public double stdev() {
        if (nround == 0)
            return nfruits;
        double avg = average();
        int sum = 0;
        for (int i = 0; i < nround; i++) {
            double diff = avg - score(i);
            sum += Math.pow(diff, 2.0);
        }
        return Math.pow((sum/nround), 0.5);

    }