each.

A visit before or a visit after

A trial that can afford one more measurement per patient can take it before treatment, to sharpen the baseline adjustment, or after it, to average the outcome. With every pair of visits correlated 0.5, the second visit after treatment removes a quarter of the variance and the second visit before it removes a twelfth, and it is never the other way round: the baseline's contribution is ρ²(1 − ρ)/(1 + ρ), which peaks at ρ⁵ = 0.090 when ρ is the golden ratio's reciprocal. Below a correlation of one half, a single extra follow-up beats any number of baselines. When the correlation fades with the time between visits, averaging an earlier baseline into the adjustment makes the trial less precise, not more.

planned · Power

Named alongside it

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

Baseline adjustmentStatistical powerChange scoreDichotomisationSample sizeAutocorrelationCorrelationData snoopingError rateExperimental designOptimismRandomisation

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