The BitMEX Weekly Historical Ether Volatility Index is referred to as the .EVOL7D Index. This index is calculated logarithmic percentage change taken from measurements taken on the ETHXBT spot price every minute.
The settlement price is calculated from 10080 snaps over the 7-day period (1440 per day).
P = Last Price (taken at 1 minute intervals)
Stdev = Sample Standard Deviation
Ln = Natural Logarithm
Sqrt = Square Root
.EVOL7D Index = Stdev(Ln(P1/P0), Ln(P2/P1), ..., Ln(P2016/P2015))