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Plus or minus two standard deviations where the standard deviations are calculated historically in a moving window estimation. Hence, the bands will widen if the most recent data is more volatile. If the prices break out of the band, this is considered a significant move.
Copyright © 2012, Campbell R. Harvey. All Rights Reserved.
In technical analysis, charts or tables that compare a security's volatility to its price over time. Bollinger Bands? consist of a simple moving average of the security's price over a given number of days (usually 20 or 21), plus one upper limit and one lower limit. The limits are calculated as the amount of the moving average plus or minus two standard deviations. The purpose of Bollinger Bands is to provide a working definition of a security's upper and lower price limit, to indicate if volatility is increasing, decreasing, or staying the same. Bollinger Bands are one of the most popular technical analysis tools. Bollinger Bands are the registered trademark of John Bollinger, who developed them.
Farlex Financial Dictionary. © 2012 Farlex, Inc. All Rights Reserved
The outer limits of the market's price variations that are used by technical analysts to determine if the market is overbought or oversold. The bands are plotted two standard deviations on each side of the moving average. The closer the market average moves to the lower band, the more oversold the market. Conversely, the closer the average to the upper band, the more overbought the market.
Wall Street Words: An A to Z Guide to Investment Terms for Today's Investor by David L. Scott. Copyright © 2003 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved. All rights reserved.