johnsrnd

Johnson system random numbers

Syntax

r = johnsrnd(quantiles,m,n)
r = johnsrnd(quantiles)
[r,type] = johnsrnd(...)
[r,type,coefs] = johnsrnd(...)

Description

r = johnsrnd(quantiles,m,n) returns an m-by-n matrix of random numbers drawn from the distribution in the Johnson system that satisfies the quantile specification given by quantiles. quantiles is a four-element vector of quantiles for the desired distribution that correspond to the standard normal quantiles [–1.5 –0.5 0.5 1.5]. In other words, you specify a distribution from which to draw random values by designating quantiles that correspond to the cumulative probabilities [0.067 0.309 0.691 0.933]. quantiles may also be a 2-by-4 matrix whose first row contains four standard normal quantiles, and whose second row contains the corresponding quantiles of the desired distribution. The standard normal quantiles must be spaced evenly.

    Note:   Because r is a random sample, its sample quantiles typically differ somewhat from the specified distribution quantiles.

r = johnsrnd(quantiles) returns a scalar value.

r = johnsrnd(quantiles,m,n,...) or r = johnsrnd(quantiles,[m,n,...]) returns an m-by-n-by-... array.

[r,type] = johnsrnd(...) returns the type of the specified distribution within the Johnson system. type is 'SN', 'SL', 'SB', or 'SU'. Set m and n to zero to identify the distribution type without generating any random values.

The four distribution types in the Johnson system correspond to the following transformations of a normal random variate:

  • 'SN' — Identity transformation (normal distribution)

  • 'SL' — Exponential transformation (lognormal distribution)

  • 'SB' — Logistic transformation (bounded)

  • 'SU' — Hyperbolic sine transformation (unbounded)

[r,type,coefs] = johnsrnd(...) returns coefficients coefs of the transformation that defines the distribution. coefs is [gamma, eta, epsilon, lambda]. If z is a standard normal random variable and h is one of the transformations defined above, r = lambda*h((z-gamma)/eta)+epsilon is a random variate from the distribution type corresponding to h.

Examples

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Generate Random Samples Using the Johnson System

This example shows several different approaches to using the Johnson system of flexible distribution families to generate random numbers and fit a distribution to sample data.

Generate random values with longer tails than a standard normal.

rng default;  % For reproducibility
r = johnsrnd([-1.7 -.5 .5 1.7],1000,1);
figure;
qqplot(r);

Generate random values skewed to the right.

r = johnsrnd([-1.3 -.5 .5 1.7],1000,1);
figure;
qqplot(r);

Generate random values that match some sample data well in the right-hand tail.

load carbig;
qnorm = [.5 1 1.5 2];
q = quantile(Acceleration, normcdf(qnorm));
r = johnsrnd([qnorm;q],1000,1);
[q;quantile(r,normcdf(qnorm))]
ans =

   16.7000   18.2086   19.5376   21.7263
   16.6986   18.2220   19.9078   22.0918

Determine the distribution type and the coefficients.

[r,type,coefs] = johnsrnd([qnorm;q],0)
r =

     []


type =

SU


coefs =

    1.0920    0.5829   18.4382    1.4494

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See Also

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