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Genetic Algorithm Performance with Different Selection
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Proceedings of the World Congress on Engineering 2011 Vol II. WCE 2011 July 6 8 2011 London U K, than proportional roulette wheel selection Julstrom 5 in TSP 10 The TSP consists a number of cities where. investigated the computing time efficiency of two types of each pair of cities has a corresponding distance The aim is. rank based selection probabilities linear ranking and to visit all the cities such that the total distance travelled will. exponential ranking probabilities and compared with be minimized Obviously a solution and therefore a. tournament selection He pointed that tournament selection chromosome which represents that solution to the TSP can. is preferred over rank based selection because repeated be given as an order that is a path of the cities. tournament selection is faster than sorting the population to The procedure for solving TSP can be viewed as a. assign rank based probabilities In addition Mashohor et al process flow given in Fig 1 The GA process starts by. 6 evaluated the performance of PCB inspection system supplying important information such as location of the city. using three GA selection method deterministic tournament maximum number of generations population size. and roulette wheel and discovered that deterministic has the probability of crossover and probability of mutation An. ability to reach the highest maximum fitness with lowest initial random population of chromosomes is generated and. number of generations for all test images This is then the fitness of each chromosome is evaluated The population. followed by roulette wheel and tournament selection is then transformed into a new population the next. Goh et al 7 in his work entitled sexual selection for generation using three genetic operators selection. genetic algorithms focused on the selection stage of GA and crossover and mutation The selection operator is used to. examined common problems and solution methods for such choose two parents from the current generation in order to. selection schemes He proposed a new selection scheme procreate a new child by crossover and or mutation The. called sexual selection and compared the performance with new generation contains a higher proportion of the. commonly used selection methods in solving the Royal road characteristics possessed by the good members of the. problem the open shop scheduling and the job shop previous generation and in this way good characteristics are. scheduling problem He claimed that the proposed selection spread over the population and mixed with other good. scheme performed either on par or better than roulette wheel characteristics After each generation a new set of. selection on average when no fitness scaling is used The chromosomes where the size is equal to the initial population. new scheme also performed better on average when size is evolved This transformation process from one. compared to tournament selection in the more difficult test generation to the next continues until the population. cases when no scaling is used Apart from that Goldberg converges to the optimal solution which usually occurs. and Deb 8 did comprehensive studies on proportional when a certain percentage of the population e g 90 has. ranking tournament and Genitor steady state selection the same optimal chromosome in which the best individual is. schemes on the basis of solutions to differential equations taken as the optimal solution. Their studies have been performed to understand the. expected fitness ratio and convergence time They found. that ranking and tournament selection outperformed. proportional selection in terms of maintaining steady. pressure toward convergence They further demonstrated. that linear ranking selection and stochastic binary. tournament selection have identical expectations but. recommended binary tournament selection because of its. more efficient time complexity, III GENETIC ALGORITHM FOR TSP. This section provides the general overview of the genetic. algorithm component and operation for solving TSP, Genetic algorithm is an optimization method that uses a. stochastic approach to randomly search for good solutions to. a specified problem These stochastic approaches use. various analogies of natural systems to build promising. solutions ensuring greater efficiency than completely. random search The basic principles of GA were first. proposed by Holland in 1975 9 The GA operation is based. on the Darwinian principle of survival of the fittest and it. implies that the fitter individuals are more likely to survive Fig 1 Genetic algorithm procedure for TSP. and have a greater chance of passing their good genetic. features to the next generation In genetic algorithm each IV SELECTION STRATEGY FOR REPRODUCTION. individual i e chromosome that is a member of the The selection strategy addresses on which of the. population represents a potential solution to the problem chromosomes in the current generation will be used to. There are a number of possible chromosome representations reproduce offspring in hopes that next generation will have. due to a vast variety of problem types The path even higher fitness The selection operator is carefully. representation is more natural to represent the chromosome formulated to ensure that better members of the population. ISBN 978 988 19251 4 5 WCE 2011, ISSN 2078 0958 Print ISSN 2078 0966 Online. Proceedings of the World Congress on Engineering 2011 Vol II. WCE 2011 July 6 8 2011 London U K, with higher fitness have a greater probability of being population because they lost a tournament.
