A value set data structure for rapid statistics queries in Java - follow-up
This is not a Set. A Set contains no duplicate elements. You should rename the class, perhaps to a Bag or a MultiSet.

I'm also worried about this bit:

/**
 * Computes the variance of this set.
 * 
 * @return the variance. 
 */
public double getVariance() {
    double step1 = squareSum - sum * sum / size;
    return step1 / (size - 1);
}

/**
 * Computes the standard deviation of this set.
 * 
 * @return the standard deviation.
 */
public strictfp double getStandardDeviation() {
    return Math.sqrt(getVariance());
}


Either strictfp cascades, and you calculate the variance twice, but in different ways for toString(), or it doesn't, and in that case, no math is being done in this method and the keyword can go.