FA-74747 / Experiment statistics / Member archive

Pre-period balance check: The difference is scaled by the control spread only · case 02

A treatment arm with extra variance changes the balance score asymmetrically.

Member previewVariant 2 · 3 implementations · 8 checks per implementation

Case contract

Standardised mean difference = (mean_t - mean_c) / sqrt((var_c + var_t) / 2) with sample (n - 1) variances; imbalance iff |SMD| > threshold. Zero pooled variance -> [0.0, False] when means are equal else [None, True]. Fewer than two values in an arm -> None. Return [round(smd, 6), imbalance].

Why this case matters

Pre-period imbalance warns that randomisation or logging went wrong before any effect is read.

One recorded failure

Sample boundary fixture

This sample comes from the broken implementation of a controlled reproducer.

Boundary fixture	Actual	Expected	Outcome
small arms	`[1.414214, true]`	`[0.894427, true]`	Failed

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