Bollinger Bands
Definition:
Investors use trading bands, lines drawn above and below the moving average, to isolate a range of prices for a given security, based on the concept that a stock generally trades within a predictable range on either side of the moving average. When a stock is near the upper or lower limits of the trading bands is when an investor should pay closest attention, according to conventional wisdom. Bollinger Bands are considered some of the most useful bands in technical analysis, for they vary in distance from the moving average of a security's price based on the security's volatility. During periods of increased fluctuation, the bands widen to take this into account, and when the fluctuation decreases, the bands are tapered for a narrower focus to the price range. The upper band is the standard deviation multiplied by a given factor above the simple moving average, and the lower band is the standard deviation multiplied by the same given factor below the simple moving average.
Interpretation:
The standard interpretation is that Bollinger Bands do not give absolute buy and sell signals, but instead indicate whether the price is relatively high or low, allowing for more informed confirmation with other technical indicators. Bollinger Bands are typically drawn two standard deviations from a twenty day simple moving average for intermediate-term analysis, ten day for short term with 1.5 standard deviations, and fifty for long-term studies with 2.5 standard deviations. According to John Bollinger, for the most accurate average "choose one that provides support to the correction of the first move up off a bottom. If the average is penetrated by the correction, then the average is too short. If, in turn, the correction falls short of the average, then the average is too long. An average that is correctly chosen will provide support far more often than it is broken."
Mr. Bollinger also contends that:
- Sharp moves tend to occur after the bands tighten to the average, when a stock is less volatile. The greater the period of less volatility, the higher the propensity for a price breakout.
- When the price hits the upper or lower bands, it is suggested to confirm with other indicators whether that price movement shows strength or weakness, respectively, which could indicate a continuation. If indicators do not confirm this movement, it can suggest a reversal.
- Tops or bottoms made outside the bands, followed by the same inside the bands, indicate a trend reversal.
- A move originating at one band tends to go to the other band.
