Daily-return association (r)
A number from −1 to +1 describing how closely two coins' daily returns moved together in a given window.
The association is computed from RETURNS (day-over-day percentage change), not price levels, and the series are aligned by UTC date, not by array position. Both details decide the outcome: raw prices rise and fall with the whole market, so on price levels almost everything looks "strongly linked", and positional alignment pairs up days from different dates whenever a coin has a hole in its history. A value of +1 means the daily returns moved identically, 0 that there was no linear relationship, −1 that they moved exactly opposite. A result is produced only when the pair has enough SHARED days; with a shorter history we give no stand-in value and skip the pair. This describes the past in one specific window, not a permanent trait of a coin.
In plain words
A measure of whether two coins travelled together. Near +1, when one rose, so did the other. Near 0, one said nothing about the other. Below zero, they moved in opposite directions. We compute it from daily percentage changes, because raw prices rise with the whole market and everything would look linked.
Example
In a 90-day window the BTC · ETH pair has r = +0.88. The daily returns of those coins essentially echoed each other. In the same window another pair comes out at r = +0.04, meaning one coin's daily returns said nothing about the other's. A third pair with only 9 shared days against a threshold of 21 does not appear at all. Instead of a zero you see a note about insufficient history.
Also known as: korelacja zwrotów, return correlation, pearson r