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The Parabolic Time/Price System, developed by Welles Wilder, is used to set
trailing price stops and is sometimes referred to as the "SAR" (stop-and-reversal),
Parabolic SAR is more popular for setting stops than for establishing direction or trend.
Wilder recommended establishing the trend first, and then trading with Parabolic SAR in
the direction of the trend. If the trend is up, buy when the indicator moves below the price.
If the trend is down, sell when the indicator moves above the price.
The formula is quite complex and beyond the scope of this definition, but interpretation
is relatively straightforward. The dotted lines below the price establish the trailing stop
for a long position and the lines above establish the trailing stop for a short position. At the
beginning of the move, the Parabolic SAR will provide a greater cushion between the price and the
trailing stop. As the move gets underway, the distance between the price and the indicator will
shrink, thus making for a tighter stop-loss as the price moves in a favorable direction.
If you are long (i.e., the price is above the SAR), the SAR will move up every
day, regardless of the direction the price is moving. The amount the SAR moves
up depends on the amount that prices move.
There are two variables: the step and the maximum step. The higher the step is set, the more sensitive
the indicator will be to price changes. If the step is set too high, the indicator will fluctuate above
and below the price too often, making interpretation difficult. The maximum step controls the adjustment
of the SAR as the price moves. The lower the maximum step is set, the further the trailing stop will be
from the price. Wilder recommends setting the step at .02 (50 periods) and the maximum step at .20 (5 periods).
Barchart.com uses a default step of 50 periods and automatically sets the maximum step to 10% of the step value (5 periods with default).
For additional information along with the calculation please refer to "New Concepts in Technical
Trading Systems" by Welles Wilder.