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java.lang.Objectorg.apache.commons.math.stat.descriptive.AbstractUnivariateStatistic
org.apache.commons.math.stat.descriptive.AbstractStorelessUnivariateStatistic
org.apache.commons.math.stat.descriptive.moment.Variance
public class Variance
Computes the variance of the available values. By default, the unbiased "sample variance" definitional formula is used:
variance = sum((x_i - mean)^2) / (n - 1)
where mean is the Mean and n is the number
of sample observations.
The definitional formula does not have good numerical properties, so this implementation does not compute the statistic using the definitional formula.
getResult method computes the variance using
updating formulas based on West's algorithm, as described in
Chan, T. F. and
J. G. Lewis 1979, Communications of the ACM,
vol. 22 no. 9, pp. 526-531.evaluate methods leverage the fact that they have the
full array of values in memory to execute a two-pass algorithm.
Specifically, these methods use the "corrected two-pass algorithm" from
Chan, Golub, Levesque, Algorithms for Computing the Sample Variance,
American Statistician, vol. 37, no. 3 (1983) pp. 242-247.increment or
incrementAll and then executing getResult will
sometimes give a different, less accurate, result than executing
evaluate with the full array of values. The former approach
should only be used when the full array of values is not available.
The "population variance" ( sum((x_i - mean)^2) / n ) can also
be computed using this statistic. The isBiasCorrected
property determines whether the "population" or "sample" value is
returned by the evaluate and getResult methods.
To compute population variances, set this property to false.
Note that this implementation is not synchronized. If
multiple threads access an instance of this class concurrently, and at least
one of the threads invokes the increment() or
clear() method, it must be synchronized externally.
| Field Summary | |
|---|---|
protected boolean |
incMoment
Boolean test to determine if this Variance should also increment the second moment, this evaluates to false when this Variance is constructed with an external SecondMoment as a parameter. |
protected SecondMoment |
moment
SecondMoment is used in incremental calculation of Variance |
| Constructor Summary | |
|---|---|
Variance()
Constructs a Variance with default (true) isBiasCorrected
property. |
|
Variance(boolean isBiasCorrected)
Constructs a Variance with the specified isBiasCorrected
property |
|
Variance(boolean isBiasCorrected,
SecondMoment m2)
Constructs a Variance with the specified isBiasCorrected
property and the supplied external second moment. |
|
Variance(SecondMoment m2)
Constructs a Variance based on an external second moment. |
|
Variance(Variance original)
Copy constructor, creates a new Variance identical
to the original |
|
| Method Summary | |
|---|---|
void |
clear()
Clears the internal state of the Statistic |
Variance |
copy()
Returns a copy of the statistic with the same internal state. |
static void |
copy(Variance source,
Variance dest)
Copies source to dest. |
double |
evaluate(double[] values)
Returns the variance of the entries in the input array, or Double.NaN if the array is empty. |
double |
evaluate(double[] values,
double mean)
Returns the variance of the entries in the input array, using the precomputed mean value. |
double |
evaluate(double[] values,
double[] weights)
Returns the weighted variance of the entries in the the input array. |
double |
evaluate(double[] values,
double[] weights,
double mean)
Returns the weighted variance of the values in the input array, using the precomputed weighted mean value. |
double |
evaluate(double[] values,
double[] weights,
double mean,
int begin,
int length)
Returns the weighted variance of the entries in the specified portion of the input array, using the precomputed weighted mean value. |
double |
evaluate(double[] values,
double[] weights,
int begin,
int length)
Returns the weighted variance of the entries in the specified portion of the input array, or Double.NaN if the designated subarray
is empty. |
double |
evaluate(double[] values,
double mean,
int begin,
int length)
Returns the variance of the entries in the specified portion of the input array, using the precomputed mean value. |
double |
evaluate(double[] values,
int begin,
int length)
Returns the variance of the entries in the specified portion of the input array, or Double.NaN if the designated subarray
is empty. |
long |
getN()
Returns the number of values that have been added. |
double |
getResult()
Returns the current value of the Statistic. |
void |
increment(double d)
Updates the internal state of the statistic to reflect the addition of the new value. |
boolean |
isBiasCorrected()
|
void |
setBiasCorrected(boolean biasCorrected)
|
| Methods inherited from class org.apache.commons.math.stat.descriptive.AbstractStorelessUnivariateStatistic |
|---|
equals, hashCode, incrementAll, incrementAll |
| Methods inherited from class org.apache.commons.math.stat.descriptive.AbstractUnivariateStatistic |
|---|
test, test |
| Methods inherited from class java.lang.Object |
|---|
clone, finalize, getClass, notify, notifyAll, toString, wait, wait, wait |
| Field Detail |
|---|
protected SecondMoment moment
protected boolean incMoment
| Constructor Detail |
|---|
public Variance()
isBiasCorrected
property.
