JAVA standard deviation

Question

Here are my methods for calculating standard deviation, which for some reason unknown by me does not work (I left my test method down here as well).

public strictfp Double getMedia(List<Double> valor) {
    try {
        return getSoma(valor) / valor.size();
    } catch (NullPointerException e) {
        throw new IllegalArgumentException("The list has null values");
    }
}

public strictfp Double getSoma(List<Double> valor) {
    Double soma = 0D;
    for (int i = 0; i < valor.size(); i++) {
        soma += valor.get(i);
    }
    return soma;
}

public strictfp Double getDesvioPadrao(List<Double> valor) {
    Double media = getMedia(valor);
    int tam = valor.size();
    Double desvPadrao = 0D;
    for (Double vlr : valor) {
        Double aux = vlr - media;
        desvPadrao += aux * aux;
    }
    return Math.sqrt(desvPadrao / tam);
}

#

Test method:

@SuppressWarnings("deprecation")
@Test
private void TesteDesvioPadrao() {
    List<Double> valor = new ArrayList<Double>();
    valor.add(13.0);
    valor.add(23.0);
    valor.add(4.0);
    valor.add(2.0);
    valor.add(11.0);
    valor.add(12.0);

    assertEquals(new Double(7.467708), ilbt.getDesvioPadrao(valor));

}

Answer 1

The error is in the calculation of the standard deviation at the end of the method getDesvioPadrao.

In it you return the square root of the variance division by the collection size. How are you calculating the standard deviation of a sample, should be the size of the collection subtracting 1.

public strictfp Double getDesvioPadrao(List<Double> valor) {
    Double media = getMedia(valor);
    int tam = valor.size();
    Double desvPadrao = 0D;
    for (Double vlr : valor) {
        Double aux = vlr - media;
        desvPadrao += aux * aux;
    }
    return Math.sqrt(desvPadrao / (tam - 1));
}

^{See working on repl.it}