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Pairs Trading Strategy

This pairs trading strategy monitors the price spread between two correlated instruments. By comparing the spread to its historical mean and standard deviation, the system attempts to exploit temporary divergences that eventually revert.

Testing indicates an average annual return of about 88%. It performs best in the stocks market.

A long spread is entered when the spread drops below its mean by more than the specified deviation multiplier. This means buying the first asset and selling the second. A short spread does the opposite when the spread rises above the mean by the same amount. Positions are closed once the spread returns to the average level.

Pairs trading appeals to market neutral traders who prefer relative-value opportunities rather than outright direction. Because both legs are hedged, volatility tends to be lower, though the strategy still uses a stop-loss on the spread to manage risk.

Details

  • Entry Criteria:
    • Long: Spread < Mean - Multiplier * StdDev
    • Short: Spread > Mean + Multiplier * StdDev
  • Long/Short: Both sides.
  • Exit Criteria:
    • Long: Exit when spread reverts to the mean
    • Short: Exit when spread reverts to the mean
  • Stops: Yes, percentage stop based on spread value.
  • Default Values:
    • LookbackPeriod = 20
    • DeviationMultiplier = 2
    • StopLossPercent = 2
    • CandleType = TimeSpan.FromMinutes(5)
    • SecondSecurity — required, no default The spread is the close of the strategy's Security minus the close of SecondSecurity on candles of the same time; Mean and StdDev are taken over the last LookbackPeriod spreads. Each leg trades Volume, and an opposite signal reverses both legs. The stop closes both legs once the spread moves StopLossPercent of its entry value against the pair, checked on candle closes; 0 disables it.
  • Filters:
    • Category: Arbitrage
    • Direction: Both
    • Indicators: Spread statistics
    • Stops: Yes
    • Complexity: Intermediate
    • Timeframe: Intraday
    • Seasonality: No
    • Neural networks: No
    • Divergence: Yes
    • Risk Level: Medium
using System;
using System.Collections.Generic;
using System.Linq;

using Ecng.Common;

using StockSharp.Algo.Strategies;
using StockSharp.BusinessEntities;
using StockSharp.Messages;

namespace StockSharp.Samples.Strategies;

/// <summary>
/// Pairs trading strategy.
/// The spread is the first instrument's close minus the second's on candles of the same time. A spread more than DeviationMultiplier
/// standard deviations below the mean of the last LookbackPeriod spreads buys the first instrument and sells the second, one more than
/// that above it does the opposite, reversing an opposite pair. Both legs close once the spread returns to the mean, or once it moves
/// StopLossPercent of its entry value against the pair.
/// </summary>
public class PairsTradingStrategy : Strategy
{
	private readonly StrategyParam<int> _lookbackPeriod;
	private readonly StrategyParam<decimal> _deviationMultiplier;
	private readonly StrategyParam<decimal> _stopLossPercent;
	private readonly StrategyParam<DataType> _candleType;
	private readonly StrategyParam<Security> _secondSecurity;

	private readonly Dictionary<DateTime, decimal> _firstCloses = [];
	private readonly Dictionary<DateTime, decimal> _secondCloses = [];
	private readonly Queue<decimal> _spreads = [];
	// 1 while long the spread, -1 while short it, 0 while flat.
	private int _side;
	private decimal _entrySpread;

	/// <summary>
	/// Number of spreads the mean and standard deviation are measured over.
	/// </summary>
	public int LookbackPeriod
	{
		get => _lookbackPeriod.Value;
		set => _lookbackPeriod.Value = value;
	}

	/// <summary>
	/// Standard deviations between the mean and an entry level.
	/// </summary>
	public decimal DeviationMultiplier
	{
		get => _deviationMultiplier.Value;
		set => _deviationMultiplier.Value = value;
	}

	/// <summary>
	/// Adverse spread move, in percent of the entry spread, that closes the pair.
	/// </summary>
	public decimal StopLossPercent
	{
		get => _stopLossPercent.Value;
		set => _stopLossPercent.Value = value;
	}

	/// <summary>
	/// Candle type.
	/// </summary>
	public DataType CandleType
	{
		get => _candleType.Value;
		set => _candleType.Value = value;
	}

	/// <summary>
	/// Second security in the pair.
	/// </summary>
	public Security SecondSecurity
	{
		get => _secondSecurity.Value;
		set => _secondSecurity.Value = value;
	}

	/// <summary>
	/// Constructor.
	/// </summary>
	public PairsTradingStrategy()
	{
		_lookbackPeriod = Param(nameof(LookbackPeriod), 20)
			.SetGreaterThanZero()
			.SetDisplay("Lookback Period", "Spreads the mean and standard deviation are measured over", "Parameters");

