不偏標準偏差と母標準偏差をApacheCommonsMathのStatで算出する

Calculator.java

import org.apache.commons.math3.stat.descriptive.SynchronizedSummaryStatistics;
import org.apache.commons.math3.util.FastMath;
import org.apache.commons.math3.util.Precision;

import java.util.List;

public class Calculator {
	protected Double calculateStandardDiviation(List<Double> scores){
		SynchronizedSummaryStatistics stats = new SynchronizedSummaryStatistics();
		for (Double score : scores){
			stats.addValue(score);
		}
		Double stdev = stats.getStandardDeviation();
		return Precision.round(stdev, 2);
	}
	protected Double calculatePopulationStandardDiviation(List<Double> scores){
		SynchronizedSummaryStatistics stats = new SynchronizedSummaryStatistics();
		for (Double score : scores){
			stats.addValue(score);
		}
		Double varp = stats.getPopulationVariance();
		Double stdevp = FastMath.sqrt(varp);
		return Precision.round(stdevp, 2);
	}
}

CalculatorSpec.groovy

import spock.lang.Specification
import spock.lang.Unroll

class CalculatorSpec extends Specification {
	@Unroll
	def "不偏標準偏差を算出する"() {
		given:
		def calc = new Calculator();
		when:
		def arg = scores.collect { it as Double };
		def stdev = calc.calculateStandardDiviation(arg);
		then:
		stdev == result;
		where:
		scores            | result
		[1, 2, 3, 4, 5]   | 1.58
		[9, 8, -1, -2, 5] | 5.07
		[0, 0, 0, 0, 0]   | 0
	}

	@Unroll
	def "母標準偏差を算出する"() {
		given:
		def calc = new Calculator();
		when:
		def arg = scores.collect { it as Double };
		def stdevp = calc.calculatePopulationStandardDiviation(arg);
		then:
		stdevp == result;
		where:
		scores            | result
		[1, 2, 3, 4, 5]   | 1.41
		[9, 8, -1, -2, 5] | 4.53
		[0, 0, 0, 0, 0]   | 0
	}
}