There are many ways to compute metrics about web accessibility. Some are discussed here

Site-wide audit

Used formula: passed / (passed + failed)

Another Formula could be: average mark for all pages, i.e. avf(mark)

Page audit

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Data available

Accessibility metrics for a page-wide audit

• number of passed / failed / NMI / NA on a given page
• number of occurrences of error for a given failed test
• number of occurrences for a given NMI test
• total number of effective tests (Tanaguru-rules dependant)
• total number of empty tests (all the Tanaguru "no-check" rules)
• total number of tests for the Reference used (e.g. 305 for AccessiWeb 2.1)

Accessibility metrics for a site-wide audit

• number of passed / failed / NMI / NA on a site-wide audit
• total number fo pages (site-wide audit)