The average directional index (ADX) was developed in 1978 by J. Welles Wilder as an indicator of trend strength in a series of prices of a financial instrument. ADX has become a widely used indicator for technical analysts, and is provided as a standard in collections of indicators offered by various trading platforms.
The ADX is a combination of two other indicators developed by Wilder, the positive directional indicator (abbreviated +DI) and negative directional indicator (-DI). The ADX combines them and smooths the result with an exponential moving average.
To calculate +DI and −DI, one needs price data consisting of high, low, and closing prices each period (typically each day). One first calculates the directional movement (+DM and −DM):
After selecting the number of periods (Wilder used 14 days originally), +DI and −DI are:
The exponential moving average is calculated over the number of periods selected, and the average true range is an exponential average of the true ranges. Then:
Variations of this calculation typically involve using different types of moving averages, such as a weighted moving average or an adaptive moving average.
The ADX does not indicate trend direction, only trend strength. It is a lagging indicator; that is, a trend must have established itself before the ADX will generate a signal that a trend is underway. ADX will range between 0 and 100. Generally, ADX readings below 20 indicate trend weakness, and readings above 40 indicate trend strength. An extremely strong trend is indicated by readings above 50.
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