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  5. Optimize by Generation (GGA)

(AI Studio Core)

Synopsis

This operator may select some attributes from the original attribute set and it may also generate new attributes from the original attribute set. GGA (Generating Genetic Algorithm) does not change the original number of attributes unless adding or removing (or both) attributes prove to have a better fitness.

Description

Sometimes the selection of features alone is not sufficient. In these cases other transformations of the feature space must be performed. The generation of new attributes from the given attributes extends the feature space. Maybe a hypothesis can be easily found in the extended feature space. This operator can be considered to be a blend of attribute selection and attribute generation procedures. It may select some attributes from the original set of attributes and it may also generate new attributes from the original attributes.

A genetic algorithm (GA) is a search heuristic that mimics the process of natural evolution. This heuristic is routinely used to generate useful solutions to optimization and search problems. Genetic algorithms belong to the larger class of evolutionary algorithms (EA), which generate solutions to optimization problems using techniques inspired by natural evolution, such as inheritance, mutation, selection, and crossover. For studying the basic algorithm of a genetic algorithm please study the description of the Optimize Selection (Evolutionary) operator.

In contrast to the simple Genetic Algorithm, the Generating Genetic Algorithm generates new attributes and thus can change the length of an individual. Therefore specialized mutation and crossover operators are being applied. Generators are chosen at random from a list of generators specified by boolean parameters. This operator is a nested operator i.e. it has a subprocess. The subprocess must return a performance vector. You need to have basic understanding of subprocesses in order to apply this operator. Please study the documentation of the Subprocess operator for basic understanding of subprocesses.

Input

  • example set in (Data table)

    This input port expects an ExampleSet. This ExampleSet is available at the first port of the nested chain (inside the subprocess) for processing in the subprocess.

Output

  • example set out (Data table)

    The genetic algorithm is applied on the input ExampleSet. The resultant ExampleSet is delivered through this port.

  • attribute weights out (Attribute Weights)

    The attribute weights are delivered through this port.

  • performance out (Performance Vector)

    This port delivers the Performance Vector for the selected attributes. A Performance Vector is a list of performance criteria values.

Parameters

  • max number of new attributesThis parameter specifies the maximum number of attributes to generate for an individual in one generation.
  • limit max total number of attributesThis parameter indicates if the total number of attributes in all generations should be limited. If set to true, the maximum number is specified by the max total number of attributes parameter.
  • max total number of attributesThis parameter is only available when the limit max total number of attributes parameter is set to true. This parameter specifies the maximum total number of attributes in all generations.
  • use local random seedThis parameter indicates if a local random seed should be used for randomization. Using the same value of local random seed will produce the same randomization.
  • local random seedThis parameter specifies the local random seed. This parameter is only available if the use local random seed parameter is set to true.
  • maximal fitnessThis parameter specifies the maximal fitness. The optimization will stop if the fitness reaches this value.
  • population sizeThis parameter specifies the population size i.e. the number of individuals per generation.
  • maximum number of generationsThis parameter specifies the number of generations after which the algorithm should be terminated.
  • use plusThis parameter indicates if the summation function should be applied for a generation of new attributes.
  • use diffThis parameter indicates if the difference function should be applied for a generation of new attributes.
  • use multThis parameter indicates if the multiplication function should be applied for a generation of new attributes.
  • use divThis parameter indicates if the division function should be applied for a generation of new attributes.
  • reciprocal valueThis parameter indicates if the reciprocal function should be applied for a generation of new attributes.
  • use early stoppingThis parameter enables early stopping. If not set to true, always the maximum number of generations are performed.
  • generations without improvalThis parameter is only available when the use early stopping parameter is set to true. This parameter specifies the stop criterion for early stopping i.e. it stops after n generations without improvement in the performance. n is specified by this parameter.
  • tournament sizeThis parameter specifies the fraction of the current population which should be used as tournament members.
  • start temperatureThis parameter specifies the scaling temperature.
  • dynamic selection pressureIf this parameter is set to true, the selection pressure is increased to maximum during the complete optimization run.
  • keep best individual If set to true, the best individual of each generation is guaranteed to be selected for the next generation.
  • p initializeThe initial probability for an attribute to be switched on is specified by this parameter.
  • p crossoverThe probability for an individual to be selected for crossover is specified by this parameter.
  • crossover typeThe type of the crossover can be selected by this parameter.
  • p generateThis parameter specifies the probability for an individual to be selected for a generation.
  • use heuristic mutation probabilityIf this parameter is set to true, the probability for mutations will be chosen as 1/n where n is the number of attributes. Otherwise the probability for mutations should be specified through the p mutation parameter
  • p mutationThe probability for an attribute to be changed is specified by this parameter. If set to -1, the probability will be set to 1/n where n is the total number of attributes.

Tutorial Processes

Applying GGA on the Polynomial data set

The 'Polynomial' data set is loaded using the Retrieve operator. A breakpoint is inserted here so that you can have a look at the ExampleSet. You can see that the ExampleSet has 5 regular attributes other then the label attribute. The Optimize by Generation (GGA) operator is applied on the ExampleSet. Optimize by Generation (GGA) is a nested operator i.e. it has a subprocess. It is necessary for the subprocess to deliver a performance vector. This performance vector is used by the underlying Genetic Algorithm. Have a look at the subprocess of this operator. The Split Validation operator is used there which itself is a nested operator. Have a look at the subprocesses of the Split Validation operator. The Linear Regression operator is used in the 'Training' subprocess to train a model. The trained model is applied using the Apply Model operator in the 'Testing' subprocess. The performance is measured through the Performance (Regression) operator and the resultant performance vector is used by the underlying algorithm. Run the process and switch to the Results Workspace. You can see that the ExampleSet that had 5 attributes, now has 4 attributes. The resultant ExampleSet with reduced attributes can be seen in the Results Workspace.

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