Current definition alt-v1. Charts stamped with it were computed by src/metrics/altseason.py, and every value plotted for a past date could have been computed on that date.
CHART_LIBRARY.md §3.3 defines it as "% of top-50 alts outperforming BTC over 90d".
Measured on market cap, not price — the basket carries capitalisation, and there is no free price series for 800 assets. For an asset with fixed supply the two are identical; for one still inflating its supply, cap return overstates price return. That biases the index up for high-emission alts. Stated here rather than hidden, and it is the reason this chart is a regime indicator, not a tradable signal.
Membership is ranked per day, so the index never back-fills today's winners into 2019.
Newest first. Each entry says what changed and whether values that had already been published moved with it, followed by the sentence from the module that decided the verdict.
top-50 by basket cap, 90d cap return vs BTC, stables excluded.
The first version of this metric. There is no earlier definition to restate.
1 chart name this metric in their readout line.