selected for mating or mutate but that worse members of the. population still have a small probability of being selected. and this is important to ensure that the search process is. global and does not simply converge to the nearest local. optimum Different selection strategies have different. methods of calculating selection probability The differing. selection techniques all develop solutions based on the. principle of survival of the fittest Fitter solutions are more. likely to reproduce and pass on their genetic material to the Fig 3 Procedure for tournament selection. next generation in the form of their offspring There are. three major types of selection schemes will be discussed and. B Proportional Roulette Wheel Selection, experimented in this study tournament selection roulette. wheel and rank based roulette wheel selection The In proportional roulette wheel individuals are selected. subsequent section will describe the mechanism of each with a probability that is directly proportional to their fitness. strategy A more detailed of selection method can be found values i e an individual s selection corresponds to a portion. of a roulette wheel The probabilities of selecting a parent. in 8 11 12 13, can be seen as spinning a roulette wheel with the size of the. A Tournament Selection segment for each parent being proportional to its fitness. Tournament selection is probably the most popular Obviously those with the largest fitness i e largest segment. selection method in genetic algorithm due to its efficiency sizes have more probability of being chosen The fittest. and simple implementation 8 In tournament selection n individual occupies the largest segment whereas the least fit. individuals are selected randomly from the larger have correspondingly smaller segment within the roulette. wheel The circumference of the roulette wheel is the sum of. population and the selected individuals compete against. all fitness values of the individuals The proportional. each other The individual with the highest fitness wins and. roulette wheel mechanism and the algorithm procedure are. will be included as one of the next generation population. depicted in Fig 4 and Fig 5 respectively In Fig 4 when the. The number of individuals competing in each tournament is wheel is spun the wheel will finally stop and the pointer. referred to as tournament size commonly set to 2 also attached to it will point on one of the segment most. called binary tournament Tournament selection also gives a probably on one of the widest ones However all segments. chance to all individuals to be selected and thus it preserves have a chance with a probability that is proportional to its. diversity although keeping diversity may degrade the width By repeating this each time an individual needs to be. convergence speed Fig 2 illustrates the mechanism of chosen the better individuals will be chosen more often than. tournament selection while Fig 3 shows the procedure for the poorer ones thus fulfilling the requirements of survival. tournament selection The tournament selection has several of the fittest Let f1 f2 fn be fitness values of individual 1. advantages which include efficient time complexity 2 n Then the selection probability Pi for individual i is. especially if implemented in parallel low susceptibility to define as. takeover by dominant individuals and no requirement for. fitness scaling or sorting 8 12 fi, The basic advantage of proportional roulette wheel. selection is that it discards none of the individuals in the. population and gives a chance to all of them to be selected. Therefore diversity in the population is preserved. However proportional roulette wheel selection has few. Fig 2 Selection strategy with tournament mechanism. major deficiencies Outstanding individuals will introduce a. bias in the beginning of the search that may cause a. In the above example the tournament size Ts is set to premature convergence and a loss of diversity For example. three which mean that three chromosomes competing each if an initial population contains one or two very fit but not. other Only the best chromosome among them is selected to the best individuals and the rest of the population are not. reproduce In tournament selection larger values of good then these fit individuals will quickly dominate the. tournament size lead to higher expected loss of diversity 12 whole population and prevent the population from exploring. 14 The larger tournament size means that a smaller portion other potentially better individuals Such a strong. of the population actually contributes to genetic diversity domination causes a very high loss of genetic diversity. making the search increasingly greedy in nature There which is definitely not advantageous for the optimization. might be two factors that lead to the loss of diversity in process On the other hand if individuals in a population. regular tournament selection some individuals might not get have very similar fitness values it will be very difficult for. sampled to participate in a tournament at all while other the population to move towards a better one since selection. individuals might not be selected for the intermediate probabilities for fit and unfit individuals are very similar. ISBN 978 988 19251 4 5 WCE 2011, ISSN 2078 0958 Print ISSN 2078 0966 Online. Proceedings of the World Congress on Engineering 2011 Vol II. WCE 2011 July 6 8 2011 London U K, Moreover it is difficult to use this selection scheme on For linear rank based selection the biasness could be.
minimization problems whereby the fitness function for controlled through the selective pressure SP such that. minimization must be converted to maximization function as 2 0 SP 1 0 and the expected sampling rate of the best. in the case of TSP Although to some degree this solves the individual is SP the expected sampling rate of the worst. selection problem it introduces confusion into the problem individual is 2 SP and the selective pressure of all other. The best chromosome in the TSP problem for instance will population members can be interpreted by linear. continually be assigned a fitness value that is the maximum interpolation of the selective pressure according to rank. of all other fitness functions and thus we are seeking the Consider n the number of individuals in the population Pos. minimum tour but the fitness maximizes the fitness value As the position of an individual in the population least fit. a consequence several other selection techniques with a individual has Pos 1 the fittest individual Pos n and SP. the selective pressure Instead of using the fitness value of an. probability not proportional to the individual s fitness values. individual the rank of individuals is used The rank for an. have been developed to encounter proportional selection. individual may be scaled linearly using the following. problem In general there are two types of such non. proportional selection operators tournament based selection. techniques which already been described in the previous Pos 1. Rank Pos 2 SP 2 SP 1 2, section and the rank based selections that assign the. probability value depending on the order of the individuals. according to their fitness values which will be discussed in. TABLE 1 contains the fitness values of the individuals for. the following section various values of the selective pressure assuming a. population of 11 individuals and a minimization problem. TABLE 1 EXAMPLE OF SCALED RANK WITH DIFFERENT SP, Individual Scaled rank Scaled rank. fitness value with SP 2 0 with SP 1 1, 1 1 2 0 1 1. 3 2 1 8 1 08, 4 3 1 6 1 06, 7 4 1 4 1 04, Fig 4 Selection strategy with roulette wheel mechanism 8 5 1 2 1 02. 9 6 1 0 1 00, 10 7 0 8 0 98, 15 8 0 6 0 96, 20 9 0 4 0 94.
30 10 0 2 0 92, 95 11 0 0 9, Rank based selection schemes can avoid premature. convergence and eliminate the need to scale fitness values. I INTRODUCTION asic genetic algorithm GA is generally composed of two processes The first process is selection of individuals for the production of the next generation and the second process is manipulation of the selected individuals to form the next generation by crossover and mutation techniques The selection mechanism determines which individuals are chosen for mating reproduction

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