Without loss of generality, a multi-objective optimization problem (MOP) can be defined in terms of maximization (although minimization is also common) as follows:
1.4. Evolutionary Multi-Objective Optimization
Multi-objective evolutionary algorithms (MOEAs) are high-level procedures designed to discover good enough solutions to MOPs (solutions that are close to the global optimum). They are especially useful with incomplete or imperfect information or a limited computing capacity (
[5]).
MOEAs address MOPs using principles from biological evolution. They use a population of individuals, each representing a solution to the MOP. The individuals in the population reproduce among them, using so-called evolutionary operators (selection, crossover, mutation), to produce a new generation of individuals. Often, this new generation of individuals is composed of both parents and children that posses the best fitness; this fitness represents the impact on the objectives of the MOP. Since each individual encodes a solution to the MOP, MOEAs can approximate a set of trade-off alternatives simultaneously.
The performance of MOEAs has been assessed in different fields (e.g.,
[6][7]). They have been widely accepted as convenient tools for addressing the problem of stock portfolio management (
[8][9][10]). The main goal of MOEAs is to find a set of solutions that approximate the true Pareto front in terms of convergence and diversity. Convergence refers to determining the solutions that belong to the PF, while diversity refers to determining the solutions that best represent all the PF. Thus, the intervention of the decision maker is not traditionally used in the process. Thus, rather little interest has been paid in the literature to choosing one of the efficient solutions as the final one in contrast to the interest paid in approximating the whole Pareto front.
Usually, two types of MOEAs are highlighted in the literature: differential evolution and genetic algorithms. Differential evolution (DE) has been found to be very simple and effective (
[11]), particularly when addressing non-linear single-objective optimization problems (
[12][13]). On the other hand, in a genetic algorithm (GA), solutions to a problem are sought in the form of strings of characters (the best representations are usually those that reflect something about the problem that is being addressed), virtually always applying recombination operators such as crossing, selection and mutation operators. GAs compose one of the most popular meta-heuristics applied to the Portfolio Optimization Problem (
[10]).
2. Computational Intelligence in Stock Portfolio Management
2.1. Portfolio Management: Price Forecasting, Stock Selection and Portfolio Optimization
In recent years, there have been plenty of contributions on price forecasting based on either statistical or computational intelligence methods (see
[8][14]). The stock market is characterized by extreme fluctuations, non-linearity, and shifts in internal and external environmental variables. Artificial intelligence techniques can detect such non-linearity, resulting in much-improved forecast results
[15].
Among the computational intelligence methods used for price forecasting are deep learning (e.g.,
[16][17][18][19]) and machine learning (e.g.,
[20][21][22]). In
[8], a hybrid stock selection model with a stock prediction stage based on an artificial neural network (ANN) trained with the extreme learning machine (ELM) training algorithm (
[23][24]) was proposed. The ELM algorithm has been tested for financial market prediction in other works (see
[25][26][27]).
There are important works on methods for stock selection, which have several different fundamental theories, from operations research methods (e.g.,
[28][29]) to approaches originating in modern portfolio theory (Mean-variance model) (e.g.,
[29][30]) and soft computing methods (e.g.,
[31][32]), including hybrid approaches (e.g.,
[8][33][34]).
The fundamental theory for portfolio optimization is Markowitz’s mean-variance model (
[35]). Its formulation marked the beginning of Modern portfolio theory (
[36]). However, Markowitz’s original model is considered too basic since it neglects real-world issues related to investors, trading limitations, portfolio size and others (
[34]). For evaluating a portfolio’s performance, the model is based on measuring the expected return and the risk; the latter is represented by the variance in the portfolio’s historical returns. Since the variance takes into account both negative and positive deviations, other risk measures have been proposed, such as the Conditional Value at Risk (CVaR) (
[37][38]). As a result, numerous works have improved the model, creating more risk measures and proposing restrictions that bring them closer to practical aspects of stock market trading (
[14]). Consequently, many optimization methods based on exact algorithms (e.g.,
[39][40][41][42][43][44][45][46]) and heuristic and hybrid optimization (e.g.,
[16][47][48][49][50][51][52][53][54][55][56]) have been proposed to solve the emerging portfolio optimization models (
[14][31][36]).
2.2. Exploiting Uptrends and Downtrends in Strategies for Stock Investment
Regarding alternative strategies to the known buy-and-hold approach for stock investment, in (
[57]), the authors propose two new trading strategies to outperform the buy-and-hold approach, which is based on the efficient market hypothesis. The proposed strategies are based on a generalized time-dependent strategy proposed in (
[58]) but propose different timing for changing the buying/selling position. According to (
[59]), the decision to adopt a long or short position in an asset requires a view of its immediate future price movements. A typical short seller would have to assess the potential future behavior of the asset price by means of evaluating several factors, such as past returns and market effects as well as technical indicators, such as market ratios (
[59]). There are a few works published in the literature to address the problem of trading strategies for the short position. An interesting work that considers not only the short position but both the short and long positions is (
[60]), in which a simultaneous long-short trading strategy (SLS) is proposed. Such a strategy is based partially on the property that a positive gain with zero initial investment is expected, which holds for all discrete and continuous price processes with independent multiplicative growth and a constant trend. Other works based on SLS are (
[61][62][63]).
Building stock portfolios with high returns and low risk is a common challenge for researchers in the financial area. Usually, the most common practice is to select the more promising stocks according to several factors, such as financial information, news of the market and technical analysis. Several approaches that use computational intelligence algorithms have been proposed in the literature to deal with the overwhelming complexity of building a stock portfolio. Usually, these approaches consider up to three activities to build a portfolio: return forecasting, stock selection and portfolio optimization. These activities decide which stocks should be supported, as well as the proportions of the investment to be allocated to them, by comparing the historical and forecasted performance of potential stock investments.
Given the approaches mentioned above for addressing the three activities or stages: return forecasting, stock selection and portfolio optimization, a comprehensive approach for portfolio bulding and optimization can be carried out by applying a combination of computational intelligence techniques in each stage. A particular effective combination proven in the literature comprises: stock price forecasting by an artificial neural network (ANN) trained by the extreme learning machine (ELM) algorithm. Stock selection is modeled as an optimization problem that seeks to determine the most plausible stocks; thus, a differential evolution is exploited on the basis of the forecasted price and a set of factors of the so-called fundamental analysis. Finally, portfolio optimization is conducted through a genetic algorithm that uses confidence intervals of the portfolio returns to determine the best stock portfolio. It is worth to mention that forecasting the price of a given stock allows the comprehensive approach to focus on uptrends or downtrends (i.e., going long or short, respectively) for that stock.