The cumulative RSI is intended to be used in cumulative RSI strategy described in Chapter 9 of Trading Strategies That Work by Larry Connors and Cesar Alvarez.
The formula is pretty simple:
CumulativeRSI = SUM(RSI(N), X)
The book recommends to use the small numbers for N and X, such as 2.
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See also CumulativeRSI signal: viewtopic.php?f=29&t=1341
The indicator was revised and updated