Automated trading system
An automated trading system (ATS), also referred to as algorithmic trading, is a computer program that creates orders and automatically submits them to a market center or exchange. Advances in computer and communications technology have led to electronic trading exchange system networks. Electronic trading exchange system networks use communications networks and computers to replicate traditional face-to-face exchange functions. The program will automatically generate orders based on predefined set of rules using a trading strategy which is often based on technical analysis but can also be based on input from other electronic sources.
Automated trading systems are often used with electronic trading in automated market centers, including electronic communication networks, "dark pools", and automated exchanges. Automated trading systems and electronic trading platforms can execute repetitive tasks at speeds with orders of magnitude greater than any human equivalent. Traditional risk controls and safeguards that relied on human judgment are not appropriate for automated trading and this has caused issues such as the 2010 Flash Crash. New controls such as trading curbs or 'circuit breakers' have been put in place in some electronic markets to deal with automated trading systems.
The automated trading system determines whether an order should be submitted based on, for example, the current market price of an option and theoretical buy and sell prices. The theoretical buy and sell prices are derived from, among other things, the current market price of the security underlying the option. A look-up table stores a range of theoretical buy and sell prices for a given range of current market price of the underlying security. Accordingly, as the price of the underlying security changes, a new theoretical price may be indexed in the look-up table, thereby avoiding calculations that would otherwise slow automated trading decisions.  A distributed processing on-line automated trading system uses structured messages to represent each stage in the negotiation between a market maker (quoter) and a potential buyer or seller (requestor). 
Advantages of Automated Trading System
- Minimizes Emotions
Automated trading systems minimize emotions throughout the trading process. By keeping emotions in check, traders typically have an easier time sticking to the plan. Since trade orders are executed automatically once the trade rules have been met, traders will not be able to hesitate or question the trade. In addition to helping traders who are afraid to "pull the trigger," automated trading can curb those who are apt to overtrade – buying and selling at every perceived opportunity. 
- Ability to Backtest
Backtesting applies trading rules to historical market data to determine the viability of the idea. When designing a system for automated trading, all rules need to be absolute, with no room for interpretation (the computer cannot make guesses). Traders can take these precise sets of rules and test them on historical data before risking money in live trading. Careful backtesting allows traders to evaluate and fine-tune a trading idea, and to determine the system's expectancy – i.e., the average amount that a trader can expect to win per unit of risk. 
- Preserves Discipline
Because the trade rules are established and trade execution is performed automatically, discipline is preserved even in volatile markets. Discipline is often lost due to emotional factors such as fear of taking a loss, or the desire to eke out a little more profit from a trade. Automated trading helps ensure that discipline is maintained because the trading plan will be followed exactly. In addition, "Pilot error" is minimized; for instance, an order to buy 100 shares will not be incorrectly entered as an order to sell 1,000 shares. 
- Achieves Consistency
One of the biggest challenges in trading is to plan the trade and trade the plan. Even if a trading plan has the potential to be profitable, traders who ignore the rules are altering any expectancy the system would have had. There is no such thing as a trading plan that wins 100% of the time – losses are a part of the game. But losses can be psychologically traumatizing, so a trader who has two or three losing trades in a row might decide to skip the next trade. If this next trade would have been a winner, the trader has already destroyed any expectancy the system had. Automated trading systems allow traders to achieve consistency by trading the plan. 
- Improved Order Entry Speed
Since computers respond immediately to changing market conditions, automated systems are able to generate orders as soon as trade criteria are met. Getting in or out of a trade a few seconds earlier can make a big difference in the trade's outcome. As soon as a position is entered, all other orders are automatically generated, including protective stop losses and profit targets. Markets can move quickly, and it is demoralizing to have a trade reach the profit target or blow past a stop-loss level – before the orders can even be entered. An automated trading system prevents this from happening. 
- Diversifies Trading
Automated trading systems permit the user to trade multiple accounts or various strategies at one time. This has the potential to spread risk over various instruments while creating a hedge against losing positions. What would be incredibly challenging for a human to accomplish is efficiently executed by a computer in milliseconds. The computer is able to scan for trading opportunities across a range of markets, generate orders and monitor trades. 
Failures/Disadvantages of Automated Trading System
Such a system is subject to uncertainties caused by the fact that a variable time is required for an order (buy or sell) message to be transmitted from the requestor to the quoter, or for a cancel (quote interrupt) message to be transmitted from the quoter to the requestor. Furthermore, it is possible that an equipment failure in the network, either in a communication link or even at the workstation of one of the traders, will prevent a small fraction of such order messages and cancel messages from reaching their intended destination within the relatively short time-frame typically associated with an on-line transaction system. 
