抛物線指標（SAR）由J. Welles Wilder 博士開發而來，並在其《技術交易系統的新概念》一書中有所描述。SAR意思為“止損和反轉”
抛物線指標至適合趨勢市場——它可以提供較好的入場和平倉位置。它使用十分獨特的繪制方式：止損點是根據前一日的數據得出。它的優勢是，可以在市場開盤之前計算出止損位置。
• 低於當前價位的止損，表明你應該做多。止損每日都會上移直到被觸發（當價格回落至止損位）。
• 高於當前價格的止損位，說明你應該做空。止損每日都會下降直到觸發（當價格上漲到止損位）。
交易信號
首先你應該確認市場是否處於單邊趨勢中：
• 利用趨勢指標，或者
• 如果你連續兩次根據SAR指標交易被掃損，那麼暫停交易。在觀察並得出突破位置後再進場交易。
當價格與止損位相交時，進場信號產生：
• 當空單被止損時，反向做多。
• 多單被止損則做空。
示例
微軟公司股票以及 SAR 指標 63日指數移動平均線
1. 忽略橫盤的交易信號（可以根據MA與價格波動關系識別）。[L] 處當價格一直位於MA上方時做多。價格接下來突破盤整區間，進一步確認了交易信號。
2. [X] 處價格觸發止損時平倉。此時不宜做空，因為MA走勢仍向上。
3. [L] 處當價格重新回到止損位上方，且MA仍然向上時，做多。
4. [X] 處價格觸發止損，平倉。MA方向向上，不做空。
5. [L] 處當價格回到止損位上方，且MA仍然向上，做多。
6. [X] 處價格觸發止損，平倉。MA向上傾斜，不做空。
在最初的系統中，每次[X] 處平倉後便做空，導致了違背趨勢的虧損交易。
設置
加速因子的默認設置為2%，最大值為20%。
評價
SAR指標引進了技術分析的一些傑出理念，但是它仍然有兩個主要的弱點：
1. 趨勢速度隨著時間的推移有所不同，不同股票間的走勢速度也不同。因此，很難有一個確認的加速因子滿足所有趨勢——該加速因子對於某些趨勢來說太快，對於另外一些來說又可能過慢。
2. SAR指標假設每次止損被觸發後，趨勢都將發生改變。任何一個交易者都明白，即使你接連幾次被掃損，趨勢也有可能持續。價格在觸發你的止損位之後又恢複原來的上升趨勢，始終讓你處於落後狀態。
SAR 公式
在解釋SAR計算公式之前，你需要了解以下幾個概念：
極點價EP
這是多頭交易期間的最高價或者是空單交易期内的最低價。
重要點位SP
重要點位是多頭期間的最高價位或是空單期間的最低價位。交易平倉後，它與極點價一致。
加速因子
一筆新交易的加速因子從2%開始，每當價格走出極點價時增加2%。當加速因子達到最大值20%時，即使價格出現極點價，加速因子數值也不會增加。
計算
• 在新交易的第一日（入場交易日），SAR值是根據前期交易的重要點位得出。如果做多，那麼SP值即為前期交易的最低價。如果做空，那麼SP值則為前期交易的最高價位。
• 計算接下來一日的SAR值：計算出EP值與當日SAR的差值，並乘以加速因子。如果做多，則加上第一日SAR值；如果做空，則SAR值減去該結果。
• 唯一的例外：SAR值不得定於當日或昨日波幅之内（兩日内的最高價與最低價之間）。如果這發生在多單交易中，則利用當日或昨日的最低價作為接下來一日的SAR值。如果做空，則利用兩日來的最高價作為接下來一日的SAR值。
• 接下來每一天的SAR都重複同樣的計算方式，每當出現極值點，則相應調整加速因子。
• 當價格與SAR相交時，平倉且進行相反方向的交易。
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.
文章翻譯由兄弟財經提供
本文來源：http://www.incrediblecharts.com/indicators/parabolic_sar.php