ARCHITECTURE Subcommand (RBF command)

The ARCHITECTURE subcommand is used to specify the neural network architecture. RBF creates a neural network with one hidden layer. There are three ways of choosing the number of hidden units:

  1. By default, RBF uses automatic architecture selection to find the “best” number of hidden units within a range. The procedure automatically computes the minimum and maximum and finds the best number of hidden units within the range.

    If a testing sample is defined; that is, if /PARTITION TESTING is specified with a number greater than zero or via /PARTITION VARIABLE, then the procedure uses the testing data criterion: The best number of hidden units is the one that yields the smallest error in the testing data. If a testing sample is not defined, then the procedure uses the Bayesian information criterion (BIC): The best number of hidden units is the one that yields the smallest BIC based on the training data.

  2. You can provide your own range by specifying the MINUNITS and MAXUNITS keywords with integers, and the procedure will find the “best” number of hidden units within that range. It is invalid to specify only MINUNITS or only MAXUNITS; both must be given. The best number of hidden units from the range is determined using the testing data criterion or the BIC.
  3. You can override the use of a range and specify a particular number of units directly using the NUMUNITS keyword. It is invalid to specify NUMUNITS with either the MINUNITS or MAXUNITS keyword.

MINUNITS Keyword

The MINUNITS keyword specifies the minimum number of units to use in the range. Value AUTO indicates that the number should be computed automatically. Alternatively, a positive integer may be specified. The integer must be less than the number specified on the MAXUNITS keyword.

The default value is AUTO.

MAXUNITS Keyword

The MAXUNITS keyword specifies the maximum number of units to use in the range. Value AUTO indicates that the number should be computed automatically. Alternatively, a positive integer may be specified. The integer must be greater than the number specified on the MINUNITS keyword.

The default value is AUTO.

NUMUNITS Keyword

The NUMUNITS keyword specifies a particular number of hidden units instead of a range. A positive integer may be specified.

  • If NUMUNITS is specified when testing data are defined (see PARTITION Subcommand (RBF command)), then the testing data are not used in determining the network architecture and a warning is issued.
  • There is no default value.

HIDDENFUNCTION Keyword

The HIDDENFUNCTION keyword specifies the Gaussian radial basis function used in the hidden layer.

NRBF. Normalized radial basis function. Uses the softmax activation function so the activations of all hidden units are normalized to sum to 1. This is the default activation function for all units in the hidden layer.

ORBF. Ordinary radial basis function. Uses the exponential activation function so the activation of the hidden unit is a Gaussian “bump” as a function of the inputs.