What is RSI? The Relative Strength Index for systematic trading

RSI compares the average size of recent up-closes to the average size of recent down-closes over a lookback period (classically 14), expressing the result on a 0–100 scale. High readings mean recent gains have dominated; low readings mean losses have. It's a measure of momentum — how forcefully price has been moving — not of price level or trend direction.

Overbought, oversold, and divergence

The textbook reading is that above 70 is “overbought” and below 30 “oversold,” hinting at a possible reversal. The subtler use is divergence: price makes a new high but RSI doesn't, suggesting weakening momentum. Both are genuinely useful — and both are traps in discretionary hands, because RSI can sit overbought through an entire trend, and divergence can persist far longer than your account can.

Why the indicator isn't the edge

Millions of traders see the exact same RSI 70/30 levels, so the level alone is not an edge. The edge, if there is one, lives in a specific, tested rule built around RSI — combined with context, confirmation, and risk — applied consistently. A discretionary trader asks “is it oversold enough?” and hesitates; after a loss they skip the next valid signal out of fear.

How a system uses RSI

A rules-based strategy doesn't interpret RSI — it executes a defined condition involving it, identically every time. RSI crosses your threshold under your other conditions, the system acts; otherwise it waits. No “close enough,” no skipping after a loss, no holding for “a bit more.” That mechanical consistency is the whole point: the system never gets bored of taking the same RSI setup for the thousandth time, which is exactly why its edge can show up over a large sample.