抛物线指标（SAR）由J. Welles Wilder 博士开发而来，并在其《技术交易系统的新概念》一书中有所描述。SAR意思为“止损和反转”

抛物线指标至适合趋势市场——它可以提供较好的入场和平仓位置。它使用十分独特的绘制方式：止损点是根据前一日的数据得出。它的优势是，可以在市场开盘之前计算出止损位置。
• 低于当前价位的止损，表明你应该做多。止损每日都会上移直到被触发（当价格回落至止损位）。
• 高于当前价格的止损位，说明你应该做空。止损每日都会下降直到触发（当价格上涨到止损位）。

交易信号
首先你应该确认市场是否处于单边趋势中：
• 利用趋势指标，或者
• 如果你连续两次根据SAR指标交易被扫损，那么暂停交易。在观察并得出突破位置后再进场交易。
当价格与止损位相交时，进场信号产生：
• 当空单被止损时，反向做多。
• 多单被止损则做空。
 
示例
微软公司股票以及   SAR 指标   63日指数移动平均线

 
1. 忽略横盘的交易信号（可以根据MA与价格波动关系识别）。[L] 处当价格一直位于MA上方时做多。价格接下来突破盘整区间，进一步确认了交易信号。
2. [X] 处价格触发止损时平仓。此时不宜做空，因为MA走势仍向上。
3. [L] 处当价格重新回到止损位上方，且MA仍然向上时，做多。
4. [X] 处价格触发止损，平仓。MA方向向上，不做空。
5. [L] 处当价格回到止损位上方，且MA仍然向上，做多。
6. [X] 处价格触发止损，平仓。MA向上倾斜，不做空。
在最初的系统中，每次[X] 处平仓后便做空，导致了违背趋势的亏损交易。

设置
加速因子的默认设置为2%，最大值为20%。

评价
SAR指标引进了技术分析的一些杰出理念，但是它仍然有两个主要的弱点：
1. 趋势速度随着时间的推移有所不同，不同股票间的走势速度也不同。因此，很难有一个确认的加速因子满足所有趋势——该加速因子对于某些趋势来说太快，对于另外一些来说又可能过慢。
2. SAR指标假设每次止损被触发后，趋势都将发生改变。任何一个交易者都明白，即使你接连几次被扫损，趋势也有可能持续。价格在触发你的止损位之后又恢复原来的上升趋势，始终让你处于落后状态。

SAR 公式
在解释SAR计算公式之前，你需要了解以下几个概念：

极点价EP
这是多头交易期间的最高价或者是空单交易期内的最低价。

重要点位SP
重要点位是多头期间的最高价位或是空单期间的最低价位。交易平仓后，它与极点价一致。

加速因子
一笔新交易的加速因子从2%开始，每当价格走出极点价时增加2%。当加速因子达到最大值20%时，即使价格出现极点价，加速因子数值也不会增加。

计算
• 在新交易的第一日（入场交易日），SAR值是根据前期交易的重要点位得出。如果做多，那么SP值即为前期交易的最低价。如果做空，那么SP值则为前期交易的最高价位。
• 计算接下来一日的SAR值：计算出EP值与当日SAR的差值，并乘以加速因子。如果做多，则加上第一日SAR值；如果做空，则SAR值减去该结果。
• 唯一的例外：SAR值不得定于当日或昨日波幅之内（两日内的最高价与最低价之间）。如果这发生在多单交易中，则利用当日或昨日的最低价作为接下来一日的SAR值。如果做空，则利用两日来的最高价作为接下来一日的SAR值。
• 接下来每一天的SAR都重复同样的计算方式，每当出现极值点，则相应调整加速因子。
• 当价格与SAR相交时，平仓且进行相反方向的交易。

Parabolic SAR
Parabolic SAR was developed by J. Welles Wilder Jr. and is described in his book New Concepts in Technical Trading Systems. SAR stands for stop and reverse.

