private VarianceWritable valeurS = new VarianceWritable();
public void reduce(NullWritable cleI,
                   Iterable<VarianceWritable> valeursI, Context context)
        throws IOException, InterruptedException
{
    valeurS.clear();
    for (VarianceWritable valeur : valeursI) {
        valeurS.add(valeur);
    }
    context.write(cleI, valeurS);
}