private double computeN(double n) {
        double p_max = (1 + skewFactor)/n;
        return Math.max(1, 1/p_max);
    }
Use StudentT distribution and estimate how many hits you need from each partition
to to get the globally top-k documents with the desired probability

@author baldersheim
container-search/src/main/java/com/yahoo/search/dispatch/TopKEstimator.java