ETH vs BTC - 90-Day Correlation
How tightly the two move together, day by day. Near 1 they are one trade wearing two tickers; when it drops, ether is being priced on its own story.
The correlation between ether's and bitcoin's DAILY RETURNS over a rolling 90-day window, from +1 (move identically) through 0 (unrelated) to -1 (opposite).
For example, 0.92 means that on almost every day both coins moved the same direction by a proportional amount - a portfolio of both is one bet, not two.
Sustained falls below 0.7 are the interesting ones: something specific to Ethereum, an upgrade or an ETF flow, is outweighing the whole-market tide.
Pearson correlation of the last 90 daily percentage changes of each coin
Correlation of RETURNS, not prices - correlating two long-rising price lines would read near 1 forever and mean nothing. It also says nothing about which coin leads the other.
Own compute - daily BTC closes (2010+) joined to daily ETH closes (2017+), no external API