org.apache.commons.math.distribution
Class PoissonDistributionImpl

java.lang.Object
  org.apache.commons.math.distribution.AbstractDistribution
      org.apache.commons.math.distribution.AbstractIntegerDistribution
          org.apache.commons.math.distribution.PoissonDistributionImpl

All Implemented Interfaces:

DiscreteDistribution, Distribution, IntegerDistribution, PoissonDistribution, Serializable

public class PoissonDistributionImpl

extends AbstractIntegerDistribution

implements PoissonDistribution, Serializable

Implementation for the @link{PoissonDistribution}

Version:

$Revision: 1.2 $ $Date: 2004/11/07 20:39:15 $

See Also:

Serialized Form

Field Summary

(package private) static long

serialVersionUID Serializable version identifier

Constructor Summary

PoissonDistributionImpl(double p)
Create a new Poisson distribution with the given the mean.

Method Summary

double	cumulativeProbability(int x) The probability distribution function P(X <= x) for a Poisson distribution.
protected int	getDomainLowerBound(double p) Access the domain value lower bound, based on p, used to bracket a CDF root.
protected int	getDomainUpperBound(double p) Access the domain value upper bound, based on p, used to bracket a CDF root.
double	getMean() Get the Poisson mean for the distribution.
double	normalApproximateProbability(int x) Calculates the Poisson distribution function using a normal approximation.
double	probability(int x) The probability mass function P(X = x) for a Poisson distribution.
void	setMean(double p) Set the Poisson mean for the distribution.

Methods inherited from class org.apache.commons.math.distribution.AbstractIntegerDistribution

cumulativeProbability, cumulativeProbability, inverseCumulativeProbability, probability

Methods inherited from class org.apache.commons.math.distribution.AbstractDistribution

cumulativeProbability

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface org.apache.commons.math.distribution.IntegerDistribution

cumulativeProbability, inverseCumulativeProbability

Methods inherited from interface org.apache.commons.math.distribution.DiscreteDistribution

probability

Methods inherited from interface org.apache.commons.math.distribution.Distribution

cumulativeProbability, cumulativeProbability

Field Detail

serialVersionUID

static final long serialVersionUID

Serializable version identifier

See Also:

Constant Field Values

Constructor Detail

PoissonDistributionImpl

public PoissonDistributionImpl(double p)

Create a new Poisson distribution with the given the mean. The mean value must be positive; otherwise an IllegalArgument is thrown.

Parameters:

p - the Poisson mean

Throws:

IllegalArgumentException - if p ≤ 0

Method Detail

getMean

public double getMean()

Get the Poisson mean for the distribution.

Specified by:

getMean in interface PoissonDistribution

Returns:

the Poisson mean for the distribution.

setMean

public void setMean(double p)

Set the Poisson mean for the distribution. The mean value must be positive; otherwise an IllegalArgument is thrown.

Specified by:

setMean in interface PoissonDistribution

Parameters:

p - the Poisson mean value

Throws:

IllegalArgumentException - if p ≤ 0

probability

public double probability(int x)

The probability mass function P(X = x) for a Poisson distribution.

Specified by:

probability in interface IntegerDistribution

Parameters:

x - the value at which the probability density function is evaluated.

Returns:

the value of the probability mass function at x

cumulativeProbability

public double cumulativeProbability(int x)
                             throws MathException

The probability distribution function P(X <= x) for a Poisson distribution.

Specified by:

cumulativeProbability in interface IntegerDistribution

Specified by:

cumulativeProbability in class AbstractIntegerDistribution

Parameters:

x - the value at which the PDF is evaluated.

Returns:

Poisson distribution function evaluated at x

Throws:

MathException - if the cumulative probability can not be computed due to convergence or other numerical errors.

normalApproximateProbability

public double normalApproximateProbability(int x)
                                    throws MathException

Calculates the Poisson distribution function using a normal approximation. The N(mean, sqrt(mean)) distribution is used to approximate the Poisson distribution.

The computation uses "half-correction" -- evaluating the normal distribution function at x + 0.5

Specified by:

normalApproximateProbability in interface PoissonDistribution

Parameters:

x - the upper bound, inclusive

Returns:

the distribution function value calculated using a normal approximation

Throws:

MathException - if an error occurs computing the normal approximation

getDomainLowerBound

protected int getDomainLowerBound(double p)

Access the domain value lower bound, based on p, used to bracket a CDF root. This method is used by AbstractIntegerDistribution.inverseCumulativeProbability(double) to find critical values.

Specified by:

getDomainLowerBound in class AbstractIntegerDistribution

Parameters:

p - the desired probability for the critical value

Returns:

domain lower bound

getDomainUpperBound

protected int getDomainUpperBound(double p)

Access the domain value upper bound, based on p, used to bracket a CDF root. This method is used by AbstractIntegerDistribution.inverseCumulativeProbability(double) to find critical values.

Specified by:

getDomainUpperBound in class AbstractIntegerDistribution

Parameters:

p - the desired probability for the critical value

Returns:

domain upper bound