Understanding age assurance accuracy

Note: you will probably want to view this post on the web because there is some math notation that uses MathJax to render. I've recently seen a number of pieces of pieces about age assurance that want to talk about the degree to which age assurance mechanisms are "accurate" or "effective". In discussions like this it's common for people to talk about accuracy and effectiveness as if they were unitary quantities that could be measured along a single axis, as in this diagram by Audrey Hingle, which shows effectiveness on the Y axis and privacy on the X axis. Age Assurance Mechanisms (Source: Hingle) I don't agree with the placement of a lot of the boxes on this diagram, but I want to focus on the placement of two, "Biometric Age Estimation" which is shown as not very effective, just above "Parental/guardian attestation" and "Digital ID/Credential-based verification", which is shown as very effective. This is a pretty commonly expressed sentiment; for example Flanagan writes:…