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Pretests for genetic-programming evolved trading programs : “zero-intelligence” strategies and lottery trading Nicolas NAVET INRIA - AIECON NCCU http://www.loria.fr/~nnavet joint work with Shu-Heng Chen AIECON NCCU http://www.aiecon.org/ ICONIP 2006 – Hong Kong – October 4, 2006

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2 Conclusions are - most often - inconclusive regarding the efficiency of GP Is it because market is efficient ? (i.e. nothing to learn on the training data) Further efforts are meaningless ! Or learning algorithm is inefficient ? Consider another ML algorithm / improvement of the GP scheme: fitness function, search intensity, high level function sets, overfitting avoidance, better genetic operators, data division scheme, etc, etc, … Pretesting may provide some evidence ! If results are not very convincing :

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3 Genetic programming : a recap Generate a population of random programs Evaluate their quality (“fitness”) Create better programs by applying genetic operators, eg - mutation - combination (“crossover”) GP is the process of evolving a population of computer programs, that are candidate solutions, according to the evolutionary principles (e.g. survival of the fittest) Solution

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4 In GP, programs are represented by trees (1/3) Trees are a very general representation form : E.g. of a trading rule : buy if expression is “true” functions terminals

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5 GP for financial trading Predicting price evolution (not discussed here) Inducing technical trading rules : Training interval Validation interval Out-of-sample interval 1 ) Creation of the trading rules using GP 2) Selection of the best resulting strategies Further selection on unseen data - One strategy is chosen for out-of-sample Performance evaluation

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6 Why GP is an appealing technique for financial trading ? Easy to implement / robust evolutionary technique Trading rules (TR) should adapt to a changing environment – GP may simulate this evolution Solutions are produced under a symbolic form that can be understood and analyzed GP may serve as a knowledge discovery tool (e.g. evolution of the market)

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7 But one may cast doubts on GP efficiency.. Highly heuristic – no theory ! Problems on which GP has been shown not to be significantly better than random search Few clear-cut successes reported in the financial literature GP embeds little domain specific knowledge yet.. Doubts on the efficiency of GP to use the available computing time : code bloat bad at finding numerical constants best solutions are sometimes found very early in the run.. Variability of the results ! e.g. returns: -0.160993, 0.0526153, 0.0526153, 0.0526153, 0.0526153, -0.0794787, 0.0526153, -0.0794787, 0.132354, 0.364311, -0.0990995, -0.0794787, -0.0855786, -0.094433, 0.0464288, -0.140719, 0.0526153, 0.0526153, -0.0746189, 0.418075, ….

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8 Possible pretest : measure of predictability of the financial time-series Possible pretest : measure of predictability of the financial time-series Serial correlation Kolmogorov complexity Lyapunov exponent Unit root analysis Comparison with results on surrogate data : “shuffled” series (e.g. Kaboudan statistics) ... Actual question : how predictable for a given horizon with a given cost function?

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9 In practice, some predictability does not imply profitability.. Volatility may not be sufficient to cover round-trip transactions costs! Not the right trading instrument at hand.. typically short selling not available Prediction horizon must be large enough!

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Part 2 : Pretests for GP evolved trading strategies

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11 Pretest methodology Compare GP with several variants of Random search algorithms “Zero-Intelligence Strategies” - ZIS Random trading behaviors “Lottery trading” - LT Statistical hypotheses testing Null : GP does not outperform ZIS Null : GP does not outperform LT Issue : how to best constraint randomness ?

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Pretest 1 : GP versus Zero-Intelligence strategies (=“Equivalent search intensity” Random Search (ERS) with validation stage) -Null hypothesis H : GP does not outperform equivalent random search - Alternative hypothesis is H -Null hypothesis H 1,0 : GP does not outperform equivalent random search - Alternative hypothesis is H 1,1

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13 Pretest 1 : GP vs zero-intelligence strategies H 1,0 cannot be rejected – interpretation : There is nothing to learn or GP is not very effective Training interval Validation interval Out-of-sample interval 1 ) Creation of the trading rules using GP 2) Selection of the best resulting strategies Further selection on unseen data - One strategy is chosen for out-of-sample Performance evaluation ERS

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14 Pretest 4 : GP vs lottery trading Lottery trading (LT) = random trading behavior according the outcome of a r.v. (e.g. Bernoulli law) Issue 1 : if LT tends to hold positions (short, long) for less time that GP, transactions costs may advantage GP.. Issue 2 : it might be an advantage or an disadvantage for LT to trade much less or much more than GP. ex: downward oriented market with no short-sell

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15 Frequency and intensity of a trading strategy Frequency : average number of transactions per unit of time Intensity : proportion of time where a position is held For pretest 4 : We impose that average frequency and intensity of LT is equal to the ones of GP Implementation : generate random trading sequences having the right characteristics 0,0,1,1,1,0,0,0,0,0,1,1,1,1,1,1,0,0,1,1,0,1,0,0,0,0,0,0,1,1,1,1,1,1,…

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16 Training interval Validation interval Out-of-sample interval 1 ) Creation of the trading rules using GP 2) Selection of the best resulting strategies Further selection on unseen data - One strategy is chosen for out-of-sample Performance evaluation Pretest 4 : implementation 0,0,1,1,1,0,0,0,0,0,1,… Lottery trading

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Answering question 1 : is there anything to learn on the training data at hand ?