- Mechanical Failures
The theory behind automated trading makes it seem simple: Set up the software, program the rules and watch it trade. In reality, however, automated trading is a sophisticated method of trading, yet not infallible. Depending on the trading platform, a trade order could reside on a computer and not a server. What that means is that if an internet connection is lost, an order might not be sent to the market. There could also be a discrepancy between the "theoretical trades" generated by the strategy and the order entry platform component that turns them into real trades. Most traders should expect a learning curve when using automated trading systems, and it is generally a good idea to start with small trade sizes while the process is refined. 
Although it would be great to turn on the computer and leave for the day, automated trading systems do require monitoring. This is due do the potential for mechanical failures, such as connectivity issues, power losses or computer crashes, and to system quirks. It is possible for an automated trading system to experience anomalies that could result in errant orders, missing orders, or duplicate orders. If the system is monitored, these events can be identified and resolved quickly. 
Traders who employ backtesting techniques can create systems that look great on paper and perform terribly in a live market. Over-optimization refers to excessive curve-fitting that produces a trading plan that is unreliable in live trading. It is possible, for example, to tweak a strategy to achieve exceptional results on the historical data on which it was tested. Traders sometimes incorrectly assume that a trading plan should have close to 100% profitable trades or should never experience a drawdown to be a viable plan. As such, parameters can be adjusted to create a "near perfect" plan that completely fails as soon as it is applied to a live market. 
The most common algorithmic trading strategies follow trends in moving averages, channel breakouts, price level movements and related technical indicators. These are the easiest and simplest strategies to implement through algorithmic trading because these strategies do not involve making any predictions or price forecasts. Trades are initiated based on the occurrence of desirable trends, which are easy and straightforward to implement through algorithms without getting into the complexity of predictive analysis. 
- For example, the following formula could be used for trend following strategy:
- Consider a complete probability space (Ω, F, P). Let denote the stock price at time satisfying the equation
- where is a two-state Markov-Chain, is the expected return rate in regime is the constant volatility, is a standard Brownian motion, and and are the initial and terminal times, respectively. 
Volume weighted average price strategy breaks up a large order and releases dynamically determined smaller chunks of the order to the market using stock-specific historical volume profiles. The aim is to execute the order close to the Volume Weighted Average Price (VWAP). It is common to evaluate the performance of traders by their ability to execute orders at prices better than the volume weighted average price (VWAP) over the trading horizon. 
- VWAP is calculated using the following formula:
- is Volume Weighted Average Price;
- is price of trade ;
- is quantity of trade ;
- is each individual trade that takes place over the defined period of time, excluding cross trades and basket cross trades. 
- Mean Reversion Strategy
Mean reversion strategy is based on the idea that the high and low prices of an asset are a temporary phenomenon that revert to their mean value (average value) periodically. Identifying and defining a price range and implementing an algorithm based on that allows trades to be placed automatically when the price of asset breaks in and out of its defined range.
- For example, a continuous mean-reverting time series can be represented by an Ornstein-Uhlenbeck process stochastic differential equation:
- where is the rate of reversion to the mean, is the mean value of the process, is the variance of the process and is a Wiener process or Brownian motion. 
Early form of Automated Trading System has been used by financial managers and brokers, software based on algorithm. These kind of software were used to automatically manage clients' portfolios. But first service to free market without any supervision from financial advisers and managers to serve clients directly was given in 2008 with the launch of Betterment by Jon Stein. Since then this system is getting improved with development in IT industry, now Automated Trading System is managing huge assets all around the globe. As of 2014[update], more than 75 percent of the stock shares traded on United States exchanges (including the New York Stock Exchange and NASDAQ) originate from automated trading system orders. ATSs can be based on a predefined set of rules which determine when to enter an order, when to exit a position and how much money to invest in each trading product. Trading strategies differ; some are designed to pick market tops and bottoms, others to follow a trend, and others involve complex strategies including randomizing orders to make them less visible in the marketplace. ATSs allow a trader to execute orders much quicker and manage their portfolio easily by automatically generating protective precautions.
Backtesting of a trading system involves programmers running the program using historical market data in order to determine whether the underlying algorithm guiding the system may produce the expected results. Developers can create backtesting software to enable a trading system designer to develop and test their trading systems using historical market data to optimize the results obtained with the historical data. Although backtesting of automated trading systems cannot accurately determine future results, an automated trading system can be backtested using historical prices to see how the system theoretically would have performed if it had been active in a past market environment.
Forward testing of an algorithm can also be achieved using simulated trading with real-time market data to help confirm the effectiveness of the trading strategy in the current market and may be used to reveal issues inherent in the computer code.
Live testing is the final stage of the development cycle. In this stage, live performance is compared against the backtested and walk forward results. Metrics compared include Percent Profitable, Profit Factor, Maximum Drawdown and Average Gain per Trade. The goal of an automated trading system is to meet or exceed the backtested performance with a high efficiency rating. 