public Variance(SecondMoment m2)
m2 - the SecondMoment (Third or Fourth moments work
here as well.)public Variance(boolean isBiasCorrected)
isBiasCorrected
property
isBiasCorrected - setting for bias correction - true means
bias will be corrected and is equivalent to using the argumentless
constructor
public Variance(boolean isBiasCorrected,
SecondMoment m2)
isBiasCorrected
property and the supplied external second moment.
isBiasCorrected - setting for bias correction - true means
bias will be correctedm2 - the SecondMoment (Third or Fourth moments work
here as well.)public Variance(Variance original)
Variance identical
to the original
original - the Variance instance to copy| Method Detail |
|---|
public void increment(double d)
If all values are available, it is more accurate to use
evaluate(double[]) rather than adding values one at a time
using this method and then executing getResult(), since
evaluate leverages the fact that is has the full
list of values together to execute a two-pass algorithm.
See Variance.
increment in interface StorelessUnivariateStatisticincrement in class AbstractStorelessUnivariateStatisticd - the new value.public double getResult()
getResult in interface StorelessUnivariateStatisticgetResult in class AbstractStorelessUnivariateStatisticDouble.NaN if it
has been cleared or just instantiated.public long getN()
getN in interface StorelessUnivariateStatisticpublic void clear()
clear in interface StorelessUnivariateStatisticclear in class AbstractStorelessUnivariateStatisticpublic double evaluate(double[] values)
Double.NaN if the array is empty.
See Variance for details on the computing algorithm.
Returns 0 for a single-value (i.e. length = 1) sample.
Throws IllegalArgumentException if the array is null.
Does not change the internal state of the statistic.
evaluate in interface UnivariateStatisticevaluate in class AbstractStorelessUnivariateStatisticvalues - the input array
IllegalArgumentException - if the array is nullUnivariateStatistic.evaluate(double[])
public double evaluate(double[] values,
int begin,
int length)
Double.NaN if the designated subarray
is empty.
See Variance for details on the computing algorithm.
Returns 0 for a single-value (i.e. length = 1) sample.
Does not change the internal state of the statistic.
Throws IllegalArgumentException if the array is null.
evaluate in interface UnivariateStatisticevaluate in class AbstractStorelessUnivariateStatisticvalues - the input arraybegin - index of the first array element to includelength - the number of elements to include
IllegalArgumentException - if the array is null or the array index
parameters are not validUnivariateStatistic.evaluate(double[], int, int)
public double evaluate(double[] values,
double[] weights,
int begin,
int length)
Returns the weighted variance of the entries in the specified portion of
the input array, or Double.NaN if the designated subarray
is empty.
Uses the formula
Σ(weights[i]*(values[i] - weightedMean)2)/(Σ(weights[i]) - 1)where weightedMean is the weighted mean
This formula will not return the same result as the unweighted variance when all weights are equal, unless all weights are equal to 1. The formula assumes that weights are to be treated as "expansion values," as will be the case if for example the weights represent frequency counts. To normalize weights so that the denominator in the variance computation equals the length of the input vector minus one, use
evaluate(values, MathUtils.normalizeArray(weights, values.length));
Returns 0 for a single-value (i.e. length = 1) sample.
Throws IllegalArgumentException if any of the following are true:
Does not change the internal state of the statistic.
Throws IllegalArgumentException if either array is null.
evaluate in interface WeightedEvaluationvalues - the input arrayweights - the weights arraybegin - index of the first array element to includelength - the number of elements to include
IllegalArgumentException - if the parameters are not valid
public double evaluate(double[] values,
double[] weights)
Returns the weighted variance of the entries in the the input array.
Uses the formula
Σ(weights[i]*(values[i] - weightedMean)2)/(Σ(weights[i]) - 1)where weightedMean is the weighted mean
This formula will not return the same result as the unweighted variance when all weights are equal, unless all weights are equal to 1. The formula assumes that weights are to be treated as "expansion values," as will be the case if for example the weights represent frequency counts. To normalize weights so that the denominator in the variance computation equals the length of the input vector minus one, use
evaluate(values, MathUtils.normalizeArray(weights, values.length));
Returns 0 for a single-value (i.e. length = 1) sample.
