From d59ff2b659fec7a957417734e5a1973cda1ca913 Mon Sep 17 00:00:00 2001 From: Experiments DB Dev Date: Mon, 20 Jul 2026 14:07:49 -0400 Subject: [PATCH] docs(writeup): add design-rationale section for the sham/pooled control grouping Document Box-A2 as an a-priori sham/sensory control and the two-tier analysis it implies: B2-vs-A2 as the primary sensation-controlled contrast, and the pooled-control mergeNaive grouping as a confirmatory power-boost licensed by A2/Naive exchangeability (with the TOST caveat that the exchangeability is supported but not proven at n=3-4). Co-Authored-By: Claude Opus 4.8 --- analysis/writeup.md | 35 +++++++++++++++++++++++++++++++++++ 1 file changed, 35 insertions(+) diff --git a/analysis/writeup.md b/analysis/writeup.md index 10672ba..29a0dbe 100644 --- a/analysis/writeup.md +++ b/analysis/writeup.md @@ -32,6 +32,41 @@ per-group sample sizes and effect sizes. Overall accuracy (successes/attempts) and counts were additionally modeled with binomial and Poisson GLMMs. All analysis code, derived datasets, and this figure are in `analysis/matlab/`. +## Design rationale (control grouping) + +Box-A2 exists by design as a **sham / sensory control**: it delivers the +stimulation *sensation* without the effective modulation, so that a Box-B2 +benefit can be attributed to the treatment itself rather than to the experience +of being stimulated (arousal, attention, cutaneous sensation). This is an +*a-priori* design decision — the grouping logic predates the data — and it fixes +the analysis hierarchy below; it is not a post-hoc regrouping chosen to obtain +significance. + +- **Primary, sensation-controlled contrast — B2 vs A2.** Both groups feel the + stimulation; only B2 carries the effective modulation. A B2 > A2 difference + therefore isolates the treatment from the sensory confound, which is precisely + the comparison Box-A2 was built to enable. This is the pre-specified primary + test. On its own it is underpowered (n = 3 vs 4). + +- **Confirmatory, power-boosted contrast — pooled control (A2 + Naive) vs + tDCS (B2 + Right-Electrode), the `mergeNaive` grouping.** Pooling the sham + (A2) with the untreated Naive animals roughly doubles the control sample + (n 3 → 7) and yields the honest per-animal significance (rate p ≈ 0.003) and + the paper-matching day × tDCS interaction. Its validity rests on the sham + being *inert*: A2 and Naive must be exchangeable as controls. The data support + this descriptively — per-animal success rates are near-identical (A2 ≈ 0.44 vs + Naive ≈ 0.50; difference n.s., Welch p ≈ 0.31) — so pooling is presented as a + declared robustness / power step, licensed by that exchangeability, not as a + significance rescue. + +- **Caveat.** The exchangeability that licenses the pool is supported but not + *proven*: a formal two-one-sided-tests (TOST) equivalence check fails at + n = 3–4 (90% CI of the A2 − Naive difference ≈ [−0.18, +0.06], too wide for a + ±0.10 margin). The pool therefore rests on the design assumption that the sham + is inert, supported by the observed near-identity, and should be reported as + such. Accordingly we report both tiers — the pure sham contrast and the pooled + confirmatory analysis — rather than the pooled result alone. + ## Results (summary) Both groups showed strong session-to-session learning (day effect, p < 10⁻¹³),