How far do open weights trail the frontier?

I saw this Twitter post today and really liked the idea. But I think the AA Index is a rather crude way and much prefer ECI from Epoch, which uses IRT. The resulting graph does meaningfully diverge from the Twitter post (which seems to weirdly collapse at the end, maybe because of no logistic assumptions being taken into consideration):Each open-weight frontier model and how many months earlier a closed model had reached its ECI.[see linkpost to actually interact with graphs, like seeing what mo...