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18 Question 1 : pretests involved Starting point: if a set of search algorithms do not outperform LT, it gives evidence that there is nothing to learn.. Pretest 4 : GP vs Lottery Trading Null hypothesis H 4,0 : GP does not outperform LT Pretest 5 : Equivalent Random Search (ZIS) vs Lottery Trading Null hypothesis H 5,0 : ERS does not outperform LT

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19 Question 1 : some answers... R means that the null hypothesis Hcannot be rejected – R means we should favor H R means that the null hypothesis H i,0 cannot be rejected – R means we should favor H i,1 H 4,0 H 5,0 Interpretation Case 1 RR RR Case 2RR Case 3R RR Case 4 RR R there is nothing to learn there is something to learn there may be something to learn - ERS might not be powerful enough there may be something to learn – GP evolution process is detrimental

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Answering question 2 : is GP effective ?

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21 Question 2 : some answers... Question 2 cannot be answered if there is nothing to learn (case 1) Case 4 provides us with a negative answer.. In case 2 and 3, run pretest 1 : GP vs Equivalent random search Null hypothesis H 1,0 : GP does not outperform ERS If one cannot reject H 1,0 GP shows no evidence of efficiency…

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Pretests at work Methodology : Draw conclusions from pretests using our own programs and compare with results in the literature [ChKuHo06] on the same time series

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23 Setup : GP control parameters - same as in [ChKuHo06]

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24 Setup : statistics, data, trading scheme Hypothesis testing with student t-test with a 95% confidence level Hypothesis testing with student t-test with a 95% confidence level Pretests with samples made of 50 GP runs, 50 ERS runs and 100 LT runs Pretests with samples made of 50 GP runs, 50 ERS runs and 100 LT runs Data : indexes of 3 stock exchanges Canada, Taiwan and Japan Data : indexes of 3 stock exchanges Canada, Taiwan and Japan Daily trading with short selling Daily trading with short selling Training of 3 years – Validation of 2 years Training of 3 years – Validation of 2 years Out-of-sample periods: 1999-2000, 2001-2002, 2003-2004 Out-of-sample periods: 1999-2000, 2001-2002, 2003-2004 Data normalized with a 250 days moving average Data normalized with a 250 days moving average

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25 Results on actual data (1/2) Evidence that there is something to learn : 4 markets out of 9 (C3,J2,T1,T3) Experiments in [ChKuHo06], with another GP implementation, show that GP performs very well on these 4 markets Evidence that there is nothing to learn : 3 (C1,J3,T2) In [ChKuHo06], there is only one (C1) where GP has positive return (but less than B&H)

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26 Results on actual data (2/2) GP effective : 3 markets out of 6 In these 3 markets, GP outperforms Buy and Hold – same outcome as in [ChKuHo06] Preliminary conclusion : one can rely on pretests.. When there is nothing to learn, no GP implementation did good (except in one case) When there is something to learn, at least one implementation did good (always) When our GP is effective, GP in [ChKuHo06] is effective too (always)

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27 Further conclusion Our GP implementation is 1. 1. is more efficient than random search : no case where ERS outperform LT and GP did not 2. 2.But only slightly more efficient … one would expect much more cases where GP does better than LT and not ERS Our GP is actually able to take advantage of regularities in data … but only of “simple” ones Ongoing work : study the correlation between predictability measures and GP performances

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28 References [ChKuHo06] S.-H. Chen and T.-W. Kuo and K.-M. Hoi, “Genetic Programming and Financial Trading: How Much about What We Know”. Handbook of Financial Engineering, Kluwers, 2006. [ChKuHo06] S.-H. Chen and T.-W. Kuo and K.-M. Hoi, “Genetic Programming and Financial Trading: How Much about What We Know”. Handbook of Financial Engineering, Kluwers, 2006. [Kab00] M. Kaboudan, “Genetic Programming Prediction of Stock Prices”, Computational Economics, vol16, 2000. [Kab00] M. Kaboudan, “Genetic Programming Prediction of Stock Prices”, Computational Economics, vol16, 2000.

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29 ?

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30 Conclusions are - most often - inconclusive regarding the efficiency of GP “Annual returns with the GP induced technical trading rules is x%” If negative, market is efficient or GP ineffective ?? If positive, mere luck or GP is effective ?? Good/bad wrt to other search techniques ?? Worth to further improve/ optimize GP ?? Pretests provide some evidence whether 1.There is something to be learned from the data 2.GP is effective at this task

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31 Equivalent search intensity Starting point : 2 search algorithms have similar search intensity if they create the same number of solutions over the course of their execution Problem : same solutions tends to be rediscovered over time and are not re-evaluated – rate of discovery strongly depends on the search technique / implementation Refined definition : similar search intensity if same number of “truly” different solutions – here truly means syntactically different

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32 Other zero-intelligence (but less meaningful) strategies one can think of Pretest 2 : GP versus equivalent random search without validation May give some insight into effectiveness of validation to fight overfitting.. but little overfitting with random search thus usefulness is dubious.. Pretest 3 : GP versus equivalent random search without training and validation : random trees applied directly out-of-sample Bias in randomness induced by the GP language..

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33 Pretest 1 : GP vs zero-intelligence strategies Implementation : Execute multiple GP runs – record average number of syntactically different individuals Random search is implemented with the initial population of GP – adjust size of population to obtain “equivalent search intensity” H 1,0 cannot be rejected – interpretation : There is nothing to learn or GP is not effective H 1,1 should be rejected – interpretation : There may be something to learn and GP may be effective..

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