Software random number generators

Models are often best understood relative to the situation they are designed to describe if their constitutive variables are allowed to fluctuate statistically in a realistic way. Once a variable has been assigned a suitable density of probability distribution and the parameters of this distribution have been chosen, the fluctuations can be conveniently produced by using random deviates from statistical tables. A random deviate is a particular value of a standard random variable. Many elementary books in statistics contain tables of deviates from uniform, normal, exponential,. .. distributions. Many high-level computation-oriented programming languages (e.g., MatLab) and spreadsheets, such as Microsoft Excel, also contain random number generators. The book by Press et al. (1986) contains software that produces random deviates for the most commonly used probability distributions. 

In modern software packages random number generators should work satisfactorily. However, if seeds are set manually, procedures should ensure that repetition of sequences is avoided. 

Monte Carlo simulation can involve several methods for using a pseudo-random number generator to simulate random values from the probability distribution of each model input. The conceptually simplest method is the inverse cumulative distribution function (CDF) method, in which each pseudo-random number represents a percentile of the CDF of the model input. The corresponding numerical value of the model input, or fractile, is then sampled and entered into the model for one iteration of the model. For a given model iteration, one random number is sampled in a similar way for all probabilistic inputs to the model. For example, if there are 10 inputs with probability distributions, there will be one random sample drawn from each of the 10 and entered into the model, to produce one estimate of the model output of interest. This process is repeated perhaps hundreds or thousands of times to arrive at many estimates of the model output. These estimates are used to describe an empirical CDF of the model output. From the empirical CDF, any statistic of interest can be inferred, such as a particular fractile, the mean, the variance and so on. However, in practice, the inverse CDF method is just one of several methods used by Monte Carlo simulation software in order to generate samples from model inputs. Others include the composition and the function of random variable methods (e.g. Ang Tang, 1984). However, the details of the random number generation process are typically contained within the chosen Monte Carlo simulation software and thus are not usually chosen by the user. 

We have recently developed a library implementing several of the parallel random number generators and statistical tests of them on the most widely available multiprocessor computers. Documentation and software are available at... 

IBM SPSS is a comprehensive, easy-to-use set of data and predictive analytics tools for business users, analysts and statistical programmers [45]. Its package has a neural network toolbox which includes both Multilayer Perceptron (MLP)-type [46] as well as RBF-type [47] models. Provisions for random number generation (seed) are also provided with this software under the Transformations option. Any data set for neural network modelling purpose has to be partitioned into three partitions ... 

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