ADX - Average Directional Index - is an indicator which fluctuates between 0 and 100. Readings below 20 indicate a weak trend or ranging market and readings above 40 indicate a strong trend. ADX does not indicate direction but rather just the strength of the trend, up or down.

Often plotted with ADX are two lines that indicate direction. The lines are +DI (positive directional indicator) and -DI (negative directional indicator). If the +DI line is above the -DI line, then the trend is considered bullish. Crosses of the +DI and -DI lines generate buy and sell signals. For example, if +DI crosses above -DI, then a buy signal is generated.

ADX = modify moving average of DX

DX = 100 x ( (+DI - -DI)/( +DI + -DI) )

+DI = +DMn / TRn , -DI = -DM / TRn

+DM = Ht - Ht-1 , -DM = Lt - Lt-1

CL = Ct - Ct-1

TR = largest of +DM,-DM ,and CL

where :

+DI = current positive directional index

-DI = current negative directional index

+DMn = current modified moving average of +DM

+DM = current positive directional movement value

Ht = current hign

Ht-1 = previous high

Lt = current low

Lt-1 = previous low

-DMn = current modified moving average of -DM

-DM = current negative directional movement value

TRn = current modified modified moving average of the true range

TR = true range

n = number of periods

DX = current DX

reference from : J.Welles Wilder

A move below 40 from above indicates that the trend is slowing. Since most option strategies rely on large price movements in short timeframes, a slowing trend is bad. Therefore an ADX move below 40 would indicate that it is time to close positions.

Conversely, an ADX indicator move above 20 from below indicates that the sideways trading is over, and a new trend is developing. This would indicate that it is time to make a move, either bullish or bearish.

Also, signals can be obtained by looking at where the positive directional index +DI (green) and negative directional index -DI (red) lines cross each other. When the +DI crosses above the -DI from below, it is a bullish signal, such as in early February. When the -DI crosses above the +DI from below, it is bearish, such as in late April.