Bollinger Band Reversion

Buy lower band, exit upper band — volatility-adaptive mean reversion.

Bollinger Bands plot two standard deviations around a 20-period SMA. Price reaching the lower band signals a statistical extreme — about 2.5% probability assuming normality. The strategy fades these extremes. Works on range-bound symbols; layer in a "no entry if bands are widening" rule to skip breakout regimes.

//@version=5
strategy("Bollinger Band Reversion", overlay=true)

length = input.int(20, "BB Length")
mult = input.float(2.0, "BB Multiplier")

basis = ta.sma(close, length)
dev = ta.stdev(close, length) * mult
upper = basis + dev
lower = basis - dev

if close < lower
    strategy.entry("Long", strategy.long)

if close > upper
    strategy.close("Long")