		_deviationMultiplier = Param(nameof(DeviationMultiplier), 2m)
			.SetGreaterThanZero()
			.SetDisplay("Deviation Multiplier", "Standard deviations between the mean and an entry level", "Parameters");

		_stopLossPercent = Param(nameof(StopLossPercent), 2m)
			.SetNotNegative()
			.SetDisplay("Stop-loss %", "Adverse spread move in percent of the entry spread", "Risk Management");

		_candleType = Param(nameof(CandleType), TimeSpan.FromMinutes(5).TimeFrame())
			.SetDisplay("Candle Type", "Type of candles to use", "General");

		_secondSecurity = Param<Security>(nameof(SecondSecurity))
			.SetDisplay("Second Security", "Second security in the pair", "General")
			.SetRequired();
	}

	/// <inheritdoc />
	public override IEnumerable<(Security sec, DataType dt)> GetWorkingSecurities()
	{
		return [(Security, CandleType), (SecondSecurity, CandleType)];
	}

	/// <inheritdoc />
	protected override void OnReseted()
	{
		base.OnReseted();
		ResetState();
	}

	/// <inheritdoc />
	protected override void OnStarted2(DateTime time)
	{
		base.OnStarted2(time);

		if (SecondSecurity == null)
			throw new InvalidOperationException("Second security is not specified.");

		ResetState();

		var firstSubscription = SubscribeCandles(CandleType);
		firstSubscription.Bind(candle => ProcessCandle(candle, _firstCloses)).Start();

		var secondSubscription = SubscribeCandles(CandleType, security: SecondSecurity);
		secondSubscription.Bind(candle => ProcessCandle(candle, _secondCloses)).Start();

		var area = CreateChartArea();
		if (area != null)
		{
			DrawCandles(area, firstSubscription);
			DrawOwnTrades(area);
		}
	}

	private void ResetState()
	{
		_firstCloses.Clear();
		_secondCloses.Clear();
		_spreads.Clear();
		_side = 0;
		_entrySpread = 0;
	}

	private void ProcessCandle(ICandleMessage candle, Dictionary<DateTime, decimal> closes)
	{
		if (candle.State != CandleStates.Finished)
			return;

		closes[candle.OpenTime] = candle.ClosePrice;

		// The spread needs both instruments' candles of the same time, whichever arrives last.
		if (!_firstCloses.TryGetValue(candle.OpenTime, out var first) || !_secondCloses.TryGetValue(candle.OpenTime, out var second))
			return;

		foreach (var stale in _firstCloses.Keys.Where(t => t <= candle.OpenTime).ToArray())
			_firstCloses.Remove(stale);

		foreach (var stale in _secondCloses.Keys.Where(t => t <= candle.OpenTime).ToArray())
			_secondCloses.Remove(stale);

		var spread = first - second;

		_spreads.Enqueue(spread);

		if (_spreads.Count > LookbackPeriod)
			_spreads.Dequeue();

		if (_spreads.Count < LookbackPeriod)
			return;

		var mean = _spreads.Average();
		var deviation = (decimal)Math.Sqrt((double)_spreads.Average(s => (s - mean) * (s - mean)));
		var upper = mean + DeviationMultiplier * deviation;
		var lower = mean - DeviationMultiplier * deviation;

		if (!IsFormedAndOnlineAndAllowTrading())
			return;

		var stopDistance = Math.Abs(_entrySpread) * StopLossPercent / 100;

		if (spread < lower && _side <= 0)
		{
			MoveLegs(1);
			_entrySpread = spread;
		}
		else if (spread > upper && _side >= 0)
		{
			MoveLegs(-1);
			_entrySpread = spread;
		}
		else if (_side > 0 && (spread >= mean || (StopLossPercent > 0 && spread <= _entrySpread - stopDistance)))
		{
			MoveLegs(0);
		}
		else if (_side < 0 && (spread <= mean || (StopLossPercent > 0 && spread >= _entrySpread + stopDistance)))
		{
			MoveLegs(0);
		}
	}

	private void MoveLegs(int side)
	{
		_side = side;

		// Long the spread holds the first instrument and is short the second, each by Volume.
		var firstChange = side * Volume - Position;

		if (firstChange > 0)
			BuyMarket(firstChange);
		else if (firstChange < 0)
			SellMarket(-firstChange);

		var secondChange = -side * Volume - (GetPositionValue(SecondSecurity, Portfolio) ?? 0m);

		if (secondChange > 0)
			BuyMarket(secondChange, SecondSecurity);
		else if (secondChange < 0)
			SellMarket(-secondChange, SecondSecurity);
	}
}