# Bollinger Bands

By Vikram

## An Introduction

Bollinger Bands (BB) are a widely popular technical analysis instrument created by John Bollinger in the early 1980’s. Bollinger Bands consist of a band of three lines which are plotted in relation to security prices. The line in the middle is usually a Simple Moving Average (SMA) set to a period of 20 days (the type of trend line and period can be changed by the trader; however a 20 day moving average is by far the most popular). The SMA then serves as a base for the Upper and Lower Bands which are used as a way to measure volatility by observing the relationship between the Bands and price.

Bollinger Bands consist of a N-period moving average (MA), an upper band at K times an N-period standard deviation above the moving average (MA + Kσ), and a lower band at K times an N-period standard deviation below the moving average .

Typical values for N and K are 20 and 2, respectively. The default choice for the average is a simple moving average, but other types of averages can be employed as needed. Exponential moving averages are a common second choice.Usually the same period is used for both the middle band and the calculation of standard deviation.

## Bollinger Bands Charts Calculation

Middle Band = 20-day simple moving average (SMA)

Upper Band = 20-day SMA + (20-day standard deviation of price x 2)

Lower Band = 20-day SMA – (20-day standard deviation of price x 2)

Bollinger Bands consist of a middle band with two outer bands. The middle band is a simple moving average that is usually set at 20 periods. A simple moving average is used because the standard deviation formula also uses a simple moving average. The look-back period for the standard deviation is the same as for the simple moving average. The outer bands are usually set 2 standard deviations above and below the middle band.

Bollinger Bands look like an envelope that forms an upper and lower band1 around the price of a stock or other security (see the chart below). Between the two bands is a moving average, typically a 20-day simple moving average (SMA).

Bollinger Bands are plotted at a standard deviation above and below a simple moving average of the price. The upper band is the moving average plus a standard deviation, and the lower band is the moving average less the standard deviation.

### Take a look at the other Trade Winx Chart Performance based on Bollinger Bands:-

In addition to these “high” and “low” relative assessments, there are a number of trading signals that are generated by how the price of the stock or security interacts with the bands. For example, when the stock breaks through the upper band (a resistance level), it generates a buy signal. When it breaks below the lower band (a support level), it’s a sell signal.

If the S&P 500 Index is in the upper band, which according to Bollinger suggests that U.S. stocks are overvalued on a short-term basis. However, if the S&P were to break above the line that forms the top of the upper band, it would be a buy signal.

## Volatility Measure

Bollinger Bands can also provide a unique assessment of volatility. Narrowing Bollinger Bands (i.e., when the bands move closer together) could suggest that volatility is decreasing—as investor sentiment potentially becomes more optimistic or complacent.
A Bollinger Band “squeeze,” as Bollinger himself describes it, occurs when volatility reaches a relative low. This squeeze can frequently be followed by a period of increased volatility, and may result in a significant move by the stock to the upside or the downside (see chart above). In the chart immediately above, the squeeze occurs around September 2016, and is followed by an increase in price.

## Advanced use of Bollinger Bands

An advanced application of Bollinger Bands involves another indicator: the Relative Strength Index (RSI). Bollinger Bands can be applied around the RSI line to generate additional buy and sell signals.

When RSI is near an extreme high (~100) or low (~0), and is touching either the high part of the upper band or the low part of the lower band, the RSI line could pull back sharply from the band. Bollinger Band analysis holds that a failure of RSI to touch the upper band on a second try generates a sell signal. At extreme lows, a failure of RSI to reach the lower band triggers a buy signal. This is similar to double top and double bottom patterns, respectively, that can occur for the price. Currently, there are no clear signals given by these indicators for the S&P 500.

## The Strategy

Most technicians will use Bollinger Bands in conjunction with other indicators, but we wanted to take a look at a simple strategy that uses only the bands to make trading decisions. It has been found that buying the breaks of the lower Bollinger Band is a way to take advantage of oversold conditions. Usually, once a lower band has been broken due to heavy selling, the price of the stock will revert back above the lower band and head toward the middle band. This is the exact scenario this strategy attempts to profit from. The strategy calls for a close below the lower band, which is then used as an immediate signal to buy the stock the next day.

