George Lane developed the Stochastic Oscillator in the late 1950s. The formula: %K = (Current Close - Lowest Low over N periods) / (Highest High over N periods - Lowest Low over N periods) × 100. The default N is 14 periods. %K oscillates between 0 and 100. A reading of 80 means the close is near the top of the 14-period range; a reading of 20 means the close is near the bottom. %D is a 3-period simple moving average of %K, creating a signal line. The Stochastic generates signals from %K crossing %D and from both lines entering or exiting the 80/20 threshold zones.

There are three variants: Fast Stochastic (raw %K), Slow Stochastic (uses the 3-period average of %K as the main line), and Full Stochastic (allows customization of all smoothing parameters). Slow Stochastic is most commonly used in practice because it filters out the whipsaws inherent in the raw %K calculation. Platforms typically default to Slow Stochastic (14,3,3) — a 14-period %K, smoothed with a 3-period average for the slow %K, and then a further 3-period average for %D.

Above 80 is conventionally labeled overbought; below 20 is oversold. The critical mistake is treating these thresholds as automatic sell/buy triggers. In a strong uptrend, the Stochastic can remain above 80 for weeks — momentum is powerful and can sustain 'overbought' readings through an entire trend phase. Lane himself emphasized that %K above 80 in an uptrend is a sign of strength, not a sell signal. The reversal signal comes when %K drops back below 80 from above — that bearish hook is the actual trigger.

%K/%D crossovers are the primary signal. When %K crosses above %D while both are below 20, it signals potential upside reversal. When %K crosses below %D while both are above 80, it signals potential downside reversal. Crossovers outside the extreme zones produce many false signals and are typically filtered out by disciplined traders. The highest-probability setups are crossovers within the extreme zones, particularly on multiple timeframe alignment (daily oversold crossover while weekly is in an uptrend).

Like other oscillators, the Stochastic generates its most reliable signals through divergence. Bullish divergence: price makes a lower low but Stochastic makes a higher low — selling momentum is waning. Bearish divergence: price makes a higher high but Stochastic makes a lower high — buying momentum is deteriorating. Divergences are most meaningful when they occur at significant price structure levels (major support or resistance zones) rather than in the middle of a range.

Multi-timeframe analysis dramatically improves Stochastic reliability. The weekly Stochastic establishes the dominant cycle; the daily Stochastic provides the entry timing. Trading long only when the weekly Stochastic is rising from oversold and the daily Stochastic crosses up from below 20 filters out most counter-trend false signals. This 'higher-timeframe context, lower-timeframe trigger' approach is one of the most robust frameworks for using any oscillator, and Stochastic is particularly effective in this structure due to its sensitivity to range position.