private VarianceWritable total = new VarianceWritable();
private DoubleWritable valeurS = new DoubleWritable();

@Override
public void reduce(NullWritable cleI,
                   Iterable<VarianceWritable> valeursI, Context context)
        throws IOException, InterruptedException
{
    total.clear();  // indispensable si plusieurs reduce
    for (VarianceWritable valeur : valeursI) {
        total.add(valeur);
    }
    valeurS.set( total.getVariance() );
    context.write(cleI, valeurS);
}