This is an indicator used by technical analysts and investors to ascertain the strength of a trend.  Most of the time, Two indicators i.e. positive directional (+DI) and negative directional (-DI) indicators are involved in showing this trend.

Three lines are therefore included by ADX to help you evaluate and choose if you should take a long or short during trading. 

Long refers to buying of securities while short refers to the selling of securities.

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