The use of Bollinger Bands varies widely among traders. Some traders buy when price touches the lower Bollinger Band and exit when price touches the moving average in the center of the bands. Other traders buy when price breaks above the upper Bollinger Band or sell when price falls below the lower Bollinger Band. Moreover, the use of Bollinger Bands is not confined to stock traders; options traders, most notably implied volatility traders, often sell options when Bollinger Bands are historically far apart or buy options when the Bollinger Bands are historically close together, in both instances, expecting volatility to revert towards the average historical volatility level for the stock.

When the bands lie close together, a period of low volatility is indicated. Conversely, as the bands expand, an increase in price action/market volatility is indicated. When the bands have only a slight slope and track approximately parallel for an extended time, the price will generally be found to oscillate between the bands as though in a channel.

Traders are often inclined to use Bollinger Bands with other indicators to confirm price action. In particular, the use of oscillator-like Bollinger Bands will often be coupled with a non-oscillator indicator-like chart patterns or a trendline. If these indicators confirm the recommendation of the Bollinger Bands, the trader will have greater conviction that the bands are predicting correct price action in relation to market volatility.

## Effectiveness

Various studies of the effectiveness of the Bollinger Band strategy have been performed with mixed results. In 2007, Lento et al. published an analysis using a variety of formats (different moving average timescales, and standard deviation ranges) and markets (e.g., Dow Jones and Forex).Analysis of the trades, spanning a decade from 1995 onwards, found no evidence of consistent performance over the standard “buy and hold” approach. The authors did, however, find that a simple reversal of the strategy (“contrarian Bollinger Band”) produced positive returns in a variety of markets.

Similar results were found in another study, which concluded that Bollinger Band trading strategies may be effective in the Chinese marketplace, stating: “we find significant positive returns on buy trades generated by the contrarian version of the moving-average crossover rule, the channel breakout rule, and the Bollinger Band trading rule, after accounting for transaction costs of 0.50 percent.” (By “the contrarian version”, they mean buying when the conventional rule mandates selling, and vice versa.) A recent study examined the application of Bollinger Band trading strategies combined with the ADX for Equity Market indices with similar results.

A paper from 2005 uses Bollinger Bands to reduce variance in a Monte Carlo simulation used to forecast the Canadian treasury bill yield curve.

In 2012, Butler et al. published an approach to fitting the parameters of Bollinger Bands using particle swarm optimization method. Their results indicated that by tuning the parameters to a particular asset for a particular market environment, the out-of-sample trading signals were improved compared to the default parameters.

Companies like Forbes suggest that the use of Bollinger Bands is a simple and often an effective strategy but stop-loss orders should be used to mitigate losses from market pressure.

## Bullish Bollinger Band Crossover

This scan finds stocks that have just moved above their upper Bollinger Band line. This scan is just a starting point. Further refinement and analysis are required.

[type = stock] AND [country = US]
AND [Daily SMA(20,Daily Volume) > 40000]
AND [Daily SMA(60,Daily Close) > 5]
AND [Daily Close x Daily Upper BB(20,2.0)]

## Bearish Bollinger Band Crossover

This scan finds stocks that have just moved below their lower Bollinger Band line. This scan is just a starting point. Further refinement and analysis are required.

[type = stock] AND [country = US]
AND [Daily SMA(20,Daily Volume) > 40000]
AND [Daily SMA(60,Daily Close) > 5]
AND [Daily Lower BB(20,2.0) x Daily Close]

## Conclusions

Bollinger Bands reflect direction with the 20-period SMA and volatility with the upper/lower bands. As such, they can be used to determine if prices are relatively high or low. According to Bollinger, the bands should contain 88-89% of price action, which makes a move outside the bands significant. Technically, prices are relatively high when above the upper band and relatively low when below the lower band. However, relatively high should not be regarded as bearish or as a sell signal. Likewise, relatively low should not be considered bullish or as a buy signal. Prices are high or low for a reason. As with other indicators, Bollinger Bands are not meant to be used as a stand-alone tool. Chartists should combine Bollinger Bands with basic trend analysis and other indicators for confirmation.