Throws IllegalArgumentException if any of the following are true:
Does not change the internal state of the statistic.
Throws IllegalArgumentException if either array is null.
evaluate in interface WeightedEvaluationvalues - the input arrayweights - the weights array
IllegalArgumentException - if the parameters are not valid
public double evaluate(double[] values,
double mean,
int begin,
int length)
Double.NaN if the designated subarray is empty.
See Variance for details on the computing algorithm.
The formula used assumes that the supplied mean value is the arithmetic mean of the sample data, not a known population parameter. This method is supplied only to save computation when the mean has already been computed.
Returns 0 for a single-value (i.e. length = 1) sample.
Throws IllegalArgumentException if the array is null.
Does not change the internal state of the statistic.
values - the input arraymean - the precomputed mean valuebegin - index of the first array element to includelength - the number of elements to include
IllegalArgumentException - if the array is null or the array index
parameters are not valid
public double evaluate(double[] values,
double mean)
Double.NaN if the array
is empty.
See Variance for details on the computing algorithm.
If isBiasCorrected is true the formula used
assumes that the supplied mean value is the arithmetic mean of the
sample data, not a known population parameter. If the mean is a known
population parameter, or if the "population" version of the variance is
desired, set isBiasCorrected to false before
invoking this method.
Returns 0 for a single-value (i.e. length = 1) sample.
Throws IllegalArgumentException if the array is null.
Does not change the internal state of the statistic.
values - the input arraymean - the precomputed mean value
IllegalArgumentException - if the array is null
public double evaluate(double[] values,
double[] weights,
double mean,
int begin,
int length)
Double.NaN if the designated subarray is empty.
Uses the formula
Σ(weights[i]*(values[i] - mean)2)/(Σ(weights[i]) - 1)
The formula used assumes that the supplied mean value is the weighted arithmetic mean of the sample data, not a known population parameter. This method is supplied only to save computation when the mean has already been computed.
This formula will not return the same result as the unweighted variance when all weights are equal, unless all weights are equal to 1. The formula assumes that weights are to be treated as "expansion values," as will be the case if for example the weights represent frequency counts. To normalize weights so that the denominator in the variance computation equals the length of the input vector minus one, use
evaluate(values, MathUtils.normalizeArray(weights, values.length), mean);
Returns 0 for a single-value (i.e. length = 1) sample.
Throws IllegalArgumentException if any of the following are true:
Does not change the internal state of the statistic.
values - the input arrayweights - the weights arraymean - the precomputed weighted mean valuebegin - index of the first array element to includelength - the number of elements to include
IllegalArgumentException - if the parameters are not valid
public double evaluate(double[] values,
double[] weights,
double mean)
Returns the weighted variance of the values in the input array, using the precomputed weighted mean value.
Uses the formula
Σ(weights[i]*(values[i] - mean)2)/(Σ(weights[i]) - 1)
The formula used assumes that the supplied mean value is the weighted arithmetic mean of the sample data, not a known population parameter. This method is supplied only to save computation when the mean has already been computed.
This formula will not return the same result as the unweighted variance when all weights are equal, unless all weights are equal to 1. The formula assumes that weights are to be treated as "expansion values," as will be the case if for example the weights represent frequency counts. To normalize weights so that the denominator in the variance computation equals the length of the input vector minus one, use
evaluate(values, MathUtils.normalizeArray(weights, values.length), mean);
Returns 0 for a single-value (i.e. length = 1) sample.
Throws IllegalArgumentException if any of the following are true:
Does not change the internal state of the statistic.
values - the input arrayweights - the weights arraymean - the precomputed weighted mean value
IllegalArgumentException - if the parameters are not validpublic boolean isBiasCorrected()
public void setBiasCorrected(boolean biasCorrected)
biasCorrected - The isBiasCorrected to set.public Variance copy()
copy in interface StorelessUnivariateStatisticcopy in interface UnivariateStatisticcopy in class AbstractStorelessUnivariateStatistic
public static void copy(Variance source,
Variance dest)
Neither source nor dest can be null.
source - Variance to copydest - Variance to copy to
NullPointerException - if either source or dest is null
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| SUMMARY: NESTED | FIELD | CONSTR | METHOD | DETAIL: FIELD | CONSTR | METHOD | |||||||||