The theory of multi-objective optimization concentrates on methods to simul- taneously optimize multiple conflicting objective functions. These methods can be applied quite universally almost in any decision making, therefore be- ing interesting from the perspective of advisory systems. Knowledge-driven multi-objective optimization combines the multi-objective optimization with knowledge bases.
3.5.1
Multi-objective optimization
In multi-objective optimization (MOO), decision making means selecting one solution out of all Pareto optimal solutions [76]. A MOO problem is generally
expressed as
minimize F(x) = {f1(x), f2(x), ..., fm(x)}
subject to x∈S (3.12)
where: fi =m (≥ 2) conflicting objectives that are simultaneously mini-
mized, fi :Rn→R
x = [x1, x2, ..., xn] = variable vector that belongs to the non-empty
feasible region S ⊂Rn
S = the feasible region formed by the constraints of the problem and the bounds of the variables. [36]
A variable vector x1 dominates x2 if
1. fi(x1)≤fi(x2) ∀i∈ {1,2, ..., m}
and 2. ∃j ∈ {1,2, ..., m} such that fj(x1)< fj(x2)
(3.13) This domination is denoted as x1 ≺x2. A vector x∗ is called Pareto optimal
if @x such that x ≺ x∗. The set of all x∗ forms the Pareto optimal set, the projection of which in the objective space, F(x∗) ∀x∗, is called the Pareto optimal front. [36]
The n-dimensional decision space formed by the variables, and the m- dimensional objective space formed by the objectives, are illustrated in Fig- ure 3.10.
Population-based evolutionary algorithms are the most common meth- ods to solve multi-objective optimization problems. These algorithms can be classified into Pareto-dominance based (e.g. MOGA, NSGA, NPGA, PAES, NSGA-II and SPEA2), decomposition-based (e.g. MOGLS, MOEA/D and NSGA-III) and indicator-based methods (e.g. HypE, IBEA and SMS-EMOA). High-dimensional objective space (m > 10) causes the curse of dimension- ality problem with Pareto-dominance based algorithms, and thus, the latter two are more suitable in that case. During the optimization, a MOO dataset is formed. Each row in the MOO dataset consists of the decision vector components, corresponding objective values and possibly some additional in- formation. Thus, the dataset can be divided into infeasible and feasible sets. [36]
According to Bandaru et al. [36], the MOO dataset can be used for ex- tracting knowledge, by using data mining methods such as descriptive statis- tics (e.g. mean, skewness, contingency), visual data mining (e.g. clustering, dimensionality reduction) and machine learning (e.g. decision trees). With these methods, it is possible to for example reveal the overall structure of the objective space and associate clusters in it to corresponding clusters in the
Figure 3.10: The decision space and the objective space (both two- dimensional) in multi-objective optimization [36].
decision space. Some methods, especially data visualization, may require ex- pert knowledge to make understanding the knowledge possible, which usually results in subjective interpretation of the extracted knowledge.
Machine learning can be used for non-visual data mining and knowl- edge extraction from MOO datasets. In supervised learning, decision tree (classification tree if the labels are discrete, or regression tree if continuous) algorithms, such as ID3 and C4.5, are used to create decision rules, e.g. in the form of a statement
if (x1 < v1)∧(x2 > v2)∧...then (Class Y) (3.14)
These algorithms recursively divide the training dataset into two subgroups, by a feature that maximizes the dissimilarity between the subgroups. Thus, explicit knowledge representation is acquired. [36] For example, an objective function in MOO can be discretized using the decision variables xi as the
division variables [77].
For implicit representation, e.g. self-organizing maps are used for visu- alization, and support vector machines (SVM) and ANNs can be used for black-box modeling [36]. For example, SVMs are used for predicting the ranks of new MOO solutions [78].
3.5.2
Knowledge-driven optimization
According to Bandaruet al. [36], interest in the application of multi-objective optimization to real-world problems is increasing, and consequently, knowl- edge-driven optimization may become an important topic in the near future. Inknowledge-driven optimization (KDO) the user or decision maker modifies the optimization problem or algorithm by using the knowledge gained in a post-optimal analysis. KDO can be performed online or offline, as illustrated in Figure 3.11. In offline KDO, data mining and the optimization algorithm are separate, and thus, any MOO algorithm can be used unchanged.
Online Knowledge-Driven Optimization
Offline Knowledge-Driven Optimization Online Implicit Knowledge (Interactive) Data Mining Multi-objective Optimization Algorithm Runtime Optimization Data Online Explicit Knowledge Optimization Data Multi-objective Optimization Problem Post-optimal Implicit Knowledge (Interactive) Data Mining Post-optimal Explicit Knowledge Knowledge Base or Expert System
Figure 3.11: A framework for knowledge-driven optimization [36].
For example in offline KDO, regression analysis can be carried out on a set of Pareto optimal solutions, to obtain analytical relationships between the variables. When these relationships are used as constraints on the original MOO problem and a local search is performed on the previously obtained solutions, a better approximation of the Pareto front can be generated. As another example, post-optimal knowledge can be used for parameter tuning of the optimization algorithm, and expert systems can be used for automatic MOO problem and algorithm modification if a knowledge base is built, as presented in Figure 3.12. [36]
Multi-objective Optimization Problem Multi-objective Evolutionary Algorithm Objective Space Decision Space
Sequential Pattern Mixing Classification Trees Clustering + Classification
Flexible Pattern Mining
Patterns Decision Rules Unsupervised Rules Visualization in Objective Space Knowledge Base or Expert System MOO Dataset
Data Mining Methods
Explicit Knowledge
Figure 3.12: A method to control the optimization algorithm and to mod- ify the original MOO problem, by using extracted knowledge in an expert system [79].
Online KDO aims at extracting knowledge during optimization, which could be used to obtain faster convergence of the Pareto optimal front. For example, dimensionality reduction can be used for identifying redundant ob- jectives in the objective space. On the other hand, online KDO can also be data stream mining: For example, if a reference point is provided by the decision maker, the input data stream can be filtered to obtain only solutions close to the reference point. [36]
In a case study by Bandaruet al. [79], a MOO problem with three objec- tive functions (including cost and buffer capacity) was solved using NSGA-II. The results for the Pareto optimal solutions for cost C and a buffer capacity
B is presented in Figure 3.13. Then DBSCAN clustering was performed in the objective space, which resulted in three clusters, Classes 1–3. Based on this, it was possible to create rules in the decision space for these clusters, as shown in Figure 3.13.