Definition
Plain language
A single number saying how many things really shared the credit, rather than how many were technically involved.
As stated in the literature
A diversity statistic (analogous to effective species number in ecology) computed from the distribution of per-component contributions; it distinguishes one dominant driver plus stragglers from genuinely diffuse contributions across all slots.
Also called: effective-count statistic, effective number
Why it matters: It tells you whether an outcome rests on a single fragile driver or is genuinely spread across many contributors, which changes how robust it is.
For example, if one ingredient explains almost the entire result and four others barely register, the effective count is close to one even though five were involved.
Heard on the show
“They measure it with an effective-count statistic, the same kind ecologists use to ask how many species are really present versus one dominant species plus a few strays.”Episode 243 — How a Hundred Meaningless Word Choices Add Up to Flip a Model's Answer