Bollinger Bands – EFS

upperBB(), middleBB(), lowerBB() Built-in Study Functions

The Bollinger Band studies (created by John Bollinger) are similar to moving average envelopes. The difference between Bollinger Bands and envelopes is that envelopes are plotted at a fixed percentage above and below a moving average, whereas Bollinger Bands are plotted at standard deviation levels above and below a moving average. Since standard deviation is a measure of volatility, the bands are self-adjusting, widening during volatile markets and contracting during calmer periods.

Syntax

upperBB( nLength , nStdev [, Series | sym() | inv() ][, nBarIndex ] )
middleBB( nLength , nStdev [, Series | sym() | inv() ][, nBarIndex ] )
lowerBB ( nLength , nStdev [, Series | sym() | inv() ][, nBarIndex ] )

Parameters

Parameter:	Description:	Default:
nLength	Required. Number of periods to use for calculation.	n/a
nStdev	Required. Number of standard deviations to use for calculation.	n/a
Series or sym() or inv()	[Optional] Series Object or function of sym() or inv() to determine symbol/interval source for the study.	Chart's sym/inv
nBarIndex	[Optional] Bar index of series to retrieve .	n/a

Return Value(s)

Returns a Series Object when nBarIndex is not specified.

Returns a single value when nBarIndex is specified.

Notes

Only available in versions 7.9 or later.

Code Examples

Single Line Indicator:

function main() {
  return new Array(upperBB(20, 2), middleBB(20, 2), lowerBB(20, 2));
}

Single Line Study-on-Study Indicator:

function main() {
  return new Array(
    upperBB(20, 2, hl2()),
    middleBB(20, 2, hl2()),
    lowerBB(20, 2, hl2())
  );
}

Retrieve single values:

function main() {
  var nValue_0 = middleBB(20, 2, 0); // Current Bar Index
  var nValue_1 = middleBB(20, 2, -1); // Prior Bar Index
  return new Array(nValue_0, nValue_1);
}

Initialize Series Objects:

//global Series Object variables
var xStudy1 = null;
var xStudy2 = null;
var xStudy3 = null;
function main() {
  if (xStudy1 == null) xStudy1 = upperBB(20, 2);
  if (xStudy2 == null) xStudy2 = middleBB(20, 2);
  if (xStudy3 == null) xStudy3 = lowerBB(20, 2); // retrieve single values for conditional statements
  var nValue1_0 = xStudy1.getValue(0); // Current Bar Index value
  var nValue2_0 = xStudy2.getValue(0); // Current Bar Index value
  var nValue3_0 = xStudy3.getValue(0); // Current Bar Index value
  var nValue1_1 = xStudy1.getValue(-1); // Prior Bar Index value
  // Plot Current Bar Index Value
  return new Array(nValue1_0, nValue2_0, nValue3_0);
}

Initialize a Series Object based on an external interval:

// global Series Object variables
var xStudy1 = null;
var xStudy2 = null;
var xStudy3 = null;
var bInit = false; // Initialization flag
function main() {
  if (bInit == false) {
    xStudy1 = upperBB(20, 2, inv(20)); // 20-min interval
    xStudy2 = middleBB(20, 2, inv(20)); // 20-min interval
    xStudy3 = lowerBB(20, 2, inv(20)); // 20-min interval
    bInit = true;
  } // retrieve single values for conditional statements
  var nValue1_0 = xStudy1.getValue(0); // Current Bar Index value
  var nValue1_1 = xStudy1.getValue(-1); // Prior Bar Index value
  // Synchronized Series plot
  return new Array(getSeries(xStudy1), getSeries(xStudy2), getSeries(xStudy3));
}

Initialize a Series Object based on an external symbol and interval:

// global Series Object variables
var xStudy1 = null;
var xStudy2 = null;
var xStudy3 = null;
var bInit = false; // Initialization flag
function main() {
  if (bInit == false) {
    xStudy1 = upperBB(20, 2, sym("IBM,20")); // IBM 20-min interval
    xStudy2 = middleBB(20, 2, sym("IBM,20")); // IBM 20-min interval
    xStudy3 = lowerBB(20, 2, sym("IBM,20")); // IBM 20-min interval
    bInit = true;
  } // retrieve single values for conditional statements
  var nValue1_0 = xStudy1.getValue(0); // Current Bar Index value
  var nValue1_1 = xStudy1.getValue(-1); // Prior Bar Index value
  // Synchronized Series plot
  return new Array(getSeries(xStudy1), getSeries(xStudy2), getSeries(xStudy3));
}