The viability equation
Longevity is fit with the Ellis & Roberts (1980) viability equation, v = Ki − p/σ, where p is time in storage, v is the probit of viability, Ki is initial probit viability, and σ is days per probit of loss. Parameters are fit by maximum likelihood on the binomial germination counts through a probit link. Lots are never pooled.
Why σ leads
Comparative studies have moved away from P50 because it is confounded by a lot's initial viability: two lots with identical decay rates but different starting viability report different P50s. σ — the probit rate of loss — is the axis that actually compares. P50, P70, and P85 remain as readouts, standardised to each lot's own fitted Ki, because re-collection schedules are written in viability thresholds, not in σ.
What P50 / P70 are relative to — the baseline question
Every P-value on this site is computed from the lot's own fitted parameters — no external or species-level baseline is ever involved. On the default absolute definition, Px = (Ki − probit(x)) · σ: the day the fitted curve crosses x% viability of all seeds. This means a lot whose fitted initial viability is already below x% — common for wild-collected material — has no Px on this definition: the curve never passes through x% going forward. The lab flags that case with the reason and the lot's fitted starting viability rather than showing a blank; σ and P50 usually remain valid.
For low-baseline lots, the calculator and batch fitter also offer a relative definition: Px (rel) = time until viability falls to x% of the lot's own fitted initial viability. This is defined for every lot, and every export labels which definition was used — the two are not interchangeable and should not be compared across studies without noting the convention.
Three tiers, never mixed
Every value is stamped measured (fit from your counts), federated (published constants), or predicted (trait/phylogeny model). The stamp travels with the number into every table, plot, and export, and the underlying claim is written to an append-only log.