In the lead saturated event study (SSDC), the uncontrolled post-adoption effect on supplemental-rebate share is +3.37 pp. After pharmacy-benefit governance controls, it attenuates to +1.28 pp. A stable-design benchmark dropping reform-dense windows gives −2.05 pp. The standalone any-pool coefficient is 0.06 pp (p = 0.95). Nominal pool membership is not separable from the governance bundle.
Three pool families (NMPI, TOP$, SSDC) emerged in the mid-2000s on a simple intuition: more covered lives ⇒ more bargaining power ⇒ bigger supplemental rebates. By 2024, 33 states belonged to one.
But pool adoption almost always travels with a broader pharmacy-benefit overhaul: uniform PDLs, pharmacy carve-outs, MCO formulary autonomy, vendor restructuring. The pool, the carve-out, and the uniform PDL move together — and nobody had tried to separate them.
| Specification | Δ rebate share |
|---|---|
| SSDC, uncontrolled saturated event study | +3.37 pp |
| SSDC, + governance controls | +1.28 pp |
| Stable-design benchmark (drop reform-dense) | −2.05 pp |
| Static bundle: SSDC alone | +1.27 pp (p=0.57) |
| Static bundle: any pool | +0.06 pp (p=0.95) |
The strongest suggestive evidence is for a specific governance bundle — pool + uniform PDL + pharmacy carve-out from MCOs:
Treated as a support-limited descriptive concentration, not a causal headline — the cell sample is small.
Built a new 50-state, 2002–2024 panel linking CMS-64 supplemental-rebate data to a hand-coded, spell-level pharmacy-benefit governance dataset — uniform PDL status, carve-out, MCO formulary autonomy, vendor type, negotiation model.
Used SSDC’s late-adopter set as the lead heterogeneity-robust saturated event study; NMPI and TOP$ as appendix extensions.
Joining a pool is not, on its own, a lever for higher rebate capture. The actionable story is the governance bundle — preferred drug lists, carve-out architecture, negotiation model — that historically traveled with pool adoption. State fiscal staff should price that bundle, not the pool label.