Parabolic SAR should only be employed in trending markets - when it provides useful entry and exit points. It is plotted in a rather unorthodox fashion: a stop loss is calculated for each day using the previous days data. The advantage is that the stop level can be calculated in advance of the market opening.
• A stop level below the current price indicates that your position is long. The stop will move up every day until activated (when price falls to the stop level).
• A stop level above the current price indicates that your position is short. The stop moves down every day until triggered (when price rises to the stop level).
Parabolic SAR: Trading Signals
Your first step is to confirm that the market is trending:
• Use a trend indicator, or
• Stop trading with the Parabolic SAR if you are whipsawed twice in a row and re-commence after you observe a breakout from the chart pattern.
A trade is signaled when the price bars and stop levels intersect:
• Go long when price meets the Parabolic SAR stop level, while short.
• Go short when price meets the Parabolic SAR stop level, while long.
 
EXAMPLE
Microsoft Corporation plotted with   Parabolic SAR and   63-day exponential moving average.
1. Ignore signals while price is ranging (identified by the fluctuations around the MA). Go long at [L] after price respects the MA. Price then breaks out of the range, confirming our signal.
2. Exit [X] when price activates the Parabolic SAR stop.
Do not go short as the MA slopes upwards.
3. Go long [L] when price crosses back above the stop. MA is still rising.
4. Exit [X] when price falls to the stop level.
Do not go short as the MA slopes upwards.
5. Go long [L] when price crosses back above the stop. MA is still rising.
6. Exit [X] when price falls to the stop level.
Do not go short as the MA still slopes upwards.
In the original system, short signals are taken at each exit point [X], resulting in unprofitable trades against the trend.
Parabolic SAR: Setup
See Indicator Panel for directions on how to set up Parabolic SAR on the price chart. The default settings are an acceleration factor of 2% and a maximum step of 20%. To alter the default settings - see Edit Indicator Settings.
Parabolic SAR: Evaluation
Parabolic SAR introduces some excellent concepts to technical analysis but leaves two major weaknesses:
1. Trend speeds vary over time and between stocks. It is difficult to arrive at one acceleration factor that suits all trends -- it will be too slow for some and too fast for others.
2. The SAR system assumes that the trend changes every time a stop has been hit. Any trader will tell you that your stops may be hit several times while the trend continues. Price merely retraces through your stop and then resumes the up-trend, leaving you lagging behind.
Parabolic SAR Formula
There are a few basic concepts that need to be addressed before we can explain how Parabolic SAR is calculated:
Extreme Point
This is the highest price recorded (to date) during a long trade or the lowest price recorded (to date) during a short trade.
Significant Point
The SP is the highest price reached in a long trade or the lowest price reached in a short trade. It is equal to the extreme point when a trade is closed.
Acceleration Factor
The Acceleration Factor starts at 2% for a new trade and increases by 2% on each day that a new extreme point is reached. The maximum acceleration factor is 20%. No further increases are made after this figure has been reached.
Parabolic SAR: Calculation
• On day 1 of a new trade (the day that the trade is entered), the Parabolic SAR is taken as the significant point from the previous trade.
If the trade is Long the SP will be the extreme Low reached in the previous trade.
If the trade is Short then the SP will be the extreme High reached in the previous trade.
• To calculate Parabolic SAR for the following day:
Take the difference between the extreme point and the SAR (on day 1) and multiply by the acceleration factor.
If the trade is Long, add the result to the SAR on day 1.
If the trade is Short, subtract the result from the SAR on day 1.
• There is one exception:
Parabolic SAR is never moved within the range of the current or previous day (highest High to lowest Low over the 2 days).
If this occurs in a long trade, use the lowest Low over the 2 days as SAR for the following day.
If short, use the highest High over the 2 days as SAR for the following day.
• Repeat the Parabolic SAR calculation for each subsequent day, adjusting the Acceleration Factor whenever a new extreme point is recorded.
• The trade is reversed when price equals the Parabolic SAR for the day.

文章翻译由兄弟财经提供

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