Improved order entry speed allows a trader to enter or exit a position as soon as the trade criteria are satisfied. Furthermore, stop losses and profit targets can be automatically generated using an automated trading system.
Market disruption and manipulation
Automated trading, or high-frequency trading, causes regulatory concerns as a contributor to market fragility.
United States regulators have published releases discussing several types of risk controls that could be used to limit the extent of such disruptions, including financial and regulatory controls to prevent the entry of erroneous orders as a result of computer malfunction or human error, the breaching of various regulatory requirements, and exceeding a credit or capital limit.
The use of high-frequency trading (HFT) strategies has grown substantially over the past several years and drives a significant portion of activity on U.S. markets. Although many HFT strategies are legitimate, some are not and may be used for manipulative trading. Given the scale of the potential impact that these practices may have, the surveillance of abusive algorithms remains a high priority for regulators. The Financial Industry Regulatory Authority (FINRA) has reminded firms using HFT strategies and other trading algorithms of their obligation to be vigilant when testing these strategies pre- and post-launch to ensure that the strategies do not result in abusive trading.
FINRA continues to be concerned about the use of so-called "momentum ignition strategies" where a market participant attempts to induce others to trade at artificially high or low prices. Examples of this activity include layering and spoofing strategies where a market participant places a nonbona fide order on one side of the market (typically, but not always, above the offer or below the bid) in an attempt to bait other market participants to react to the non-bona fide order and trade with another order on the other side of the market.
Other examples of problematic HFT or algorithmic activity include order entry strategies related to placing orders near the open or close of regular trading hours that involve distorting disseminated market imbalance indicators through the entry of non-bona fide orders and/or aggressive trading activity near the open or close.
FINRA also continues to focus concern on the entry of problematic HFT and algorithmic activity through sponsored participants who initiate their activity from outside of the United States. In this regard, FINRA reminds firms of their surveillance and control obligations under the SEC's Market Access Rule and Notice to Members 04-66, as well as potential issues related to treating such accounts as customer accounts, anti-money laundering and margin levels, as highlighted in Regulatory Notice 10-18  and the SEC's Office of Compliance Inspections and Examination's National Exam Risk Alert dated September 29, 2011.
FINRA conducts surveillance to identify cross-market, cross-product manipulation of the price of underlying equity securities, typically through abusive trading algorithms, and strategies used to close out pre-existing option positions at favorable prices or establish new option positions at advantageous prices.
In recent years, there have been a number of algorithmic trading malfunctions that caused substantial market disruptions. These raise concern about firms' ability to develop, implement and effectively supervise their automated systems. FINRA has stated that it will assess whether firms' testing and controls related to algorithmic trading and other automated trading strategies and trading systems are adequate in light of the U.S. Securities and Exchange Commission and firms' supervisory obligations. This assessment may take the form of examinations and targeted investigations. Firms will be required to address whether they conduct separate, independent and robust pre-implementation testing of algorithms and trading systems and whether the firm's legal, compliance and operations staff are reviewing the design and development of the algorithms and trading systems for compliance with legal requirements. FINRA will review whether a firm actively monitors and reviews algorithms and trading systems once they are placed into production systems and after they have been modified, including procedures and controls used to detect potential trading abuses such as wash sales, marking, layering and momentum ignition strategies. Finally, firms will need to describe their approach to firm-wide disconnect or "kill" switches, as well as procedures for responding to catastrophic system malfunctions.
Examples of recent substantial market disruptions include the following:
- On May 6, 2010, the Dow Jones Industrial Average declined about 1,000 points (about 9 percent) and recovered those losses within minutes. It was the second-largest point swing (1,010.14 points) and the largest one-day point decline (998.5 points) on an intraday basis in the Average's history. This market disruption became known as the Flash Crash and resulted in U.S. regulators issuing new regulations to control market access achieved through automated trading.
- On August 1, 2012, between 9:30 a.m. and 10:00 a.m. EDT, Knight Capital Group lost four times its 2011 net income. Knight's CEO Thomas Joyce stated, on the day after the market disruption, that the firm had "all hands on deck" to fix a bug in one of Knight's trading algorithms that submitted erroneous orders to exchanges for nearly 150 different stocks. Trading volumes soared in so many issues, that the SPDR S&P 500 ETF (SYMBOL: SPY), which is generally the most heavily traded U.S. security, became the 52nd-most traded stock on that day, according to Eric Hunsader, CEO of market data service Nanex. Knight shares closed down 62 percent as a result of the trading error and Knight Capital nearly collapsed. Knight ultimately reached an agreement to merge with Getco, a Chicago-based high-speed trading firm.
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This supports regulatory concerns about the potential drawbacks of automated trading due to operational and transmission risks and implies that fragility can arise in the absence of order flow toxicity.
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