Reasoning
Early case-finding disproportionately tested people with a market link, so market-associated positives were over-represented, creating the illusion that the market was the source (everyone testing positive had been there). Weissman marshals additional strands against a market spillover: (i) the more-ancestral lineage A appeared in locations away from the market while the derived lineage B clustered at the market - the reverse of the ordering expected if the market were the spillover site; (ii) SARS-CoV-2 RNA in market environmental swabs showed no tendency to co-occur with DNA of candidate non-human host species, unlike genuine animal coronaviruses whose RNA correlated with their hosts’ DNA (Bloom), undercutting an infected-animal source at the market; (iii) early social-media (Weibo) illness reports clustered on the south side of the Yangtze near the Wuhan CDC and WIV rather than the market; and (iv) Levin’s spatiotemporal Bayesian reanalysis found ~27:1 odds favouring a research-side source over the market. Together these support H-19 (the clustering is an ascertainment artifact) and, by removing the market-zoonosis evidence, shift the overall odds in H-17 toward a research-related origin.
Validity
Reconstruction. Premises (assumed): early case definitions preferentially tested people with a market link; (i) ancestral lineage A appeared away from the market while derived lineage B clustered at it; (ii) market environmental RNA did not co-occur with candidate host DNA (Bloom); (iii) early Weibo illness reports clustered near CDC/WIV; (iv) Levin’s spatiotemporal reanalysis gave ~27:1 for a research-side source. Load-bearing step: market-link-biased testing manufactures a market-centred cluster, and (i)-(iv) each remove or reverse the market-spillover reading, so ascertainment bias is the better explanation and, with market-zoonosis evidence subtracted, the odds shift toward lab.
Checked. The ascertainment mechanism validly produces over-representation of market-linked positives conditional on the premise that early testing was market-biased. Each strand is a valid conditional inference in the stated direction: (i) an ancestral-lineage-away / derived-at-market pattern is the reverse of the market-as-source expectation; (ii) RNA-without-host-DNA undercuts an infected-animal source; (iii)/(iv) directionally support a non-market/research-side locus. The comparative conclusion (“better explained by ascertainment than by true spillover”) follows once these strands neutralise the market evidence. Probed the strongest defeater — Worobey/Pekar’s finding that early cases without market links also cluster spatially on the market, which market-link-biased testing alone would not generate, and Pekar’s two-introduction reading of the lineage A/B split. These contest the truth of the ascertainment premise and of strand (i), not the validity of the inference conditional on the stated premises, so they are priced at step 8; strand (iii) (Weibo/CDC proximity) is individually weak but only additive here. No premise-preserving defeater blocks the conditional step. Approved.