Spotting Extreme Stock Price Moves Calculator
Use the Spotting Extreme Stock Price Moves Calculator to display the status of the last ten daily price changes for any stock, exchange-traded fund (ETF) and mutual fund listed on a major U.S. stock exchange and supported by Alpha Vantage. Some stocks traded on non-U.S. exchanges are also supported. Indexes are not supported.
Output Explained
Each day is independently evaluated against computed Gumbel thresholds. If one or more days in the last 10 were an anomaly (like a 12% drop or a 15% gain), those specific days will display Orange and "HISTORICAL EXTREME."
Most investors look at a daily chart to see where the price is going up or down, but the Gumbel analysis looks at a day to see if the stock had made a statistical extreme up or down move.
How to Read the 10-Day Extreme Analysis
While a standard stock chart shows you the "what," this 10-day breakdown shows you the "how rare." By evaluating each of the last 10 trading sessions independently against the Gumbel distribution, the calculator can categorize the stock's behavior into two distinct states:
1. Normal Range (Slate Blue)When a day's display is Slate Blue, it indicates that the price movement for that day—whether it was a gain or a loss—falls within the expected historical boundaries. The stock is "behaving" normally. Even if the price dropped, it stayed above the Normal Bad Day (Mode) plus the typical volatility buffer.
2. Historical Extreme (Orange)When a day's display turns Orange, the math has detected a statistical anomaly. This means the daily move was so large that it exceeded the 95th percentile of historical extremes. (the Mode + 2.5 Beta units). This is a "Black Swan" event or a significant momentum shift.
In a Breakout: Multiple Orange days with positive gains suggest a powerful new trend or a period of irrational exuberance.
In a Crash: Orange days with negative returns signals panic selling. Historically, these are the moments where the "Floor" is being tested or broken.
Why 10 Days?
Looking at 10 individual "stress tests" enables you to see if volatility is clustering.
Isolated Extreme: One orange day in a sea of blue is often the result of an "exceptional" news event (like an earnings surprise).
Clustered Extremes: If 3 or 4 out of the 10 days are Orange, the stock has entered a high-gain or loss volatility state.
Note that the Thresholds listed on each day are not numbers you'll find in daily news sources. They are the result of analyzing the last 20 years of "worst-case" and "best-case" monthly scenarios. When today's move crosses that threshold, you aren't just seeing a "big move"—you are seeing a move that is historically significant.
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